CN113390899B - Microwave reflectometer with online automatic calibration function - Google Patents

Abstract

The invention relates to a microwave reflectometer with an online automatic calibration function, which comprises a microwave reflectometer body and a calibration assembly. The second data acquisition module is configured to obtain a dynamic working curve formed by working voltages of the swept frequency microwave source, the time-to-digital converter is configured to measure a first time delay between the trigger signal and the swept frequency microwave signal and a second time delay between the trigger signal and the beat frequency signal, the first data acquisition module is configured to measure a third time delay between the reference signal and the reflection signal, and the controller is configured to be connected with the arbitrary waveform generator (102), and use the dynamic working curve and modify the sweep frequency control voltage output by the arbitrary waveform generator (102) based on the first, second and third time delays, so that the beat frequency signal is modified, and the calibration accuracy is improved.

Description

A microwave reflectometer with online automatic calibration function

technical field

The invention belongs to the technical field of plasma diagnosis, and in particular relates to a microwave reflector with an online automatic calibration function, which belongs to the technical category of microwave diagnosis.

Background technique

A microwave reflectometer is a tool for measuring plasma density distribution and is often used in various fusion devices. The most commonly used microwave reflector at present is the continuous wave frequency modulation method. Its working method is to generate a microwave signal with a continuously changing frequency through a frequency sweeping microwave source, and transmit the signal to the plasma. Since the reflection cross sections of microwave signals of different frequencies in the plasma are related to the plasma density, by calculating the flight time of different microwave signals in the plasma, the positions of the different reflection cross sections can be obtained, thus corresponding to the different plasma densities. is the plasma density distribution. Due to the limited transmission distance, the total flight time of a microwave signal is usually in nanoseconds. It is relatively difficult to directly measure the flight time. Usually, the transmitted signal and the received signal are mixed and filtered to obtain the beat frequency signal. By measuring the frequency of the beat signal, combined with the sweep speed, the time of flight of the microwave signal can be calculated.

In the continuous wave frequency modulation microwave reflectometer, the emission measurement needs to have a certain accuracy, otherwise it will cause a large error in the calculation of the plasma density distribution, which needs to be avoided as much as possible. Errors in emission measurement are mainly caused by the following aspects:

1. The output of the microwave source is nonlinear. Due to the nonlinearity between the input voltage and the output frequency of the microwave source, there is an error in the output frequency.

Second, the transmission line has dispersion characteristics. In transmission lines, especially in waveguides, microwaves of different modes and frequencies have different group velocities, which leads to dispersion in microwave transmission, which causes beat frequency drift.

3. Frequency drift caused by temperature and aging. Due to the change of the working temperature of the electronic device and the prolongation of the working time, the working characteristic of the electronic device deviates from the original calibration value, resulting in a measurement error.

The calibration of the emission frequency of the microwave reflector is mainly carried out through the following steps:

1. Input the control signal whose voltage is a constant value to the microwave source, and measure its output frequency. Then the voltage of the control signal is increased according to a certain step value, and its output frequency is measured respectively to obtain the voltage-frequency relationship curve of the microwave source. Perform piecewise linear fitting on the voltage-frequency relationship curve, calculate the corresponding working voltage according to the equal frequency interval, and finally generate the sweep-frequency control voltage curve. This step mainly corrects the nonlinearity of the microwave source.

2. Align the microwave reflector with a fixed metal plane for frequency sweep emission. Since the frequency sweeping speed is constant, ideally, the frequency of the beat frequency signal of the microwave reflectometer should be a constant value, but due to the above-mentioned reasons, the beat frequency will drift. By measuring the error between the actual beat frequency and the ideal value, the sweep frequency control voltage curve is adjusted until a beat frequency signal with a smaller error is generated.

In the traditional method, the delay time of the control signal and the microwave signal in the transmission line system is not considered, resulting in a certain phase error in the calibration data during the calibration process. When the frequency sweep speed is slow, the effect of this phase error is not obvious, but when the frequency sweep speed is increased to 1MHz, because the frequency changes too fast, a small phase error will bring several MHz or even a few MHz on the beat frequency. The influence of tens of MHz will affect the calibration results, and even the calibration will fail.

In order to ensure the calibration accuracy, the traditional calibration is usually carried out in the laboratory and is realized by manual offline operation. , a new error will be generated. Moreover, the above calibration process involves the disassembly and assembly of the microwave reflectometer equipment, and the process is cumbersome. Therefore, the calibration cycle is usually long, sometimes as long as several months. During this period, the working state of the equipment may have changed, resulting in the generation of measurement results. error.

SUMMARY OF THE INVENTION

The technical problem to be solved by the present invention is to improve the deficiencies of the prior art for the microwave reflector of the continuous wave frequency modulation mode, and to provide a microwave reflector with an online automatic calibration function. The calibration accuracy is improved by correcting the swept-frequency control voltage based on various delay times of the signal in the transmission line system by the calibration component.

The technical problem solved by the present invention can be realized by the following technical solutions:

A microwave reflectometer with on-line automatic calibration function includes a microwave reflector body, and the microwave reflector body includes a frequency sweep microwave source, an arbitrary waveform generator, a first directional coupler, a local oscillator source, and a single sideband modulator, A power divider, a first mixer, a first in-phase and quadrature demodulator, a first data acquisition module, and a controller, wherein the swept-frequency microwave source is generated under the control of the swept-frequency control voltage signal generated by the arbitrary waveform generator The frequency sweep microwave signal, the local oscillator source generates a baseband signal with a fixed frequency, the power divider divides the baseband signal into multiple outputs, and the first directional coupler divides the sweep frequency microwave signal into two parts: a detection signal and a reference signal, The single sideband modulator synthesizes the baseband signal and the probe signal to generate an upper sideband signal, the upper sideband signal is transmitted to the plasma, and the receiving antenna receives the reflected signal reflected from the plasma cut-off layer; the first mixer combines the reference signal with the The reflected signal is mixed to obtain an intermediate frequency signal, and the first in-phase quadrature demodulator performs complex mixing of the intermediate frequency signal and the baseband signal to obtain a beat frequency signal, which is characterized in that it also includes a calibration component, and the calibration component includes The second directional coupler, the frequency synthesis source, the second mixer, the second data acquisition module, the time-to-digital converter, and the clock synchronization module, wherein the second directional coupler is from the swept-frequency microwave signal output by the swept-frequency microwave source A part is separated as a calibration signal, the frequency synthesis source generates a microwave signal with a fixed frequency, and the second mixer is used to mix the calibration signal and the microwave signal with a fixed frequency to generate a difference frequency signal between the two. The second data acquisition module is configured to obtain a dynamic working curve composed of the working voltage of the frequency sweep microwave source based on the difference frequency signal, the clock synchronization module is controlled by the controller to generate a trigger signal, and the time-to-digital converter is configured to measure the trigger signal and the sweep frequency. the first time delay between the frequency microwave signal and the second time delay between the trigger signal and the beat frequency signal, the first data acquisition module is configured to measure the third time delay between the reference signal and the reflected signal, the controller is configured to be connected to an arbitrary waveform generator, and to modify the beat frequency by modifying the sweep frequency control voltage output by the arbitrary waveform generator using the dynamic operating curve and based on the first, second and third time delays Signal.

The swept-frequency microwave source generates a swept-frequency microwave signal whose frequency varies with the voltage under the control of the swept-frequency control voltage signal generated by the arbitrary waveform generator.

The microwave reflector body also includes a first frequency multiplier, a second frequency multiplier, and a third frequency multiplier. The first frequency multiplier is used for frequency multiplication of the upper sideband signal, and the second frequency multiplier is used for frequency multiplication of the reference signal. The third frequency multiplier is used to multiply the baseband signal. The upper sideband signal after frequency multiplication is transmitted to the plasma. The first frequency mixer mixes the reference signal output by the second frequency multiplier with the reflected signal. The in-phase quadrature demodulator performs complex mixing of the intermediate frequency signal and the baseband signal output by the third frequency multiplier.

Wherein, the calibration component further includes a programmable delay line, which is used to adjust the delay time of the reference signal output by the first directional coupler, which is switched by two microwave switches to realize coaxial cables of different lengths switch.

Wherein, the beat signal has an in-phase component and a quadrature component, and the first data acquisition module collects the in-phase component, the quadrature component and the frequency sweep control voltage signal, and outputs the signal to the controller.

Wherein, the calibration component further includes a narrowband filter, a second in-phase quadrature demodulator, the second mixer outputs to the narrowband filter, and the passband center frequency of the narrowband filter is equal to the generation frequency of the local oscillator source, The second in-phase and quadrature demodulator is used to extract the in-phase component and the quadrature component of the output signal of the narrowband filter, and the second data acquisition module is used to digitize the signal output by the second in-phase and quadrature demodulator and transmit it to the controller .

Among them, the core component of the swept-frequency microwave source is a voltage-controlled oscillator.

After the microwave reflectometer is installed on the fusion device, the calibration operation can be performed during the operation of the device. Since there is no plasma in the device at this time, the microwave signal is directly reflected by the inner wall of the device after being emitted, and the measurement result is relatively constant. The calibration operation is carried out in this environment. The steps of online automatic calibration of microwave reflectometer are as follows:

Step 1: Measure the time delay between the trigger signal, the frequency sweep microwave signal, and the beat frequency signal. A pulse waveform with a height of V ₁ and a width of T ₁ is generated by the controller and output to the arbitrary waveform generator, which is loaded into the memory. The controller instructs the clock synchronization module to generate a trigger signal and output it to the arbitrary waveform generator and the time-to-digital converter. The arbitrary waveform generator outputs the pulse waveform in the memory immediately after receiving the trigger signal. At the same time, the time-to-digital converter starts timing with the trigger signal as the start signal in channel 1, and stops the timing with the pulse signal output by the arbitrary waveform generator as the stop signal. In channel 2, the time-to-digital converter uses the trigger signal as a start signal to start timing, and the in-phase signal output by the first in-phase quadrature demodulator is screened and used as a stop signal to stop timing. The timing result T _{D_SWEEP} obtained by the time-to-digital converter in channel 1 is the time delay between the trigger signal and the frequency sweep signal, and the timing result T _{D_BEAT} obtained in channel 2 is the time delay between the trigger signal and the beat signal. In order to reduce the error, the above measurement process needs to be repeated several times, and the average result is taken.

Step 2: Measure the time delay between the reference signal and the reflected signal. Usually, in order to reduce the IF frequency, it is necessary to reduce the time delay between the reference signal and the reflected signal, so the reference signal needs to reach the mixer through the delay line, but too small time delay brings great difficulty to the measurement. For calibration needs, modify the delay line to be a programmable delay line that can be programmed to switch between long and short coaxial cables, controlled by the controller to switch. In this step, the programmable delay line is instructed by the controller to switch from a long coaxial cable to a short coaxial cable, thereby increasing the time delay between the reference signal and the reflected signal. A narrow pulse waveform with height V ₂ and width T ₂ is generated by the controller and output to the arbitrary waveform generator, which is loaded into the memory, where T ₂ should be less than the estimated time delay between the transmitted signal and the reflected signal. The controller instructs the clock synchronization module to generate a trigger signal and output it to the arbitrary waveform generator and the first data acquisition module. After the arbitrary waveform generator receives the trigger signal, the narrow pulse waveform is output immediately, and the first data acquisition module starts to collect data at the same time. The reference signal first reaches the mixer through the delay line, while the reflected signal is transmitted through the waveguide and reflected in the device before reaching the mixer. After mixing, two clusters of waveforms are generated on the beat signal. After the beat signal is collected by the first data acquisition module, the upper envelope of the signal is extracted, and peak-finding calculation is performed. The time interval _{TD_TOF_NL} between the two peaks is the time delay between the reference signal and the reflected signal. In order to reduce the error, the above measurement process needs to be repeated several times, and the average result is taken. Finally, the controller instructs the programmable delay line to switch from the short coax back to the long coax. The time delay between the reference signal and the reflected signal is also obtained from T _{D_TOF_NL} minus the delay T _LINE due to the difference in cable length, ie T _{D_TOF} = T _{D_TOF_NL -} T _LINE .

Step 3: Measure the dynamic working curve of the microwave source. The controller generates a linear sweep waveform swept from 0V to V _FS , transmits it to the arbitrary waveform generator, and stores it, where V _FS corresponds to the control voltage corresponding to the highest output frequency of the swept-frequency microwave source. The controller instructs the frequency synthesis source to output a fixed frequency microwave signal. The controller instructs the clock synchronization module to generate a trigger signal and output the trigger signal to the arbitrary waveform generator and the second data acquisition module. The arbitrary waveform generator outputs a linear sweep waveform immediately after receiving the trigger signal. Because the second mixer mixes the swept-frequency signal outputted by the swept-frequency microwave source with the frequency F _P (T) and the fixed-frequency signal output by the frequency synthesis source with the frequency F _FIX , and the output is set with a narrow-band filter, Therefore, only the signal whose output frequency of the mixer falls within the passband range (F _BC -ΔB/2≤|F _P (T)-F _FIX |≤F _BC +ΔB/2) can be collected by the second data acquisition module . After the signal has passed through the second in-phase quadrature demodulator, the in-phase and quadrature components of the signal are extracted. The second data acquisition module is responsible for acquiring the in-phase component I(T), the quadrature component Q(T), and the linear scanning signal V(T). After the in-phase component and the quadrature component are combined into a complex signal, the amplitude value of the complex signal is extracted to generate an amplitude signal A(T). There are two peaks in A(T), and the peak-finding algorithm is used to obtain the moments of the two peaks. The linear scanning signal V(T) is queried, and the scanning voltages corresponding to the two peaks are V _S1 and V _S2 respectively. V _S1 corresponds to the frequency F _P (T)=F _FIX -F _BC , V _S2 corresponds to the frequency F _P (T)=F _FIX +F _BC , so the center point voltage between the two voltages V _S =(V _S1 +V _S2 )/2 corresponds to the frequency F _FIX . From this, it can be concluded that the working voltage of the swept-frequency microwave source when the output frequency is F _FIX is V _S . In order to reduce the error, the above measurement process needs to be repeated many times, and the average value is taken as the measurement result. After completing the measurement of the working voltage corresponding to the frequency F _FIX , the controller controls the frequency integrated source to increase F _S according to a certain step value, and repeat this step until the working voltage measurement within the entire output frequency range of the frequency sweep microwave source is completed, thus The dynamic working curve of the swept-frequency microwave source is obtained.

Step 4: Calibrate the beat frequency signal. The controller uses the dynamic working curve of the swept-frequency microwave source obtained in step 3 to generate the sweep-frequency control waveform W _SWEEP according to the sweep-frequency period T _SWEEP and equal frequency intervals, wherein the number of points n of the waveform is determined by the sweep-frequency period T _SWEEP and the arbitrary waveform generator. The data rate of F _DAC is determined by n=T _SWEEP * F _DAC . The controller sends the W _SWEEP to the arbitrary waveform generator and it stores it. The controller instructs the clock synchronization module to generate a trigger signal and output it to the arbitrary waveform generator and the first data acquisition module. The arbitrary waveform generator outputs W _SWEEP immediately after receiving the trigger signal. After the first data acquisition module receives the trigger signal, it immediately starts to collect the sweep frequency control voltage signal output by the arbitrary waveform generator and the beat frequency signal output by the in-phase quadrature demodulator. The acquisition time T _SAMPLE should be greater than or equal to T _{D_BEAT} +T _{D_TOF} + T _SWEEP , so that the complete beat signal can be recorded. The collected sweep frequency control voltage signal is recorded as S _SWEEP , the collected beat frequency signal is recorded as S _BEAT , and the number of sampling points is determined by the sampling duration T _SAMPLE and the sampling rate F _ADC of the first data acquisition module, namely T _SAMPLE *F _ADC . In order to align and analyze the data, first truncate the data with the length of T _{D_SWEEP} *F _ADC in front of S _SWEEP , and then take the first M=T _SWEEP *F _ADC points from the remaining data, so that the intercepted data is recorded as S _{SWEEP_S} , this data saves all the information of the sweep control voltage. The beat signal is obtained by mixing the reference signal and the reflected signal. Since there is a time delay _{TD_TOF} between the two signals, the length of the beat signal will increase by a period of time _{TD_TOF compared to the frequency sweep period TSWEEP .} For the entire beat frequency signal, the first signal with a duration of _{TD_TOF} is obtained by mixing the reference signal and the reflected fixed-frequency signal, and the last signal with a duration of _{TD_TOF} is a signal that has become a fixed frequency. The reference signal and the reflected swept frequency signal are mixed, and neither has the actual meaning of calibration, so these two signals will not be used in the calibration process. In order to align with the frequency sweep control voltage signal, the first T _{D_BEAT} *F _ADC data in S _BEAT is cut off, and then in the remaining signal, the first M=T _SWEEP *F _ADC points are cut off, so that the cut data is recorded as S _{BEAT_S} . In S _{BEAT_S} , a signal with a length of _{TD_TOF} in front of the beat signal is retained, and a signal with a length of _{TD_TOF} in the back is cut off. This approach is to facilitate alignment with the frequency sweep control voltage signal, which is conducive to analysis. . Perform fast Fourier transform on S _{BEAT_S} , add window, window width w, step size 1, and obtain its time-frequency spectrum FS _{BEAT_S} . Make a copy of S _{SWEEP_S} and record it as S _{SWEEP_FIX} to store the corrected sweep frequency control waveform. The number of data points in FS _{BEAT_S} and S _{SWEEP_S} is M, starting from the Mth data, and searching for the peak of FS _{BEAT_S} (M) to obtain the largest component in its frequency spectrum, its corresponding frequency F _B (M) is is the beat frequency. The beat frequency is actually obtained by mixing the reference signal and the reflected signal at point M. The frequency of the reference signal at point M is determined by the sweep control voltage S _{SWEEP_S} (M), while the frequency of the reflected signal is determined by the frequency before _{TD_TOF} . The sweep frequency control voltage S _{SWEEP_S} (M-Δm) is determined, where Δm=T _{D_TOF} *F _ADC . Due to the dispersion of the reflected signal after being transmitted through the waveguide, the frequency drift occurs, and we need to correct the beat frequency by modifying S _{SWEEP_S} (M-Δm). The specific correction process is as follows. Now, according to the corrected sweep frequency control voltage S _{SWEEP_FIX} (M) and the microwave source sweep frequency control curve, the ideal frequency of the reference signal at point M can be calculated as F _{P_LO_FIX} (M), combined with the ideal beat frequency F _{B_FIX} =( dF _P /dt)*T _{D_TOF} can obtain the ideal frequency of the reflected signal here should be F _{P_RF_FIX} (M)=F _{P_LO_FIX} (M)-F _{B_FIX} . According to the sweep frequency control voltage S _SWEEP (M) and the sweep frequency control curve of the microwave source, the actual frequency of the reference signal at point M can be calculated as F _{P_LO} (M), and combined with the actual beat frequency F _B , the reflected signal here can be obtained The actual frequency of is F _{P_RF} (M)=F _{P_LO} (M)-F _B . The difference between the ideal frequency of the reflected signal and the actual frequency is F _{P_RF_FIX} (M)-F _{P_RF} (M). Since the difference is formed by the accumulation of _{TD_TOF} , the correction frequency should be spread over Δm frequency sweep points to obtain the correction coefficient: η=1+[F _{P_RF_FIX} (M)-F _{P_RF} (M)]/(F _{P_RF} (M)Δm. According to the sweep frequency control voltage S _{SWEEP_S} (M-Δm) and the microwave source sweep frequency control curve, the original detection frequency at this place is obtained as F _P (M-Δm), which is multiplied by the correction coefficient η to obtain the corrected The frequency is F _{P_FIX} (M-Δm)=η*F _P (M-Δm), query the microwave source sweep frequency control curve again, convert F _{P_FIX} (M-Δm) into a voltage value, and fill in the corrected sweep frequency Control voltage S _{SWEEP_FIX} (M-Δm). At this point, the beat frequency correction at the Mth point is completed, and so on, and the M-1, M-2th points are corrected one by one until the Δm+1th point. The reason why it is at Δm It ends at +1 point, because at the Δm+1th point, the actual correction is S _{SWEEP_FIX} (1), which has been corrected. Finally, the controller writes the obtained S _{SWEEP_FIX} into the arbitrary waveform generator, and its Store. Since the effect of a single calibration may not meet the requirements, this step needs to be repeated several times until the beat frequency error of each detection point is less than the threshold. At this point, all correction steps are completed.

The beneficial effects of the present invention are as follows:

1. During the calibration process of the present invention, various delays of signals in the transmission line system are fully considered, and the calibration data is precisely aligned, thereby improving the accuracy of calibration.

2. The calibration structure proposed by the present invention can automatically complete the calibration process by programming the controller, during which there is no circuit connection switching and no manual intervention is required.

3. The present invention is completely integrated into the original system of the microwave reflector, forming a whole with the reflector, and the calibration component does not affect the detection performance of the microwave reflector itself.

Description of drawings

In order to make the objectives, technical solutions and advantages of the present invention clearer, the present invention is described in detail with reference to the accompanying drawings.

Fig. 1 is the overall structure diagram of the microwave reflectometer with online automatic calibration function according to the present invention.

Detailed ways

Embodiments of the present invention will be described below with reference to the accompanying drawings. It should be understood, however, that these descriptions are exemplary only, and are not intended to limit the scope of the present invention. Hereinafter, an embodiment of the present invention will be described with a specific microwave reflectometer device and its calibration process.

Fig. 1 is the overall structure diagram of the microwave reflectometer with online automatic calibration function according to the present invention. The microwave reflectometer with on-line automatic calibration function adds a calibration component on the basis of the microwave reflectometer body. The microwave reflectometer body is a typical continuous wave frequency modulation working mode. Specifically, the microwave reflectometer body consists of a frequency sweep microwave source 101, an arbitrary waveform generator 102, a first directional coupler 103, a local oscillator source 104, a single sideband Modulator 105, power divider 106, first frequency multiplier 107, second frequency multiplier 108, third frequency multiplier 109, power amplifier 110, first mixer 111, first in-phase quadrature demodulator 112 , a transmitting antenna 113 , a receiving antenna 114 , a first data acquisition module 115 , and a controller 116 are composed. The working mode of the microwave reflector body is as follows: the core component of the frequency-sweeping microwave source 101 is a voltage-controlled oscillator. The transmit frequency is F _P (t). The local oscillator source 104 generates a baseband signal having a fixed frequency, the frequency of which is F _L . The power divider 106 is used to divide the local oscillator signal into multiple outputs. The first directional coupler 103 is used to divide the microwave signal generated by the swept-frequency microwave source into two parts, most of the power is called the detection signal, which is used to transmit to the plasma, and a small part of the power is called the reference signal, which is used to compare with the echo. The signal is mixed to realize heterodyne measurement. The single sideband modulator 105 is used for synthesizing the baseband signal of frequency _FL into the swept-frequency microwave signal to generate an upper sideband signal of frequency _FP (t)+ _FL . The frequency multiplier is used to multiply the frequency of the microwave signal by a certain coefficient M, and the first frequency multiplier 107 is used to multiply the frequency of the upper sideband signal F _P (t)+F _L to the frequency M*F _P (t)+M*F _L , the second frequency multiplier 108 is used to multiply the reference signal frequency FP (t) to M* _FP (t), and the third frequency multiplier 109 is used to multiply the local oscillator signal frequency _FL to M* _{F L.} The power amplifier 110 is used to amplify the power of the microwave signal for transmission. The transmit antenna 113 is used to transmit the detection signal to the plasma. When the detection signal is transmitted to the plasma cut-off layer, reflection occurs. The receiving antenna 114 is used to receive the reflected signal reflected from the plasma cutoff layer. The first mixer 111 is used to mix the reference signal and the reflected signal. Due to the transmission, the frequency of the reflected signal becomes M* _FP (t+Δt)+M* _FL , and the frequency of the reference signal is M*F _P (t), then an intermediate frequency signal with a frequency of M*F _P (t+Δt)+M*F _L -M*F _P (t) can be obtained after mixing. The first in-phase quadrature demodulator 112 is used for further complex mixing of the intermediate frequency signal, and the baseband signal is mixed with the intermediate frequency signal to obtain a frequency of F _{B =} M*FP (t+Δt)-M* The beat signal of F _P (t). After complex mixing, the output beat signal has an in-phase component and a quadrature component, which facilitates further time-frequency analysis and reduces noise caused by signal aliasing. The first data acquisition module 115 collects the in-phase and quadrature signals output by the first in-phase and quadrature demodulator 112 and the voltage control signal generated by the arbitrary waveform generator 102 , and outputs them to the controller 116 .

The calibration component includes a second directional coupler 201, a frequency synthesis source 202, a second mixer 203, a narrowband filter 204, a second in-phase quadrature demodulator 205, a second data acquisition module 206, a time-to-digital converter 207, Programmable delay line 208, clock synchronization module 209. The second directional coupler 201 is mainly used to separate a small part of the power from the signal with the frequency F _P (t) output from the swept-frequency microwave source as a calibration signal. The frequency synthesis source 202 is used to generate a microwave signal of a fixed frequency F _S . The second mixer 203 is configured to mix the calibration signal of frequency F _P (t) and the fixed frequency signal of frequency F _S to generate a difference frequency signal of the two, the frequency of which is | _FP (t)- F _S |. The passband center frequency of the narrowband filter 204 is F _BC and the passband width is ΔB. To facilitate in-phase and quadrature demodulation with the signal of the local oscillator source 104, F _BC should be equal to the frequency _FL of the local oscillator source 104 . Only when the frequency of the difference frequency signal output by the second mixer ₂₀₃ satisfies |F _P (t) _{-F S} | The second in-phase quadrature demodulator 205 is used to extract the in-phase and quadrature components of the signal. The second data acquisition module 206 is used to digitize the signal output by the narrowband filter 204 and transmit it to the controller 116 . The time-to-digital converter 207 is used to measure the time delay between the trigger signal, the sweep frequency signal, and the beat frequency signal. The programmable delay line 208 is used to adjust the delay time of the reference signal, which is switched by two microwave switches to realize the adjustment of coaxial cables of different lengths. The clock synchronization module 209 is used to provide a trigger signal and a clock signal for the arbitrary waveform generator 102 and the first data acquisition module 115, so that the signals and data can be aligned on the time axis.

To ensure work synchronization, the clock synchronization module 209 is connected with the arbitrary waveform generator 102 , the second data acquisition module 206 , the time-to-digital converter 207 , and the second data acquisition module 115 using coaxial cables of the same length. The arbitrary waveform generator 102 is connected with the frequency swept microwave source 101 and the data acquisition module 206 by using a coaxial cable of usual length.

In order to improve the system integration, the second data acquisition module 206, the time-to-digital converter 207, the clock synchronization module 209 and the arbitrary waveform generator 102 are integrated on a printed circuit board, which can reduce the space of the calibration device for the microwave reflectometer system. occupied. The circuit board takes a field programmable logic array (FPGA) chip as the core, and realizes the above-mentioned module functions in combination with peripheral chips. Specifically, the second data acquisition module 206 is composed of an off-chip amplification and filter circuit, an analog-to-digital converter (ADC), an off-chip memory, and on-chip write logic, buffers, and read logic. The arbitrary waveform generator 102 is composed of an off-chip amplifying filter circuit, a digital-to-analog converter, and on-chip read logic and memory. The clock synchronization module consists of off-chip output buffers, clock buffers, and on-chip timing generation logic and clock modules. The time-to-digital converter 207 is composed of off-chip comparators, input buffers and on-chip counters, delay chains, encoders, and buffers. The specific working mode of the time-to-digital converter is as follows: the counter is responsible for counting according to the rising edge of the clock signal to record the rough moment of the signal input, and the time resolution is determined by the clock cycle. The delay chain consists of a long carry chain structure that is connected at the end. When a signal is input between two clock cycles, its rising edge propagates in the chain, and the output ports of each carry chain structure on the chain pass through the rising edge. Changes from 0 to 1 in turn. When the signal is transmitted to a certain position in the delay chain, the rising edge of the clock arrives and the output port of the carry chain is locked. By calculating the number of 1s on the output port of the carry chain, we can know that a clock How many carry chain structures have passed in the cycle, and the delay of each carry chain structure is roughly equal, so it can be estimated that the input signal arrives at the delay chain entry and the specific time from the rising edge of the previous clock. Timing results, you can know the exact time a signal arrives.

Among them, the sweep frequency range of the above scanning microwave instrument is 50-70GHz, the working frequency range of the sweep frequency microwave source is 12.5 to 17.5GHz, the corresponding control voltage range is 0 to 20V, the frequency multiplication factor of the frequency multiplier is 4, and the frequency sweep period is 8 microseconds, the sweep repetition frequency is 100KHz, and the design intermediate frequency is 50MHz. The sampling rate of the arbitrary waveform generator 102 is 250 MSPS. The sampling rate of the second data acquisition module 115 is 250 MSPS, and the input bandwidth is 125 MHz.

The parameters of the calibration components are as follows: the coupling degree of the directional coupler 201 is -15dB, the output range of the frequency synthesis source 202 is 100KHz to 20GHz, the operating frequency range of the second mixer 203 is 2-18GHz, the center frequency of the narrowband filter 204 is 100MHz, and the bandwidth is 100MHz. 20MHz, the sampling rate of the second data acquisition module 206 is 1GSPS, and the input bandwidth is 500MHz. In the time-to-digital converter, the coarse count clock is 250MHz, the time resolution is 4ns, and the average delay of a single carry chain is 40ps, so the minimum time resolution is 40ps. The microwave switch used in the programmable delay line 208 operates at a frequency of DC-40GHz. The loss is 0.4dB@18GHz, the switching time is 15ms, and the lengths of the two coaxial cables are 9.0m and 0.5m respectively. The clock synchronization module 209 has a built-in constant temperature crystal oscillator, and the output synchronization clock frequency is 10MHz.

The following are the specific steps for the automatic calibration of the microwave reflectometer:

Step 1: Measure the time delay between the trigger signal, the sweep frequency signal, and the beat frequency signal. A pulse waveform with a height of 1V and a width of 50ns is generated by the controller and output to the arbitrary waveform generator 102, which is then loaded into the memory. The controller 116 instructs the clock synchronization module 209 to generate a trigger signal and output it to the arbitrary waveform generator 102 and the time-to-digital converter 207 . The arbitrary waveform generator 102 outputs the pulse waveform in the memory immediately after receiving the trigger signal. At the same time, the time-to-digital converter 207 starts timing with the trigger signal as the start signal on channel 1, and stops the timing with the pulse signal output by the arbitrary waveform generator as the stop signal. After taking the average of multiple measurements, the measured time interval is T _{D_SWEEP} = 1.240n. In channel 2, the time-to-digital converter 207 uses the trigger signal as a start signal to start timing, and the in-phase signal output by the first in-phase quadrature demodulator 112 is screened as a stop signal to stop timing, and after multiple measurements are averaged , the measured time interval is T _{D_BEAT} =42.360ns.

Step 2: Measure the time delay between the reference signal and the reflected signal. The controller instructs 116 the programmable delay line 208 to switch from 9.0 meters of coaxial cable to 0.5 meters of coaxial cable, thereby increasing the time delay between the reference signal and the reflected signal. A narrow pulse waveform with a height of 1V and a width of 10ns is generated by the controller 116 and output to the arbitrary waveform generator 102, which is then loaded into the memory. The controller 116 instructs the clock synchronization module to generate 209 a trigger signal and output it to the arbitrary waveform generator 102 and the first data acquisition module 115 . The arbitrary waveform generator 102 outputs the narrow pulse waveform immediately after receiving the trigger signal, and at the same time, the first data acquisition module 115 starts to collect data at a sampling rate of 1 GSPS. The reference signal first reaches the mixer through the delay line, while the reflected signal is transmitted through the waveguide and reflected in the device before reaching the mixer. After mixing, two clusters of waveforms are generated on the beat signal. After the beat signal is collected by the first data collection module 115, the upper envelope of the signal is extracted, and peak-finding calculation is performed. The time interval _{TD_TOF_NL} between the two peaks is the time delay between the reference signal and the reflected signal. After multiple measurements are averaged, T _{D_TOF_NL} =60.132ns. Finally, the controller 116 instructs the programmable delay line 208 to switch from the short coaxial cable back to the long coaxial cable. The cable length difference is 8.5 meters, and the delay per meter is 4.7 ns, resulting in a delay T _LINE = 39.95 ns, that is, T _{D_TOF} = 60.132-39.950 = 20.182 ns.

Step 3: Measure the dynamic working curve of the microwave source. The controller 116 generates a linear sweep waveform sweeping from 0V to 20V, transmits it to the arbitrary waveform generator, and stores it there. The controller 116 instructs the frequency synthesis source 202 to output a microwave signal with a fixed frequency of 12.625 GHz. The controller 116 instructs the clock synchronization module 209 to generate a trigger signal and output the trigger signal to the arbitrary waveform generator 102 and the second data acquisition module 206 . The arbitrary waveform generator 102 immediately outputs a linear sweep waveform after receiving the trigger signal, while the second data acquisition module 206 collects the mixed signal output by the second mixer 203 and passed through the narrowband filter 204, and the arbitrary waveform generator 02 The resulting linear scan signal. On the collected complex signal, there are two peaks in its signal amplitude, and the position of the two peaks is obtained by using the peak-seeking algorithm, and the scanning voltages at the moments corresponding to the two peaks are 0.110V and 0.923V, respectively. Among them, 0.110V corresponds to the frequency 12.625-0.100=12.525GHz, and 0.923V corresponds to the frequency 12.625+0.100=12.725GHz, so the center point voltage between the two voltages V _S =(0.110+0.923)/2=0.517V corresponds to The frequency is 12.625GHz. From this, it can be concluded that the working voltage of the swept-frequency microwave source is 0.517V when the output frequency is 12.625GHz. In order to reduce the error, the above measurement process needs to be repeated many times, and the average value is taken as the measurement result. The frequency integrated source 202 is controlled by the controller 116 to increase F _S according to the step value of 0.005GHz, and this step is repeated until the working voltage measurement in the entire output frequency range of the swept-frequency microwave source is completed, thereby obtaining the working voltage of the swept-frequency microwave source and the frequency. The corresponding relationship between the output frequencies, that is, the dynamic working curve.

Step 4: Calibrate the beat frequency signal. The controller 116 uses the dynamic working curve of the swept-frequency microwave source obtained in step 3 to generate a swept-frequency control waveform W _SWEEP with a length of 2000 points by interpolation at equal frequency intervals. Since the sampling rate of the arbitrary waveform generator is 1 GSPS, the output The sweep period corresponding to the waveform is 8us. The controller 116 sends the _WSWEEP to the arbitrary waveform generator and stores it there. The controller 116 instructs the clock synchronization module 209 to generate a trigger signal and output it to the arbitrary waveform generator 102 and the first data acquisition module 115 . The arbitrary waveform generator 102 outputs W _SWEEP immediately after receiving the trigger signal. Immediately after receiving the trigger signal, the first data acquisition module 115 starts to acquire the sweep frequency control voltage signal output by the arbitrary waveform generator 102 and the beat frequency signal output by the in-phase quadrature demodulator 112, and the acquisition duration is 10us. The collected sweep frequency control voltage signal is marked as S _SWEEP , the collected beat frequency signal is marked as S _BEAT , and the number of sampling points is 2500. The data alignment is performed below. Since T _{D_SWEEP} = 1.240ns is less than the sampling period of 4ns, the front-end data of S _SWEEP does not need to be processed. It is only necessary to intercept the first T _SWEEP *F _ADC = 8us*250MSPS = 2000 points in S _SWEEP , so that the intercepted The data is recorded as S _{SWEEP_S} , and all information of the frequency sweep control voltage is stored in this data. In order to align with the frequency sweep control voltage signal, the first _{TD_BEAT} *F _ADC = 42.360ns*250MSPS≈10 data points in S _BEAT are cut off, and then the first 2000 points are cut off in the remaining signal, so that the cut data record for S _{BEAT_S} . Perform fast Fourier transform on S _{BEAT_S} , add a window, the window width is 32, the step size is 1, and its time-frequency spectrum FS _{BEAT_S} is obtained. Make a copy of S _{SWEEP_S} and record it as S _{SWEEP_FIX} to store the corrected sweep frequency control waveform. Starting from the 2000th data, find the peak of FS _{BEAT_S} (2000) to obtain the largest component in its frequency spectrum, and its corresponding frequency FB (2000)= _48.325MHz is the beat frequency. The beat frequency is actually obtained by mixing the reference signal and the reflected signal at 2000 o'clock. The reference signal frequency at 2000 o'clock is determined by the sweep control voltage S _{SWEEP_S} (2000), and the reflected signal frequency is before the 2000 o'clock. Δm=T _{D_TOF} *F _ADC = 5 points of sweep control voltage S _{SWEEP_S} (1995). Due to the dispersion of the reflected signal after being transmitted through the waveguide, the frequency shift occurs, and we need to correct the beat frequency by modifying S _{SWEEP_S} (1995). The specific correction process is as follows. Now, according to the corrected sweep frequency control voltage S _{SWEEP_FIX} (2000) and the microwave source sweep frequency control curve, the ideal frequency of the reference signal at point 2000 can be calculated as F _{P_LO_FIX} (2000)=17.5GHz, combined with the ideal beat frequency of 50MHz , it can be obtained that the ideal frequency of the reflected signal should be F _{P_RF_FIX} (2000)=17.5GHz-0.050GHz=17.450GHz. According to the sweeping control voltage S _SWEEP (2000) and the sweeping control curve of the microwave source, the actual frequency of the reference signal at point 2000 can be calculated to be F _{P_LO} (2000)=17.5GHz, and combined with the actual beat frequency of 48.325MHz, we can get this The actual frequency of the reflected signal at _{FP_RF} (M)=17.5GHz-0.048325GHz=17.451675GHz. The difference between the ideal frequency of the reflected signal and the actual frequency is F _{P_RF_FIX} (2000)-F _{P_RF} (2000)=17.450GHz-17.451675GHz=-0.001675GHz. Since the difference is formed by accumulation of _{TD_TOF} , the correction frequency should be evenly distributed to 5 frequency sweep points, and the correction coefficient η=1+[F _{P_RF_FIX} (M)-F _{P_RF} (M)]/(F _{P_RF} ( M)Δm)=1-0.001675GHz/(17.451675GHz*5)=0.99998. According to the sweep frequency control voltage S _{SWEEP_S} (1995) and the sweep frequency control curve of the microwave source, the original detection frequency at this location is F _P (1995), and the corrected frequency is F _{P_FIX} (1995)= 0.99998F _P (1995), query the microwave source sweep frequency control curve again, convert F _{P_FIX} (1995) into a voltage value, and fill in the corrected sweep frequency control voltage S _{SWEEP_FIX} (1995). So far, the beat frequency correction at the 2000th point has been completed, and so on, and the 1999th and 1998th points are corrected one by one until the sixth point. The reason why it ends at 6 points is because at the 6th point, the actual correction is S _{SWEEP_FIX} (1), which has been corrected. It is worth mentioning that when correcting the 1995th point, the S _{SWEEP_FIX} (1995) used to calculate the ideal frequency of the reference signal has been modified at the 2000th point. 1995 points for correction, which can speed up the correction compared to using unmodified data for correction. Finally, the controller 116 writes the obtained S _{SWEEP_FIX} into the arbitrary waveform generator 102 and stores it. Since the effect of a single calibration may not meet the requirements, this step needs to be repeated several times until the beat frequency error of each detection point is less than 1%. At this point, all correction steps are completed.

The above are only specific embodiments of the present invention, but the protection scope of the present invention is not limited thereto, and these embodiments are only for the purpose of illustration, not for limiting the scope of the present invention. The scope of the invention is defined by the appended claims and their equivalents. Without departing from the scope of the present invention, those skilled in the art can make various substitutions and modifications, and these substitutions and modifications should all fall within the scope of the present invention.

Claims (8)

1. A microwave reflectometer with an online automatic calibration function comprises a microwave reflectometer body, wherein the microwave reflectometer body comprises a sweep frequency microwave source (101), an arbitrary waveform generator (102), a first directional coupler (103), a local vibration source (104), a single-sideband modulator (105), a power divider (106), a first mixer (111), a first in-phase quadrature demodulator (112), a first data acquisition module (115) and a controller (116), wherein the sweep frequency microwave source (101) generates a sweep frequency microwave signal under the control of a sweep frequency control voltage signal generated by the arbitrary waveform generator (102), the local vibration source (104) generates a baseband signal with fixed frequency, the power divider (106) divides the baseband signal into multiple paths for output, the first directional coupler (103) divides the sweep frequency microwave signal into a detection signal and a reference signal, the single-sideband modulator (105) synthesizes the baseband signal and the detection signal to generate an upper sideband signal, the upper sideband signal is transmitted to the plasma, and the receiving antenna (114) receives a reflected signal reflected back from the plasma cut-off layer; a first mixer (111) mixes the reference signal with a reflected signal to obtain an intermediate frequency signal, and a first in-phase quadrature demodulator (112) complex mixes the intermediate frequency signal with a baseband signal to obtain a beat signal, characterized in that:

the calibration assembly comprises a second directional coupler (201), a frequency synthesis source (202), a second mixer (203), a second data acquisition module (206), a time-to-digital converter (207) and a clock synchronization module (209), wherein the second directional coupler (201) separates a part of a frequency-swept microwave signal output by the frequency-swept microwave source (101) to be used as a calibration signal, the frequency synthesis source (202) generates a microwave signal with a fixed frequency, the second mixer (203) is used for mixing the calibration signal and the microwave signal with the fixed frequency to generate a difference frequency signal of the calibration signal and the microwave signal, the second data acquisition module (206) is configured to obtain a dynamic working curve formed by working voltages of the frequency-swept microwave source (101) based on the difference frequency signal, and the clock synchronization module (209) is controlled by the controller to generate a trigger signal, the time-to-digital converter (207) is configured to measure a first time delay between the trigger signal and the swept frequency microwave signal and a second time delay between the trigger signal and the beat frequency signal, the first data acquisition module (115) is configured to measure a third time delay between the reference signal and the reflected signal, and the controller (116) is configured to be connected to the arbitrary waveform generator (102) and to modify the beat frequency signal by using the dynamic operating curve and by modifying the sweep frequency control voltage output by the arbitrary waveform generator (102) based on the first, second and third time delays.

2. The microwave reflectometer with on-line automatic calibration function as in claim 1, wherein: the frequency sweep microwave source (101) generates the frequency sweep microwave signal with the frequency changing along with the voltage under the control of the frequency sweep control voltage signal generated by the arbitrary waveform generator (102).

3. The microwave reflectometer with on-line automatic calibration function as in claim 1, wherein: the microwave reflectometer body further comprises a first frequency multiplier (107), a second frequency multiplier (108) and a third frequency multiplier (109), wherein the first frequency multiplier (107) is used for multiplying the frequency of an upper sideband signal, the second frequency multiplier (108) is used for multiplying the frequency of a reference signal, the third frequency multiplier (109) is used for multiplying the frequency of a baseband signal, the frequency-multiplied upper sideband signal is transmitted to plasma, the reference signal output by the second frequency multiplier and a reflection signal are mixed by the first frequency mixer (111), and an intermediate frequency signal and the baseband signal output by the third frequency multiplier are subjected to complex mixing by the first in-phase quadrature demodulator (112).

4. The microwave reflectometer with on-line automatic calibration function as in claim 1, wherein: the calibration component further comprises a programmable delay line (208), the programmable delay line (208) is used for adjusting the delay time of the reference signal output by the first directional coupler (103), and the two microwave switches are switched to realize the switching of coaxial cables with different lengths.

5. The microwave reflectometer with on-line automatic calibration function as in claim 1, wherein: the beat frequency signal has an in-phase component and a quadrature component, and the first data acquisition module (115) acquires the in-phase component, the quadrature component and the sweep frequency control voltage signal and outputs the same to the controller (116).

6. The microwave reflectometer with on-line automatic calibration function as in claim 1, wherein: the calibration component further comprises a narrow-band filter (204), a second in-phase and quadrature demodulator (205), the second mixer (203) outputs to the narrow-band filter (204), the center frequency of the passband of the narrow-band filter (204) is equal to the frequency generated by the local oscillator source (104), the second in-phase and quadrature demodulator (205) is used for extracting the in-phase component and the quadrature component of the output signal of the narrow-band filter (204), and the second data acquisition module (206) is used for digitizing the signal output by the second in-phase and quadrature demodulator (205) and transmitting the digitized signal to the controller (116).

7. The microwave reflectometer with on-line automatic calibration function as in claim 1, wherein: the core component of the sweep frequency microwave source (101) is a voltage controlled oscillator.

8. The microwave reflectometer with on-line automatic calibration function as in claim 1, wherein: the second data acquisition module (206), the time-to-digital converter (207), the clock synchronization module (209) and the arbitrary waveform generator (102) are integrated on a printed circuit board.

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