WO2024216317A1

WO2024216317A1 - Electrical oscillation circuit and method for driving an oscillation element

Info

Publication number: WO2024216317A1
Application number: PCT/AT2024/060154
Authority: WO
Inventors: Hajrudin BESIC; Silvan SCHMID
Original assignee: Invisible-Light Labs Gmbh
Priority date: 2023-04-17
Filing date: 2024-04-17
Publication date: 2024-10-24

Abstract

The invention relates to an electrical oscillation circuit (1) for driving an oscillation element (4) at an oscillation frequency (f_osc), comprising: a circuit input (2) for receiving an oscillation signal (O) from the oscillation element (4); a phase detection unit (10) configured to detect a predetermined phase (φ₀) of the oscillation signal (O); a pulse generator unit (15) configured to generate pulses (16); a circuit output (3) for outputting a driving signal (D) to the oscillation element (4); and a filter unit (7) arranged prior to the phase detection unit (10) and having a filter (8) with a filter frequency range (9) containing the oscillation frequency (f_osc), the filter (8) being configured to suppress signal components of the oscillation signal (O) outside the filter frequency range (9) before the oscillation signal (O) reaches the phase detection unit (10). The invention further relates to a method for driving an oscillation element (4) at an oscillation frequency (f_osc) •

Description

Electrical oscillation circuit and method for driving an oscillation element

The present invention relates to an electrical oscillation circuit for driving an oscillation element at an oscillation frequency, in particular at a resonance frequency, the electrical oscillation circuit comprising : a circuit input configured to receive an oscillation signal from the oscillation element ; a phase detection unit configured to detect a predetermined phase of the oscillation signal , preferably zero-crossings of the oscillation signal , and configured to generate a phase detection signal ; a pulse generator unit configured to generate pulses based on the phase detection signal ; a circuit output configured to output a driving signal to the oscillation element containing the pulses .

The invention also relates to a method for driving an oscillation element at an oscillation frequency, in particular at a resonance frequency, the method comprising the following steps : receiving an oscillation signal from the oscillation element ; detecting a predetermined phase of the oscillation signal , preferably zero-crossings of the oscillation signal , and generating a phase detection signal ; generating pulses based on the phase detection signal ; and outputting a driving signal to the oscillation element containing the pulses .

The invention further relates to an oscillation system, a photothermal detection system for detecting infrared radiation, a method for tracking an oscillation frequency and a method for detecting infrared radiation .

Many measurement applications rely on driving an oscillation element , such as a micro- or nanomechanical resonator, at a speci fic oscillation frequency and monitoring changes in its behavior over time . Such changes can be indicative of a physical quantity of interest acting on the oscillation element , enabling its measurement . Often, the oscillation element is driven at one of its resonance frequencies , which may shi ft when a physical quantity starts to act on the oscillation element or changes . Therefore , shi fts in the resonance frequency of an oscillation element can be used to measure or detect physical quantities or their changes . This measurement principle is widely used in micro- and nanomechanical sensing applications , including gas sensors , particle sensors , and thermal detectors .

In many applications , such as the already mentioned measurement applications , it is essential to drive the oscillation element at or very close to one of its resonance frequencies . To achieve this , electrical oscillation circuits as described above , which are referred to as sel f-sustaining oscillator circuits ( SSO- circuits ) , can be utili zed . Advantageously, SSO-circuits have very good speed performances in comparison to , for example , feedback free approaches . In SSO-circuits , the oscillation signal is positively fed back to the oscillation element to maintain the oscillation at its resonance frequency . A disadvantageous feature of SSO-circuits is that , in linear approximations , SSO-circuits are unstable systems , and the amplitude of the oscillation of an unstable system grows exponentially with time until nonlinearities become important and limit the amplitude . Thus , the advantage of fast performance is , to some extent , bought with the disadvantage of instable behavior, which has to be compensated by taking further measures .

Electrical oscillation circuits of the above-described type are for example known from WO 2013/ 123348 Al , which discloses an oscillation driver circuit with an ampli fication element adapted to be coupled to an oscillator output of an oscillator and a driver element adapted to be coupled to an oscillator input of the oscillator . A pulse generator is configured to receive an ampli fied oscillator output and to generate a pulsed output signal which is in phase with the ampli fied oscillator output .

Bl okhina E. , Pons, J. , Ri cart , J. , Feel , 0. , & Pumar, M. D. (2010) . Control of MEMS vibration inodes with pulsed digital oscillators: Part I— Theory. IEEE transactions on circuits and systems I: regular papers, 57(8) , 1865-1878 relates to a pulsed digital oscillator (PDO) with a 1-bit quantizer and a digital feedback filter which allows for the monitoring of resonance frequencies .

Schmid, S. (2009) . Electrostatically actuated all-polymer microbeam resonators: Characterization and application (Vol. 6) . ETH Zurich discloses an oscillator circuit for driving a polymer micro resonator by means of positive feedback. In the setup shown in Fig. 9.9, a laser-Doppler vibrometer measures the movement of the resonator and feeds the oscillation signal into the circuit. A comparator detects the positive or negative phase of the oscillation signal. In order to take account of the delay of the comparator, the decreasing slope of the rectangular signal is triggered with a high-pass filter placed in front of a Schmitt-trigger . The switching point of the Schmitt-trigger can be controlled by changing the cut-off frequency of the high pass filter .

The documents

- ZOU XUDONG ET AL, Non-Linear Frequency Noise Modulation in a Resonant MEMS Accelerometer, IEEE SENSORS JOURNAL, IEEE, USA, vol. 17, no. 13, pages 4122-4127,

- XU LIU ET AL, Effect of Joule heating on the performance of micromechanical piezoresistive oscillator, SENSORS AND ACTUATORS A: PHYSICAL, ELSEVIER BV, NL, vol. 333,

- E-Y LEE J ET AL, A Single-Crystal-Silicon Bulk-Acoustic-Mode Microresonator Oscillator, IEEE ELECTRON DEVICE LETTERS, IEEE, USA, vol. 29, no. 7, and

- DEEPAK K AGRAWAL ET AL, Modelling non-linearities in a MEMS square wave oscillator, FREQUENCY CONTROL SYMPOSIUM (FCS) , 2012 IEEE, INTERNATIONAL, IEEE, disclose oscillation circuits comprising comparators for detecting predetermined phases in an oscillation signal and also for the generation of pulses. One disadvantage of the disclosed oscillation circuits is that with this design the flexibility of the oscillation circuits is limited .

Typically, oscillation elements have multiple excitable modes with di f ferent resonance frequencies . However, the modes of an oscillation element may be af fected di f ferently by a physical quantity . To ensure accurate measurements , it is desirable to track resonance frequencies of speci fic modes of an oscillation element . The detection of shi fts of a resonance frequency of one mode can become challenging or even impossible i f it interferes with other modes .

However, SSO-circuits known from the prior art tend to excite multiple modes of an oscillation element . Another disadvantage of electrical oscillation circuits based on SSO-schemes known from the prior art is that detection noise can introduce errors into the measurements .

Accordingly, it is an obj ective of the present invention to eliminate or at least alleviate at least some of the disadvantages of the prior art . Preferably it is an obj ective of the present invention to improve the tracking of an oscillation frequency of an oscillation element driven by an electrical oscillation circuit of the type mentioned above , thereby increasing the measurement accuracy when the electrical oscillation circuit is used in measurement setups .

The obj ective is solved by an electrical oscillation circuit according to claim 1 and a method for driving an oscillation element according to claim 25. An oscillation system is claimed in claim 22 . Claim 24 relates to a photothermal detection system for detecting infrared radiation . Claim 29 is directed to a method for tracking an oscillation frequency . A method for detecting infrared radiation is claimed in claim 30 .

The invention according to independent claim 1 provides for an electrical oscillation circuit of the above described type which is characteri zed by a filter unit arranged prior to the phase detection unit and having at least one filter with a filter frequency range containing the oscillation frequency, the at least one filter being configured to suppress signal components of the oscillation signal outside the filter frequency range before the oscillation signal reaches the phase detection unit .

Advantageously, by means of the filter unit the excitation of unwanted modes of the oscillation element can be damped or avoided so that the inventive electrical oscillation circuit only or mainly stimulates and detects desired modes , preferably essentially only one single desired mode , of the oscillation element . Further, the filter unit reduces detection noise in the circuit , which also avoids the stimulation and detection of unwanted modes of the oscillation element and enhances the measurement accuracy when the electrical oscillation circuit is used for tracking an oscillation frequency . In a preferred embodiment , the electrical oscillation circuit may be entirely built on one single carrier element , in particular on a circuit board . In another embodiment of the invention, the units of the electrical oscillation circuit , such as the phase detection unit , the pulse generator unit and the filter unit , may also be disposed on separate carrier elements , in particular circuit boards , which can be electrically connected with each other to form the electrical oscillation circuit . Said units of the electrical oscillation circuit may be integrated circuits or may use integrated and/or discrete electrical components . In general , within the scope of the claims , the units can be functional units that may be implemented by one and the same physical unit or two or more physical units , each performing one or more of the functions . In another embodiment of the invention, the entire electrical oscillation circuit may be one single integrated circuit ( IC ) , which may also be disposed on a carrier element , such as a circuit board . The circuit input and the circuit output may comprise at least one electrical contact surface for receiving the oscillation signal and outputting the driving signal . The circuit output can be electrically connected to the oscillation element or to an actuator for actuating the oscillation element , for example , with wires and/or circuit traces . The circuit input can be electrically connected to the oscillation element or a measurement device for measuring an oscillation of the oscillation element , for example , with wires and/or circuit traces . The phase detection unit is configured to detect a predetermined phase of the oscillation signal after the oscillation signal has passed the filter unit . The phase detection unit may be directly connected with the filter unit . Directly connected means , in this context , that no further unit is arranged between the filter unit and the phase detection unit . However, directly connected with each other still comprises connection of the filter unit with the phase detection unit , for example , via wires or traces of a circuit board . Further, the phase detection unit is configured to generate a phase detection signal based on the oscillation signal . In one embodiment of the invention, the phase detection unit may be a comparator or a Schmitt-trigger . The phase detection signal generated by the phase detection unit contains information about the occurrence of the predetermined phase of the oscillation signal and communicates this information to the pulse generator unit . To this end, the phase detection unit and the pulse generator unit may be directly connected with each other . The predetermined phase of the oscillation signal may be , for example , a certain voltage level or a certain current level . In a preferred embodiment of the invention, the predetermined phase of the oscillation signal may be a zero crossing, a minimum or a maximum of the oscillation signal . With regard to periodic oscillation signals , the predetermined phase of an oscillation signal may also be described as phase angle from 0 to 2 *pi or 0 ° to 360 ° of a fundamental wave of the oscillation signal . The phase detection signal may contain a trigger to indicate when the oscillation signal has a predetermined phase so that the pulse generator unit generates a pulse . A trigger may be , for example , a rising or falling signal edge of a signal step or a pulse . In one embodiment of the invention, upon receipt of a trigger, the pulse generator unit may generate a pulse . Preferably, upon receipt of each trigger, the pulse generator unit may generate a pulse . I f the trigger is a pulse , in one embodiment of the invention, this pulse can be fed back to the oscillation element as driving signal . In this case , the pulse generator unit thus can be considered as being included into the phase detection unit . In other words , the phase detection unit and the pulse generator unit may be one single unit . The pulse generator unit is configured to generate pulses based on the phase detection signal . In a preferred embodiment of the invention, the pulse generator unit may be configured to set the form, in particular the height and/or the length, of the pulses according to predefined setpoint values, which may be adjusted. In one embodiment of the invention, the pulse generator unit may comprise a switch, such as a transistor, which may be triggered by the phase detection signal. The transistor may be used in a common-emitter amplifier or in a common-source amplifier, by means of which the pulses may be generated. In another embodiment of the invention, the pulse generator unit may comprise a preferably monostable multivibrator. In a preferred embodiment, such a multivibrator may be implemented in an FPGA (Field-Programmable Gate Array) . However, also an analog implemented monostable multivibrator may be used. The phase detection signal triggers the generation of pulses. The pulse generator unit may generate preferably essentially rectangular pulses. However, also other signal forms, for example, triangular shaped pulses or sine half-wave shaped pulses may be used. Signal forms may be stored in and retrieved from Look-up tables. The driving signal and the phase detection signal may be individual signals. In particular, the driving signal may be a different signal than the phase detection signal, i.e., the driving signal and the phase detection signal in this case are not the same signal. The triggers contained in the phase detection signal may be different from the pulses in the driving signal. Preferably, the phase detection unit and the pulse generator unit are distinct units, in particular separate units, which are, in a preferred embodiment, directly connected with each other. Directly connected with each other means, in this context, that no further unit is arranged between the phase detection unit and the pulse generator unit. However, directly connected with each other still comprises connection of the phase detection unit with the pulse generator unit, for example, via wires or traces of a circuit board. As phase detection unit, a comparator may be used. The pulse generator unit may comprise a switch, such as a transistor, which may be triggered by the phase detection signal. In this way, the pulses may be generated independent from the oscillation frequency of the oscillation signal. A digital-to-analogue converter (DAG) may transduce the digital signal into an analogue signal. The pulses may each comprise a rising pulse edge and a falling pulse edge. The time between the rising and the falling pulse edge may be referred to as pulse width. The pulse width may be adjustable, e.g., by a counter or a timer . Once the width is adj usted, the pulse generator unit may generate the pulses with the adj usted width, independent from the oscillation signal . The pulse generator unit is configured to generate pulses with the same frequency as the oscillation signal . Hence, the driving signal has the same frequency as the oscillation signal . The pulse width may depend on the period of the oscillation signal . The pulse width of the pulses is preferably shorter than the hal f of a period of the oscillation signal . The pulses generated by the pulse generator unit are contained in the driving signal and fed back to the oscillation element . Due to its arrangement prior to the phase detection unit , the filter unit attenuates unwanted modes so that the phase detection unit only detects a predetermined phase of a desired mode . Additionally, by means of the filter unit , noise components of the oscillation signal are suppressed and hence not forwarded to the phase detection unit . The filter unit comprises at least one filter . The filter unit may comprise a plurality of filters arranged in series and/or in parallel . Preferably, the at least one filter may be a biquad filter . Multiple filters arranged in series can reinforce the filter ef fect . Parallel filters with di f ferent filter frequency ranges allow for the use of di fferent oscillation elements with the electrical oscillation circuit . In exemplary embodiments of the invention, the at least one filter of the filter unit may be a high pass filter, a low pass filter or a band pass filter . I f the filter unit comprises several filters , di fferent types of filters , e . g . , a high pass and a low pass filter, may be combined to achieve a desired filter ef fect . The at least one filter suppresses signal components outside the filter frequency range , preferably by a factor of at least 20 dB . The filter frequency range contains the oscillation frequency, which is preferably a resonance frequency of a desired mode of the oscillation element . Thus , signal components with frequencies close to the oscillation frequency, i . e . , frequencies inside the filter frequency range, can pass the filter unit , whereas signal components with frequencies far away from the oscillation frequency, i . e . , frequencies outside the filter frequency range , are filtered out before the oscillation signal reaches the phase detection unit . The filter unit , the phase detection unit and pulse generator unit are preferably arranged in the same signal path of the oscillation signal . Preferably, the components of the circuit are arranged in the following order : filter unit , phase detection unit and pulse generator unit . The electrical oscillation circuit may be implemented at least partially digitally and/or at least partially analogue . For digital implementation, for example a microprocessor , a microcontroller or an FPGA may be used . In one embodiment of the invention, the phase detection unit , the pulse generator unit and/or filter unit may be implemented in a microprocessor . In another embodiment of the invention, the phase detection unit , the pulse generator unit and/or the filter unit are implemented in an FPGA.

In one embodiment of the invention, a form of the pulses is adj ustable .

Preferably, the form of the pulses comprises at least one of a height of the pulses and a width of the pulses . In one embodiment , the form of the pulses may also comprise the contour of the pulses .

In one embodiment of the invention, the pulse generator unit is configured to set the form of the pulses . In this way, the form of the pulses is set upon generation of the pulses .

In one embodiment of the invention, the pulse generator unit is configured to set the form of pulses independently of the oscillation signal , in particular of the oscillation frequency or period of the oscillation signal . This may be achieved by generating the pulses upon detection of triggers contained in the phase detection signal , not certain signal levels of the oscillation signal as it would the case with the sole usage of comparators . In contrast to the sole usage of comparators , as known from the prior art , the separation of the detection of a predetermined phase in the oscillation signal and the generation of pulses allows for generating pulses whose form is independent of the oscillation signal . Advantageously, in particular the width and/or the height of the pulses may be set independently of the oscillation signal , in particular independently of its frequency or period . However, although the form of the pulses may be set independently of the oscillation signal, the oscillation frequency and the frequency of the driving signal, in particular the frequency of the pulses, may still be the same .

In one embodiment of the invention, the pulse generator unit is configured to set the form of the pulses according to at least one setpoint value, which is adjustable. The at least one setpoint value may be fed into the pulse generator unit. By adjusting the at least one setpoint value, the form of the pulses may be adjusted. A height setpoint value may be used to set the height of the pulses. A width setpoint value may be used to set the width of the pulses.

In one embodiment, the phase detection signal and the pulses are individual signals. In other words, the phase detection signal and the pulses are distinct signals. In other words, the pulses generated by the pulse generator unit and the phase detection signal are not the same. Hence, the pulses generated by the pulse generator unit are not contained in the phase detection signal .

In one embodiment, the pulse generator unit and the phase detection unit are distinct units. Preferably, the pulse generator unit and the phase detection unit are separate units connected in particular directly with each other.

In a preferred embodiment of the invention, the at least one filter is a band-pass filter with an upper cut-off frequency and a lower cut-off-frequency defining the frequency range of the filter. The upper cut-off frequency and the lower cut-off frequency may be adjustable between 1 kHz and 25 MHz, depending on the application. In one exemplary embodiment of the invention, the upper cut-off frequency may be set below 25 MHz, preferably below 20 MHz, below 15 MHz, below 10 MHz, below 5 MHz, below 3 MHz, below 2.5 MHz or below 2 MHz. The lower cutoff frequency may be set higher than 500 Hz, preferably higher than 1 kHz, higher than 2 kHz, higher than 3 kHz, higher than 5 kHz, higher than 10 kHz, higher than 15 kHz or higher than 20 kHz. In one exemplary embodiment of the invention, the difference between the upper cut-off frequency and the lower cut-of f frequency of the filter, which may be also referred to as bandwidth, is smaller than 100 kHz , preferably smaller than 50 kHz , smaller than 20 kHz or smaller than 10 kHz . The bandwidth between the upper cut-of f frequency and the lower cutof f frequency may be defined by a quality factor of the at least one filter . The band-pass filter may attenuate the signal components outside the filter frequency range by a factor of at least 6 dB . The at least one filter may be a digital filter or an analogue filter . In a preferred embodiment of the invention, the at least one filter is a digital filter, preferably implemented in a microprocessor or in an FPGA. One advantage of digital filters is that the filter frequency range can be altered by adj usting the parameters of the filter . Thus , it is not necessary to use multiple filters in combination . On the other hand, analogue filters exhibit a fast signal processing .

In one embodiment of the invention, the filter unit comprises at least two filters with di f ferent frequency ranges arranged in parallel electrical paths , wherein a switch allows for switching between the electrical paths . This embodiment is in particular advantageous i f the filters are analogue filters . In one embodiment of the invention, the filter unit comprises at least two analogue band-pass filters with di f ferent filter frequency ranges arranged in parallel . The filter unit may also comprise a switch which allows for selecting one of the parallel band-pass filters to be applied to the oscillation signal . In this way, it is possible to apply di f ferent filter frequency ranges to the oscillation signal .

I f the oscillation element is driven close to or at the resonance frequency, the amplitude of the oscillation element can rise until non-linearities confine the further growth of the oscillation . However, nonlinearities may lead to the excitation of unwanted modes and have detrimental ef fects on the measurement results i f the electrical oscillation circuit is used for measurement applications . Thus , in one embodiment of the invention, the electrical oscillation circuit comprises an amplitude regulation unit configured to adj ust the amplitude of the driving signal . By adj usting the height of the pulses generated by the pulse generator unit , the energy pumped into the oscillation element can be controlled . The amplitude of the driving signal may be reduced, i f the amplitude of the oscillation signal is greater than an upper limit . I f the oscillation signal is lower than a lower limit , the amplitude of the driving signal may be increased . In one embodiment of the invention, the amplitude regulation unit may be included into the pulse generator unit . In one embodiment of the invention, the amplitude regulation unit may be implemented digitally, preferably in an FPGA.

Preferably, the amplitude regulation unit comprises a feedback controller configured to detect the amplitude of the oscillation signal , preferably between the filter unit and the phase detection unit , and to adj ust the amplitude of the driving signal in order to control the amplitude of the oscillation signal according to an oscillation signal amplitude setpoint value . The feedback controller may be , for example , a PI- or a PID-controller . To avoid detection of signal components of unwanted modes of the oscillation signal , the amplitude of the oscillation signal is preferably detected in a signal path part between the filter unit and the phase detection unit . The amplitude of the driving signal may be adj usted by means of a variable gain element . The variable gain element may be arranged after the pulse generator unit and prior to the circuit output . In the present disclosure , "prior" ( or "upstream" ) and " after" ( or "downstream" ) etc . generally refer to the information and signal flow during signal processing; in other words , an input of the latter unit (which is " after" ) is an output or dependent on an output of the former unit (which is "prior" ) . Alternatively, the amplitude can be adj usted by adj usting the height of the pulses directly at the pulse generator unit . Preferably, the amplitude setpoint value may be chosen such that the oscillation of the oscillation element has an amplitude between 10 pm and 100 pm .

In the connected state, the oscillation element and the electrical oscillation circuit form a closed loop which can introduce phase shi fts to the oscillation signal and the driving signal , respectively . To ef ficiently drive an oscillation signal , the driving signal should have a certain phase di fference with the oscillation signal at the output of the oscillation element , depending on the oscillation element and the oscillation frequency . To compensate for phase shi fts introduced into the closed loop, the electrical oscillation circuit preferably comprises a time delay unit configured to introduce a phase shi ft to the driving signal by inducing a time delay . The time delay unit may be a digital delay counter . The time delay unit may be implemented in an FPGA. In one embodiment of the invention, the time delay unit may be included into the pulse generator unit , thereby delaying the generation of the pulses by the time delay . I f the pulse generator unit is digitally implemented, the time delay unit may be a digital delay counter . By introducing a time delay, the pulses of the driving signal may be in phase with a periodic energy trans fer within the oscillation signal . In a preferred embodiment , the time delay unit is arranged after the phase detection unit , preferably also after the pulse generator unit . In other words , preferably, the pulse generator unit is arranged prior to the time delay unit . In this way, only the generated pulses will be delayed, not the measured oscillation signal . In another embodiment , the time delay unit may be included into the pulse generator unit .

In a preferred embodiment of the invention, the time delay is adj ustable to a time delay setpoint value . The time delay setpoint value may be chosen such that phase shi fts introduced by the trans fer function of the oscillation element and the electrical circuit element as well as latencies can be compensated . Typical time delay setpoint values may be between 1 ns and 1 s , preferably between 1 ns and 1 ms , between 1 ns and 500 ps or between 1 ns and 1 ps .

According to one embodiment of the invention, a pulse width of the pulses is adj ustable to a pulse width setpoint value . In this way, the pulses can be adj usted to dif ferent oscillation elements and oscillation frequencies . Preferably, the pulse generator unit is configured to adj ust the pulse width of the pulses to the pulse width setpoint value . Additionally or alternatively, the pulse generator unit may be configured to adj ust the height of the pulses to a pulse height setpoint value . Pulse width setpoint value and a pulse height setpoint value may be adj ustable .

It is advantageous , i f the pulse width setpoint value is set so that the pulse width covers between 5 % and 40 % , preferably between 7 % and 38 % or between 20 % and 35 % or between 25 % and 35 % , of a period of the driving signal . For pulses with nonvertical rising and falling edges , such as triangular shaped pulses or sine hal f-wave shaped pulses , the pulse width may be expressed as FWHM ( Full Width at Hal f Maximum) . The driving signal has the same frequency as the oscillation signal .

When the electrical oscillation circuit is utili zed for measurement applications , it is advantageous i f the electrical oscillation circuit comprises a frequency counter unit configured to detect the frequency and/or a period of the oscillation signal . By means of the frequency counter unit , the frequency and/or a period of the oscillation signal can be tracked . In one embodiment of the invention, the frequency counter unit may comprise a time measurement unit which is configured to determine the duration of one or more periods of the oscillation signal . To this end, the time measurement unit may comprise a clock with a predetermined clock cycle time and a counter for counting the clock cycles . The time measurement unit may be configured to count the number of clock cycles fitting into one or more periods of the oscillation signal , i . e . , count the number of clock cycles between predetermined phases of the oscillation signal , and to output a signal representative of the one or more periods of the oscillation signal . The frequency counter unit may comprise a frequency divider at the front end prior to the time measurement unit . The signal output by the time measurement unit may be a time signal or a number of clock cycles . Based on the signal output by the time measurement unit , a frequency of the oscillation signal may be detected . In one exemplary embodiment of the invention, the clock cycle may be between 30 ps and 10 ns , for instance 50 ps . In order to enhance precision, interpolation may be used . This may be carried out by a zero-order-hold element . In another embodiment of the invention, the frequency counter unit may comprise a phase- locked loop . The frequency counter unit may be provided with the oscillation signal , preferably with the oscillation signal after passing the filter unit .

Preferably, the frequency counter unit comprises a further filter configured to reduce fluctuations of the detected oscillation frequency and/or the detected period of the oscillation signal , preferably wherein the frequency counter unit is configured to calculate the oscillation frequency of the oscillation signal based on a signal representative of one or more periods of the oscillation signal after passing the further filter . The further filter may be configured to filter the signal output by the time measurement unit , which signal is representative of one or more periods of the oscillation signal . Preferably, the signal output by the time measurement unit may be filtered by the further filter prior to determining the frequency of the oscillation signal . In other words , the frequency of the oscillation signal may be calculated based on the signal of the time measurement unit after passing the further filter . It can be shown experimentally and mathematically that filtering the ( time ) signal before determining the frequency of the oscillation signal enhances measurement accuracy . Of course , also a detected period of the oscillation signal based on the filtered signal may be output by the frequency counter unit . The further filter may be configured to filter the signal of the time measurement unit and extract an average time of one or more periods of oscillation signal . The oscillation frequency may be detected on basis of the average time of the one or more periods of the oscillation signal . In other words , the further filter may average the measured one or more periods of the oscillation signal and thus also the frequency of the oscillation signal . The at least one further filter may be a digital or analogue filter . In one embodiment of the invention, the further filter may comprise several sub filters arranged in series or arranged in parallel . The further filter is preferably a low-pass filter implemented, e . g . , by a moving average , or finite impulse response ( FIR) or infinite impulse response ( HR) filter . In one embodiment , the cut-of f frequency of such a low-pass filter may be set , preferably, to any frequency value smaller than the expected oscillation frequency and as low as 0 . 01 Hz . In one embodiment of the invention, the electrical oscillation circuit comprises a pre-amplifier unit configured to ampli fy the oscillation signal , preferably by an amplification factor between 1 and 500000 . The pre-ampli fier unit may be arranged prior to the filter unit or between the filter unit and the phase detection unit . The pre-ampli fier unit amplifies weak oscillation signals stemming from the output of the oscillation element . Preferably, the pre-ampli fier is an analogue ampli fier .

Preferably, the electrical oscillation circuit is configured to drive a resonator . The resonator may be , for example , an electromechanical , an optomechanical , an electric, or a mechanical resonator . The resonator may be driven directly by the electrical oscillation circuit or indirectly by an additional actuator connected to the electrical oscillation circuit . The driving signal of the electrical oscillation circuit can be fed into the actuator . The actuator may, for instance , generate a mechanical oscillation of the resonator . In one example , the actuator may be a piezo-electric actuator or a magnetic actuator . Preferably, the resonator is a NEMS-resonator (NEMS = nanoelectromechanical system) or a MEMS-resonator (MEMS = microelectromechanical system) . NEMS-resonators are typically made up of components between 1 nm and 1 mm in si ze . MEMS- resonators are typically made up of components between 1 pm and 10 mm in si ze . The smallest dimension of a resonator defines i f a resonator is a NEMS or a MEMS resonator . In general , a resonator is a device that exhibits resonance or resonant behavior when excited by a driving signal . Preferably, the resonator is a mechanical or electromechanical resonator . Such a resonator may comprise a movable membrane , string, beam, or a cantilever which can be brought in resonance , either directly by the electrical oscillation circuit or indirectly by means of an additional actuator . The oscillation of the resonator , which may be a mechanical oscillation of a membrane , string, beam, or cantilever, may be measured by means of a measurement device , such as an optical measurement device . The measurement device may transduce the measured oscillation into the oscillation signal which can be forwarded to the electrical oscillation circuit . Alternatively, the oscillation element may output an electrical oscillation signal which can be used by the electrical oscillation circuit .

The invention also relates to an oscillation system, comprising : an oscillation element , preferably a resonator, in particular a NEMS-resonator or a MEMS-resonator ; and an electrical oscillation circuit coupled to the oscillation element , wherein the electrical oscillation circuit is designed as described above .

As described above , the oscillation element may be , for example , an electromechanical , an optomechanical , an electric, or a mechanical oscillation element . The electrical oscillation element may be driven directly by the electrical oscillation circuit or indirectly by an additional actuator connected to the electrical oscillation circuit . Thus , the oscillation system may additionally comprise an actuator connected to the circuit output . The driving signal of the electrical oscillation circuit can be fed into the actuator . The actuator may, for instance , generate a mechanical oscillation of the oscillation element . In one example , the actuator may be a piezo-electric actuator or a magnetic actuator . Preferably, the oscillation element is a NEMS-resonator or a MEMS-resonator . An oscillation element may comprise a movable membrane , string, beam, or a cantilever which can be brought in resonance , either directly by the electrical oscillation circuit or indirectly by means of an additional actuator . The oscillation element may generate an electrical oscillation signal that may be forwarded to the electrical oscillation circuit . Alternatively, the oscillation of the oscillation element may be measured by means of a measurement device , such as an optical measurement device . Thus , the oscillation system may further comprise a measurement device connected to the circuit input . The measurement device may transduce the measured oscillation into the oscillation signal which can be fed into the electrical oscillation circuit .

In a preferred embodiment of the invention, the oscillation element is an oscillation element of a thermal detector, preferably an oscillation element of a photothermal infrared detector . Such an oscillation element of a photothermal detector element may comprise a frame and a membrane or a trampoline supported by the frame . The membrane or trampoline may comprise an absorption area configured to absorb an incident radiation . In one embodiment of the invention, the oscillation element may generate an electrical oscillation signal which may be used by the electrical oscillation circuit . In another embodiment , the oscillation system may further comprise a preferably optical measurement device for detecting the oscillation of the oscillation element and generating an oscillation signal which can be used by the electrical oscillation device . An optical measurement device may be configured to detect the membrane or trampoline oscillation by means of a probing light beam, which is radiating onto the membrane or trampoline . The oscillation element can be driven directly by the driving signal or by means of an additional actuator . Such an actuator may be connected to the circuit output and may generate an oscillation of the membrane or trampoline based on the driving signal of the electrical oscillation circuit .

The invention also relates to a photothermal detection system for detecting infrared radiation, comprising : an oscillation system as described above ; a detection unit configured to detect infrared radiation, in particular to determine power and/or energy of absorbed infrared radiation, preferably wherein the infrared radiation is absorbed by a sample located on the oscillating element , based on changes in the oscillation frequency .

The detection unit may be implemented in the FPGA, in a computer, or in a microprocessor, for example .

Absorbed electromagnetic radiation causes a temperature increase of the nanomechanical resonator which in turn detunes its resonance frequency . The resulting frequency detuning has a linear dependence to the photothermal heating of the oscillating element ( Schmid, Silvan, Luis Guillermo Villanueva, and Michael Lee Roukes . Fundamental s of nanomechani cal resona tors . Springer, 2016 . ) . In a preferred embodiment , a sample , in particular a chemical sample , such as environmental nanoparticles or pharmaceutical nanoparticles , may be disposed or collected on the oscillation element . The sample may absorb incident infrared radiation and detune the resonance frequency of the oscillation element .

The above formulated obj ective is also solved by a method for driving an oscillation element at an oscillation frequency, in particular at a resonance frequency, according to claim 25 , the method comprising the following steps : receiving an oscillation signal from the oscillation element ; detecting a predetermined phase of the oscillation signal , preferably zero-crossings of the oscillation signal , and generating a phase detection signal ; generating pulses based on the phase detection signal ; outputting a driving signal to the oscillation element containing the pulses , wherein signal components of the oscillation signal outside a filter frequency range containing the oscillation frequency are suppressed prior to detecting the predetermined phase of the oscillation signal .

The advantages described in connection with the electrical oscillation circuit above also apply to the method for driving an oscillation element at an oscillation frequency . Thus , features of the electrical oscillation circuit are trans ferable to the method for driving an oscillation element at an oscillation frequency . The oscillation signal may be received via the circuit input . The driving signal may be output via the circuit output . The predetermined phase of the oscillation signal may be detected by the phase detection unit . The pulses may be generated by the pulse generator unit . Signal components outside a filter frequency range may be suppressed by the filter unit .

Preferably, a phase shi ft is introduced to the driving signal by inducing a time delay . The time delay may be induced by a time delay unit as described above . Preferably, the time delay is introduced to the generated pulses . In one embodiment of the invention, the amplitude of the driving signal may be adj usted . The driving signal may be adj usted by an amplitude regulation unit as described above .

Preferably, the amplitude of the oscillation signal is controlled according to an oscillation signal amplitude setpoint value by adj usting the amplitude of the driving signal . To this end, the amplitude of the oscillation signal may be detected, preferably after signal components of the oscillation signal outside a filter frequency range containing the oscillation frequency are suppressed and prior to the detection the phase of the oscillation signal .

The invention also relates to a method for tracking an oscillation frequency, in particular a resonance frequency, of an oscillation element , comprising the following steps :

Driving an oscillation element by applying a method for driving an oscillation element at an oscillation frequency as described above ; and detecting the oscillation frequency of the oscillation signal .

Detecting of the oscillation frequency of the oscillation signal and/or a period of the oscillation signal may be carried out by a frequency counter unit . In order to reduce fluctuations of the detected oscillation frequency, the frequency counter unit may comprise a further filter, preferably a low-pass filter . The further filter may average a signal output by a time measurement unit of the frequency counter unit representative of one or more periods of the oscillation signal . This filtered signal may be further processed to calculate a frequency of the oscillation signal . In other words , signal representative of one or more periods of the oscillation signal may be filtered prior to the calculation of the frequency of the oscillation signal .

The invention also provides for a method for detecting infrared radiation, comprising the following steps :

Tracking the oscillation frequency, in particular the resonance frequency, of an oscillation element by applying a method for tracking the oscillation frequency as described above ; and

Detecting infrared radiation, in particular determining the power and/or energy of absorbed infrared radiation, based on changes in the oscillation frequency .

By absorbing the infrared radiation, the oscillation frequency may be shi fted . Preferably, the infrared radiation is absorbed by a sample , in particular a chemical sample , located on the oscillation element . The sample may be environmental or pharmaceutical nanoparticles . The sample may be , for example , placed or collected on a membrane of the oscillation element .

In a more general aspect , the invention can be described with the following embodiments . The advantages and features described above may also apply and hence be trans ferred to the embodiments described in the following . In the context of the embodiments , it shall be noted that the filter unit and the suppression of signal components of the oscillation signal outside a filter frequency range containing the oscillation frequency prior to detecting the predetermined phase of the oscillation signal do not represent essential , but optional features .

Embodiment 1 :

Electrical oscillation circuit for driving an oscillation element at an oscillation frequency, in particular at a resonance frequency, comprising : a circuit input configured to receive an oscillation signal from the oscillation element ; a phase detection unit configured to detect a predetermined phase of the oscillation signal , preferably zero-crossings of the oscillation signal , and configured to generate a phase detection signal ; a pulse generator unit configured to generate pulses based on the phase detection signal ; a circuit output configured to output a driving signal to the oscillation element containing the pulses .

Embodiment 2 :

Electrical oscillation circuit according to embodiment 1 , further comprising a filter unit arranged prior to the phase detection unit and having at least one filter with a filter frequency range containing the oscillation frequency, the at least one filter being configured to suppress signal components of the oscillation signal outside the filter frequency range before the oscillation signal reaches the phase detection unit .

Embodiment 3 :

Electrical oscillation circuit according to embodiment 2 , wherein the at least one filter is a band-pass filter with an upper cut-of f frequency and a lower cut-of f- frequency defining the filter frequency range of the at least one filter .

Embodiment 4 :

Electrical oscillation circuit according to embodiment 2 or 3 , wherein the filter unit comprises at least two filters with di fferent frequency ranges arranged in parallel electrical paths , wherein a switch allows for switching between the electrical paths .

Embodiment 5 :

Electrical oscillation circuit according to one of embodiments 1 to 4 , further comprising an amplitude regulation unit configured to adj ust the amplitude of the driving signal .

Embodiment 6 :

Electrical oscillation circuit according to embodiment 5 , wherein the amplitude regulation unit comprises a feedback controller configured to detect the amplitude of oscillation signal , preferably between the filter unit and the phase detection unit , and to adj ust the amplitude of the driving signal in order to control the amplitude of the oscillation signal according to an oscillation signal amplitude setpoint value .

Embodiment 7 :

Electrical oscillation circuit according to one of embodiments 1 to 6 , further comprising a time delay unit configured to introduce a phase shi ft to the driving signal by inducing a time delay, wherein the pulse generator unit is preferably arranged prior to the time delay unit . Embodiment 8 :

Electrical oscillation circuit according to embodiment 7 , wherein the time delay is adj ustable to a time delay setpoint value .

Embodiment 9 :

Electrical oscillation circuit according to one of embodiments 1 to 8 , wherein a pulse width of the pulses is adj ustable to a pulse width setpoint value .

Embodiment 10 :

Electrical oscillation circuit according to embodiment 9 , wherein the pulse width setpoint value is set so that the pulse width covers between 5 % and 40 % , preferably between 7 % and 38 % or between 20 % and 35 % or between 25 % and 35 % , of a period of the driving signal .

Embodiment 11 :

Electrical oscillation circuit according to one of embodiments 1 to 10 , further comprising a frequency counter unit configured to detect the oscillation frequency of the oscillation signal .

Embodiment 12 :

Electrical oscillation circuit according to embodiment 11 , wherein the frequency counter unit comprises a further filter configured to reduce fluctuations of the detected oscillation frequency .

Embodiment 13 :

Electrical oscillation circuit according to one of embodiments 1 to 12 , wherein the electrical oscillation circuit is configured to drive a resonator, preferably a NEMS-resonator or a MEMS- resonator .

Embodiment 14 :

Oscillation system, comprising : an oscillation element , preferably a resonator, in particular a NEMS-resonator or a MEMS-resonator ; and an electrical oscillation circuit coupled to the oscillation element , wherein the electrical oscillation circuit is designed according to any of embodiments 1 to 13 .

Embodiment 15 :

Oscillation system according to embodiment 14 , wherein the oscillation element is an oscillation element of a thermal detector, preferably an oscillation element of a photothermal infrared detector .

Embodiment 16 :

Photothermal detection system for detecting infrared radiation, comprising : an oscillation system according to embodiment 15 ; a detection unit configured to detect infrared radiation, in particular to determine power and/or energy of absorbed infrared radiation, preferably wherein the infrared radiation is absorbed by a sample located on the oscillating element , based on changes in the oscillation frequency, preferably wherein the detection unit is further configured to determine a temperature based on the detected infrared radiation .

Embodiment 17 :

Method for driving an oscillation element at an oscillation frequency, in particular at a resonance frequency, comprising the following steps : receiving an oscillation signal from the oscillation element ; detecting a predetermined phase of the oscillation signal , preferably zero-crossings of the oscillation signal , and generating a phase detection signal ; generating pulses based on the phase detection signal ; and outputting a driving signal to the oscillation element containing the pulses .

Embodiment 18 :

Method for driving an oscillation element according to embodiment 17 , wherein signal components of the oscillation signal outside a filter frequency range containing the oscillation frequency are suppressed prior to detecting the predetermined phase of the oscillation signal .

Embodiment 19 :

Method for driving an oscillation element according to embodiment 17 or 18 , wherein a phase shi ft is introduced to the driving signal by inducing a time delay, wherein the time delay is preferably introduced to the generated pulses .

Embodiment 20 :

Method for driving an oscillation element according to one of embodiments 17 to 19 , wherein the amplitude of the oscillation signal is controlled according to an oscillation signal amplitude setpoint value by measuring the amplitude of the oscillation signal and adj usting the amplitude of the driving signal .

Embodiment 21 :

Method for tracking an oscillation frequency, in particular a resonance frequency, of an oscillation element , comprising the following steps : driving an oscillation element by applying a method for driving an oscillation element at an oscillation frequency according to any one of embodiments 17-20 ; and detecting the oscillation frequency of the oscillation signal .

Embodiment 22 :

Method for detecting infrared radiation, comprising the following steps : tracking the oscillation frequency, in particular the resonance frequency, of an oscillation element by applying a method for tracking the oscillation frequency according to embodiment 21 ; and detecting infrared radiation, in particular determining the power and/or energy of absorbed infrared radiation, based on changes in the oscillation frequency, preferably wherein the infrared radiation is absorbed by a sample located on the oscillating element . In the following, the exemplary embodiments of the invention are described with reference to the drawings , which the invention shall not be restricted to , however .

The drawings show :

Fig . 1 a block diagram of an electrical oscillation circuit ;

Fig . 2A an oscillation signal at the circuit input of the electrical oscillation circuit ;

Fig . 2B an oscillation signal after passing a filter unit of the electrical oscillation circuit ;

Fig . 2C a phase detection signal ;

Fig . 2D pulses of a driving signal ;

Fig . 3 a schematic of an electrical oscillation circuit ;

Fig . 4A a first embodiment of a photothermal infrared detector with an oscillation element ;

Fig . 4B a second embodiment of a photothermal infrared detector with an oscillation element ; and

Fig . 5 a filter unit with multiple filters arranged in parallel .

Fig . 1 shows a block diagram of an electrical oscillation circuit 1 with a circuit input 2 and a circuit output 3 . The electrical oscillation circuit 1 is connected to an oscillation element 4 in the form of a NEMS-resonator 5 , which can be driven with a driving signal D at an oscillation frequency f_osc, preferably at one of its resonance frequencies f_res , by the electrical oscillation circuit 1 . It shall be noted that the oscillation frequency f_osc of the oscillation element 4 , in particular the resonance frequency f_res , is not fixed but may shift due to ambient influences , such as changes in incident infrared radiation I_IR . The driving signal D may be output via the circuit output 3 to the oscillation element 4 or a driving apparatus for bringing the oscillation element 4 into oscillation (not shown) . The mechanical oscillation of the oscillation element 4 can be transduced into an electrical oscillation signal 0, either directly by the oscillation element 4 or by means of a measurement device (not shown) , which can measure the oscillation of the oscillation element 4 . The oscillation signal 0 can be guided into the electrical oscillation circuit 1 via the circuit input 2 .

In the embodiment shown, the electrical oscillation circuit 1 comprises a pre-ampli fier unit 6 configured to ampli fy the oscillation signal 0, preferably by an ampli fication factor between 1 and 50 0000 , in particular between 100 and 10 000 . This can be advantageous when the oscillation signal 0 at the circuit input 2 is a weak signal . After passing the preampli fier, the oscillation signal 0 may also be referred to as ampli fied oscillation signal 0.

In order to attenuate the excitation of unwanted modes of the oscillation element 4 and to reduce detection noise , the electrical oscillation circuit 1 further comprises a filter unit 7 having at least one filter 8 . The at least one filter 8 has a filter frequency range 9 containing the oscillation frequency f_OSc - Signal components of the oscillation signal 0 outside the filter frequency range are suppressed . In the embodiment shown, the at least one filter is a band-pass filter 50 with a lower cut-of f frequency flower at 2 kHz and an upper cut-of f frequency fupper at 20 MHz . By means of the at least one filter 8 , signal components of the oscillation signal 0 outside the filter frequency range 9 are suppressed before being processed by other units of the electric circuit as described below . In particular, the filter unit 7 is arranged prior to a phase detection unit 10 of the electric oscillation circuit 1 . In this way, only predetermined phases <p₀ ( see Fig . 2B ) of desired modes are detected . After passing the filter unit 7 , the oscillation signal 0 may also be referred to as filtered oscillation signal 0.

The filter unit 7 may be implemented digitally, for example in a FPGA 56 ( see Fig . 3 ) . In this way, by changing the filter parameters , the filter frequency range 9 can be easily altered . I f the filter unit 7 is built up with analogue components , it may be advantageous i f the filter unit 7 comprises multiple filters 8a, 8b, 8c with dif ferent filter frequency ranges 9a, 9b, 9c arranged in parallel . This is shown in Fig . 5 . By means of a switch 51 it is possible to select a filter 8 a, 8b, 8c to be applied to the oscillation signal 0.

The electrical oscillation circuit 1 further comprises a phase detection unit 10 . The phase detection unit 10 is configured to detect a predetermined phase <p₀ of the oscillation signal 0, such as a zero-crossing 11 of the oscillation signal 0. The phase detection unit 10 may be , for example , a comparator or a Schmitt-trigger . The phase detection unit 10 is configured to generate a phase detection signal P which contains information about the occurrence of the predetermined phase <p₀ in the filtered oscillation signal 0. The phase detection signal P may contain triggers 12 ( see Fig . 2C ) that indicate the occurrence of the predetermined phase <p₀ in the oscillation signal 0, such as rising 13 and/or falling signal edges 14 .

The electrical oscillation circuit 1 further comprises a pulse generator unit 15 which is configured to generate pulses 16 ( see Fig . 2D) based on the phase detection signal P . The pulse generator unit 15 is preferably an individual unit independent from the phase detection unit 10 and arranged after the phase detection unit 10 . The pulses 16 are included in the driving signal D . The driving signal D has the same frequency as the oscillation signal 0. The pulses 16 preferably have an essentially rectangular form . The pulses 16 may have solely a positive polarity . Alternatively, the pulses 16 may have a solely negative polarity or an alternating polarity . The height 17 of the pulses 16 and/or the pulse width T_w may be adj ustable ( see Fig . 2D) . The pulse generator unit 15 is preferably configured to adj ust the height 17 of the pulses 16 and/or the pulse width T_w . In a preferred embodiment , the pulse width T_w may be adj ustable to a pulse width setpoint value T_{w re}f . Preferably, the pulse width setpoint value T_{w re}f is set so that the pulse width T_w covers between 5 % and 40 % , preferably between 7 % and 38 % or between 20 % and 35 % or between 25 % and 35 % of a period T_period of the driving signal D and the oscillation signal 0, respectively . In one example , the pulses may have a height below 100 pV . In this way, the pulses 16 may be di fferent to the triggers 12 . The driving signal D and the phase detection signal P may be separate signals and di f ferent to each other .

I f the phase detection signal P consists of pulses , the pulse generator unit 15 may be considered included into the phase detection unit 10 in an alternative embodiment of the invention, as these pulses may be used for the driving signal D .

In order to compensate for time delays and phase shi fts introduced by latencies and trans fer functions of the units of the electrical oscillation circuit 1 and the oscillation element 4 , a time delay unit 18 may be disposed in the electrical oscillation circuit 1 . The time delay unit 18 is configured to introduce a phase shi ft to the driving signal D by inducing a time delay T_d according to a time delay setpoint value T_d-ref ( see Fig . 2D) . The time delay setpoint value T_d-ref may be calculated based on latency-times and phase shi fts of trans fer functions . Alternatively, the time delay setpoint value T_d-ref may be determined experimentally . The time delay setpoint value T_d-ref may be chosen such that the driving signal D is in phase with an energy trans fer inside the oscillation element 4 . The time delay setpoint value T_d-ref may be determined by measuring the oscillation amplitude and varying the time delay setpoint value T_d_ref - The ideal time delay setpoint value T_d-ref maximizes the oscillation amplitude of the oscillation element 4 . In one exemplary embodiment of the invention, the time delay setpoint value T_d-ref may be may be between 5 ns and 100000 ns . Preferably, the time delay unit 18 may arranged after the pulse generator unit 15 .

In one embodiment of the invention, the time delay unit 18 is included into the pulse generator unit 15 .

In order to regulate the amplitude of the oscillation signal , the electrical oscillation circuit 1 may comprise an amplitude regulation unit 19 . The amplitude regulation unit 19 may comprise a feedback controller 20 , preferably a PI - or a PID controller 20a, which is configured to detect , preferably measure , the amplitude of the oscillation signal 0 before the oscillation signal 0 reaches the phase detection unit 10 . In the shown embodiment , the feedback controller 20 detects the amplitude of the oscillation signal 0 in a signal path part between the filter unit 7 and the phase detection unit 10 . The feedback controller 20 may adj ust the amplitude of the driving signal D by means of a variable gain so that the amplitude of the oscillation signal 0 corresponds to an oscillation signal amplitude setpoint value A_ref . In this way, a stable oscillation can be achieved .

In one embodiment of the invention, the amplitude regulation unit 19 is at least partially included into the pulse generator unit 15 . In this way, the pulse generator unit 15 generates pulses 16 with a height 17 that leads to an amplitude of the oscillation signal 0 which corresponds to the oscillation signal amplitude setpoint value A_ref .

Due to its structure and functionality, the electrical oscillation circuit 1 may be referred to as an SSO-type ( SSO : self-sustaining oscillator ) circuit .

In order to track the oscillation frequency f_osc, the electrical oscillation circuit 1 may further comprise a frequency counter unit 21 configured to detect the oscillation frequency f_osc of the oscillation signal 0 and/or a period of the oscillation signal 0. The oscillation signal 0 may be fed into the frequency counter unit 21 after the oscillation signal 0 has passed the filter unit 7 . In other words , the filtered oscillation signal 0 between the filter unit 7 and the phase detection unit 10 may be fed into the frequency counter unit 21 . The frequency counter unit 21 may comprise a time measurement unit 21a configured to measure the duration of one or more periods of the oscillation signal 0. The time measurement unit 21a may comprise a clock with a given clock cycle and a counter, which may be configured to count the number of clock cycles which fit into one or more periods of the oscillation signal 0. The time measurement unit 21a may output a signal representative of one or more periods of the oscillation signal 0, for example a number of clock cycles fitting into one or more periods of the oscillation signal 0 or a time calculated from the number of clock cycles . To reduce fluctuations of the detected oscillation frequency, the frequency counter unit 21 may comprise a further filter 22 , which may be a low-pass filter with a cut-of f frequency between 0 . 1 Hz and 100 kHz , depending on the application . The further filter 22 may filter the signal representative of one or more periods of the oscillation signal 0 output by the time measurement unit 21a, thereby averaging detected periods of the oscillation signal 0. After filtering the signal from the time measurement unit 21a by the further filter 22 , a frequency calculator 21b may determine the frequency f_osc of the oscillation signal 0 based on the filtered signal output by the further filter 22 . The oscillation frequency f_osc may be fed into a detection unit 53 as will be described below .

Fig . 2A shows an oscillation signal 0 at the circuit input 2 of the electrical oscillation circuit 1 . One can see that the oscillation signal 0 contains multiple signal components with multiple di f ferent frequencies . Some of the signal components belong to unwanted modes of the oscillation element 4 .

Fig . 2B schematically shows the oscillation signal 0 after passing the filter unit 7 of the electrical oscillation circuit 1 . The abscissa represents the time t in seconds . The ordinate represents a normali zed voltage U . The filtered signal is depicted as a sine wave with one single frequency . Of course , the filtered oscillation 0 signal may still comprise attenuated higher harmonics . The zero-crossings 11 of the oscillation signal 0 with a rising signal edge represent a predetermined phase cpo • In Fig . 2B, also the period T_period of the oscillation signal 0, which corresponds essentially to the period of the driving signal D, is shown .

Fig . 2C schematically shows a phase detection signal P derived from the oscillation signal 0 depicted in Fig . 2B . The abscissa represents the time t in seconds . The ordinate represents a normali zed voltage U . The phase detection signal P may be generated by a comparator into which the phase detection signal P is fed . Fig. 2D schematically shows pulses 16 (solid line) generated by the pulse generator unit 15. The pulses 16 have a pulse width T_w and a height 17. As outlined above, the pulses 16 may be delayed by a time delay Td in order to compensate for phase shifts in the oscillation element 4 and the electrical oscillation circuit 1. This is represented in Fig. 2D, in which the pulses 16 are delayed to the phase detection signal P (dashed line) . The pulses 16 form the driving signal D.

Fig. 3 shows a schematic of an electrical oscillation circuit 1. In the shown embodiment, the electrical oscillation circuit 1 of Fig. 3 is connected to an oscillation element 4. The preamplifier unit 6 is an analog amplifier and may be implemented, for example, as instrumentation amplifier. After amplification by the pre-amplifier unit 6, the oscillation signal 0 is then digitalized by an analog-to-digital converter (ADC) 55. The oscillation signal 0 is then fed into an FPGA 56 which contains at least the pulse generator unit 15 and the filter unit 7, preferably also the time delay unit 18. After passing the filter unit 7, the filtered digital oscillation signal 0 is converted into an analog signal by means of a digital-to-analog-converter (DAC) 57a. The output of the DAC 57a is fed into the phase detection unit 10, which may be a comparator. The phase detection signal P from the phase detection unit 10 is then fed back into the FPGA 56, in particular into the pulse generator unit 15. Further, the phase detection signal P from the phase detection unit 10 is fed into a TDC 58 (TDC = Time-to-Digital- Converter) . The TDC 58 may be configured to interpolate the clock of the time measurement unit 21a as described above to enhance time measurement accuracy. The driving signal D at the output of the FPGA 56 is then converted into an analog signal by the DAC 57b. In the embodiment shown, the analog driving signal D is attenuated by optional attenuator 59 to protect the oscillation element 4 from voltage levels higher than 100 pV. After passing the attenuator 59, the driving signal D is then forwarded to the oscillation element 4. Instead of an attenuator 59, also a gain may be used. Fig . 4A shows a first embodiment photothermal infrared detector 23 comprising an oscillation element 4 in the form of a resonator . The photothermal infrared detector 23 is connected to the electrical oscillation circuit 1 . The combination of the electrical oscillation circuit 1 and the oscillation element 4 may be referred to as oscillation system 52 . The oscillation element 4 is arranged between magnets generating a static magnetic field B as indicated by arrows 24 and in a vacuum chamber . The vacuum chamber may create a pressure below 1 Pascal . The vacuum chamber also features a window that is transparent to the used probing radiation ( e . g . , infrared radiation I_IR) . The oscillation element 4 comprises a frame 25 and a membrane 26 supported by the frame 25 . The membrane 26 is arranged to oscillate within the magnetic field B . The frame 25 may be thermally connected to the vacuum chamber . The photothermal infrared detector 23 can be temperature controlled with a Peltier element (not shown) thermally connected to the frame 25 , i . e . , placed between the frame 25 and the vacuum chamber . The membrane 26 has an absorption area 27 . The absorption area 27 may feature a broadband IR absorber . The membrane 26 may be chemically treated to become hydrophobic at least in the absorption area 27 . The membrane area may be quadratic and the absorption area 27 may be a circular area provided in the center of the membrane area . The membrane 26 is preferably pervious in the absorption area 27 as schematically indicated by a mesh . At the absorption area 27 , alternatively or additionally a sample 32 , in particular a chemical sample , may be placed ( see Fig . 4B) .

The electrical oscillation circuit 1 is connected to the photothermal infrared detector via contacts 28 , 29 . The two electrical contacts 28 , 29 are arranged on an axis 30 orthogonal to the magnetic field B . The electrons within the membrane 26 are moved through the magnetic field B by the membrane oscillation (perpendicular to the drawing plane of Fig . 4A) and the thereby induced voltage may be picked up via the electrical contacts 28 , 29 at opposite sides of the membrane 26 . In order to allow electrons to flow between the contacts 28 , 29 , at least one layer of the membrane 26 may be electrically conductive . The electrical contacts 28 , 29 may be for example wire bonds or spring-loaded pins . In the embodiment shown, the contact 29 is connected to the circuit input 2 of the electrical oscillation circuit 1 . The oscillation of the membrane 26 can be induced by the electrical oscillation circuit 1 , as in this example , by putting one electrical contact 28 to ground 31 and applying the driving signal D to the other electrical contact 29 . In general , the membrane 26 may also be driven by other means , e . g . , with a piezoelectric element .

Incident radiation, such as infrared radiation I_IR, may change the oscillation frequency f_osc, in particular the resonance frequency f_res , of the oscillation element 4 . This frequency change may be tracked and processed by the detection unit 53 depicted in Fig . 1 . The detection unit 53 may be configured to detect infrared radiation I_IR, in particular power and/or energy of infrared radiation I_IR, based on changes in the oscillation frequency f_osc . An oscillation system 52 comprising a detection unit 53 may be also referred to as photothermal detection system 54 .

Fig . 4B shows a second embodiment of the photothermal infrared detector 23 comprising an oscillation element . In the following, primarily the di f ferences to Fig . 4A will be described . A sample 32 , in particular a chemical sample , may be located on the membrane 26 . The sample 32 may be environmental or pharmaceutical nanoparticles . The sample 32 may absorb incident radiation I_IR . Absorption of incident radiation I_IR may cause a shift of the oscillation frequency f_osc which can be detected by detection unit 53 . In Fig . 4B, the drum 26 comprises electrically conductive traces or wires 33a, 33b, which are oriented at least partially obliquely to the magnetic field B . The traces or wires 33a, 33b may be connected to the electrical oscillation circuit 1 via contacts 29a, 29b . Contacts 28a, 28b may be put to ground 31 . The driving signal D of the electrical oscillation circuit 1 may be applied to the contact 29a . The oscillation signal 0 may be picked up at contact 29b and fed back into the electrical oscillation circuit 1 . In this way, a driving force can be generated by means of the trace or wire 33a and an oscillation signal 0 can be derived from the induced voltage of trace or wire 33b .

Claims

Claims :

1. Electrical oscillation circuit (1) for driving an oscillation element (4) at an oscillation frequency (f_osc) , in particular at a resonance frequency (f_res) , comprising: a circuit input (2) configured to receive an oscillation signal (0) from the oscillation element (4) ; a phase detection unit (10) configured to detect a predetermined phase (<p₀) of the oscillation signal (0) , preferably zero-crossings (11) of the oscillation signal (0) , and configured to generate a phase detection signal (P) ; a pulse generator unit (15) configured to generate pulses (16) based on the phase detection signal (P) ; a circuit output (3) configured to output a driving signal (D) to the oscillation element (4) containing the pulses (16) ; characterized by a filter unit (7) arranged prior to the phase detection unit (10) and having at least one filter (8) with a filter frequency range (9) containing the oscillation frequency (f_osc) , the at least one filter (8) being configured to suppress signal components of the oscillation signal (0) outside the filter frequency range (9) before the oscillation signal (0) reaches the phase detection unit (10) .

2. Electrical oscillation circuit (1) according to claim 1, characterized in that a form of the pulses (16) is adjustable.

3. Electrical oscillation circuit (1) according to claim 2, characterized in that the form of the pulses (16) comprises at least one of a height (17) of the pulses (16) and a width (T_w) of the pulses (16) .

4. Electrical oscillation circuit (1) according to one of claims claim 2 or 3, characterized in that the pulse generator unit (15) is configured to set the form of the pulses (16) .

5. Electrical oscillation circuit (1) according to claim 4, characterized in that the pulse generator unit (15) is configured to set the form of pulses (16) independently of the oscillation signal (0) , in particular of the oscillation frequency (f_osc) .

6. Electrical oscillation circuit (1) according to one of claims 4 or 5, characterized in that the pulse generator unit (15) is configured to set the form of the pulses (16) according to at least one setpoint value, which is adjustable.

7. Electrical oscillation circuit (1) according to one of claims 1 to 6, characterized in that the phase detection signal (P) and the pulses (16) are individual signals.

8. Electrical oscillation circuit (1) according to one of claims 1 to 7, characterized in that the pulse generator unit (15) and the phase detection unit (10) are distinct units.

9. Electrical oscillation circuit (1) according to one of claims 1 to 8, characterized by a time delay unit (18) configured to introduce a phase shift to the driving signal (D) by inducing a time delay (T_d) .

10. Electrical oscillation circuit (1) according to claim 9, characterized in that the pulse generator unit (15) is arranged prior to the time delay unit (18) or that the time delay unit (18) is included into the pulse generator unit (15) .

11. Electrical oscillation circuit (1) according to claim 9 or

10, characterized in that the time delay (T_d) is adjustable to a time delay setpoint value (T_{d re}f) .

12. Electrical oscillation circuit (1) according to one of claims 1 to 11, characterized in that a pulse width (T_w) of the pulses is adjustable to a pulse width setpoint value (T_{w re}f) .

13. Electrical oscillation circuit (1) according to claim 12, characterized in that, the pulse width (T_w) of the pulses (16) is adjustable to the pulse width setpoint value (T_{w re}f) by the pulse generator unit (15) .

14. Electrical oscillation circuit (1) according to claim 12 or 13, characterized in that the pulse width setpoint value (T_{w re}f) is set so that the pulse width (T_w) covers between 5 % and 40 %, preferably between 7% and 38 % or between 20 % and 35 % or between 25 % and 35 %, of a period (T_period) of the driving signal (D) .

15. Electrical oscillation circuit (1) according to one of claims 1 to 14, characterized by a frequency counter unit (21) configured to detect the oscillation frequency (f_osc) and/or a period of the oscillation signal (0) .

16. Electrical oscillation circuit (1) according to claim 15, characterized in that the frequency counter unit (21) comprises a further filter (22) configured to reduce fluctuations of the detected oscillation frequency (f_osc) and/or the detected oscillation periods of the oscillation signal (0) , preferably wherein the frequency counter unit (21) is configured to calculate the oscillation frequency (f_osc) based on a signal representative of one or more periods of the oscillation signal (0) after passing the further filter (22) .

17. Electrical oscillation circuit (1) according to one of claims 1 to 16, characterized in that the electrical oscillation circuit (1) is configured to drive a resonator, preferably a NEMS-resonator (5) or a MEMS-resonator .

18. Electrical oscillation circuit (1) according to one of claims 1 to 17, characterized in that the at least one filter (8) is a band-pass filter (50) with an upper cut-off frequency ( fupper) and a lower cut-off-frequency ( flower) defining the filter frequency range (9) of the at least one filter (8) .

19. Electrical oscillation circuit (1) according to one of claims 1 to 18, characterized in that the filter unit (7) comprises at least two filters (8) with different frequency ranges (9) arranged in parallel electrical paths, wherein a switch (51) allows for switching between the electrical paths.

20. Electrical oscillation circuit (1) according to one of claims 1 to 19, characterized by an amplitude regulation unit (19) configured to adjust the amplitude of the driving signal

(D) .

21. Electrical oscillation circuit (1) according to claim 20, characterized in that the amplitude regulation unit (19) comprises a feedback controller (20) configured to detect the amplitude of oscillation signal (0) , preferably between the filter unit (7) and the phase detection unit (10) , and to adjust the amplitude of the driving signal (D) in order to control the amplitude of the oscillation signal (0) according to an oscillation signal amplitude setpoint value (A_ref) .

22. Oscillation system (52) , comprising: an oscillation element (4) , preferably a resonator, in particular a NEMS-resonator (5) or a MEMS-resonator ; and an electrical oscillation circuit (1) coupled to the oscillation element (4) , characterized in that the electrical oscillation circuit (1) is designed according to any of claims 1 to 21.

23. Oscillation system (52) according to claim 22, characterized in that the oscillation element (4) is an oscillation element

(4) of a thermal detector, preferably an oscillation element (4) of a photothermal infrared detector (23) .

24. Photothermal detection system (54) for detecting infrared radiation, comprising: an oscillation system according to claim 23; a detection unit (53) configured to detect infrared radiation (I_IR) , in particular to determine power and/or energy of absorbed infrared radiation (I_IR) , preferably wherein the infrared radiation (I_IR) is absorbed by a sample located on the oscillating element (4) , based on changes in the oscillation frequency (f_osc) •

25. Method for driving an oscillation element (4) at an oscillation frequency (f_osc) , in particular at a resonance frequency (f_res) , comprising the following steps: receiving an oscillation signal (0) from the oscillation element ( 4 ) ; detecting a predetermined phase (<p₀) of the oscillation signal (0) , preferably zero-crossings (11) of the oscillation signal (0) , and generating a phase detection signal (P) ; generating pulses (16) based on the phase detection signal (P) ; and outputting a driving signal (D) to the oscillation element (4) containing the pulses (16) , characterized in that signal components of the oscillation signal (0) outside a filter frequency range (9) containing the oscillation frequency (f_osc) are suppressed prior to detecting the predetermined phase (<p₀) of the oscillation signal (0) .

26. Method for driving an oscillation element (4) according to claim 25, characterized in that a phase shift is introduced to the driving signal (D) by inducing a time delay (Td) .

27. Method according to claim 26, wherein the time delay (T ) is introduced to the generated pulses (16) .

28. Method for driving an oscillation element (2) according to one of claims 25 to 27, characterized in that the amplitude of the oscillation signal (0) is controlled according to an oscillation signal amplitude setpoint value (A_ref) by measuring the amplitude of the oscillation signal (0) and adjusting the amplitude of the driving signal (D) .

29. Method for tracking an oscillation frequency (f_osc) , in particular a resonance frequency (f_rer) , of an oscillation element (4) , comprising the following steps:

Driving an oscillation element (4) by applying a method for driving an oscillation element (4) at an oscillation frequency (f_Osc) according to any of claims 25-28; and detecting the oscillation frequency (f_osc) of the oscillation signal (0) .

30. Method for detecting infrared radiation, comprising the following steps:

Tracking the oscillation frequency (f_osc) , in particular the resonance frequency (f_ref) , of an oscillation element (4) by applying a method for tracking the oscillation frequency (f_osc) according to claim 29; and

Detecting infrared radiation, in particular determining the power and/or energy of absorbed infrared radiation, based on changes in the oscillation frequency (f_osc) , preferably wherein the infrared radiation (I_IR) is absorbed by a sample located on the oscillating element (4) .