CN103674315B - Four-channel noise thermometer with quantum voltage as reference - Google Patents

Abstract

The invention relates to a four-channel noise thermometer with quantum voltage as a reference, which includes: a noise source to be measured, a reference quantum voltage pseudo-noise source, and a switch conversion circuit connected with the noise source to be measured and the reference quantum voltage pseudo-noise source , characterized in that: a four-way amplification filter circuit connected to the switch connection circuit, the four-way amplification filter circuit is correspondingly connected to the four-way acquisition channels of the data acquisition and processing part. The four-channel noise thermometer can effectively reduce the integration time required for measurement and reduce system deviation, and will play a huge role in the research of thermodynamic temperature and Boltzmann constant measurement.

Description

A Noise Thermometer with Four Channels Referenced by Quantum Voltage

technical field

The invention belongs to the field of temperature measurement, in particular to a four-channel noise thermometer with quantum voltage as a reference.

Background technique

Temperature is a physical quantity that characterizes the degree of hot or cold of an object, and it is a macroscopic manifestation of the intensity of thermal motion of a large number of microscopic particles. Many physical properties of matter are closely related to temperature, so accurate temperature measurement is of great significance for scientific experiments, industrial production and people's lives.

The premise of accurate temperature measurement is to establish a temperature scale. A temperature scale is a scale that digitizes temperature. It specifies a set of methods for digitizing temperature and defines the unit of temperature. After the development of the temperature scale has gone through three historical stages, it is currently the international practical temperature scale widely used in the world. The basis for the realization of the international temperature scale depends on the accurate measurement of thermodynamic temperature. Although in the past few decades, researchers from all over the world have developed various temperature measurement methods and research methods to accurately measure thermodynamic temperature, but so far, the world Only a few metrological institutions and research units in developed countries have the conditions to use reference thermometers to measure thermodynamic temperature.

Johnson noise thermometer is one of the few temperature measurement methods that can directly measure thermodynamic temperature. Johnson noise is also called thermal noise. When the temperature is above absolute zero, due to the random thermal disturbance of the internal charge or carrier in the conductor There will be potential fluctuations caused by thermal noise at both ends of the conductor, and the Johnson noise thermometer obtains the thermodynamic temperature by measuring this thermal noise voltage. Compared with the temperature measurement method in the current international temperature scale, the Johnson noise thermometer does not need to be divided, the measurement has nothing to do with the material of the sensor, and is not affected by the pressure of the measurement environment, and the resistance value of the sensor hardly affects the measurement accuracy. An ideal temperature measurement method.

Johnson noise is a random signal with a constant power spectral density. Accurate measurement of it requires a long statistical averaging time to eliminate random fluctuations of the Johnson noise signal. Long-term statistical averaging is the main factor that restricts the real-time measurement of noise thermometers. Most of the traditional dual-channel noise thermometers use two thermal noise sources, one is the reference source and the other is the source to be measured. The statistical average time of the two Johnson noise sources prolongs the measurement time, and the power and impedance cannot be matched between the two noise sources at the same time, and the nonlinearity of the measurement circuit limits the improvement of its measurement level.

Contents of the invention

In view of the above problems, the object of the present invention is to provide a set of four-channel noise thermometers calibrated with quantum voltage pseudo-noise sources.

The four-channel noise thermometer of the present invention includes: a noise source to be measured, a reference quantum voltage pseudo-noise source, and a switch conversion circuit connected with the noise source to be measured and the reference quantum voltage pseudo-noise source, and is characterized in that: The four-way amplification filter circuit connected with the switch connection circuit is correspondingly connected with the four-way acquisition channel of the data acquisition and processing part.

Wherein, the amplification and filtering circuit includes a preamplification circuit, a first-stage buffer circuit, a second-stage buffer circuit and at least one filter.

Wherein, the filters are two passive low-pass filters.

Wherein, the switching conversion circuit is a four-input-four-output switching conversion circuit

Wherein, the switching conversion circuit includes a relay switching circuit and a switch control circuit.

Wherein, the data acquisition and processing part includes a data acquisition circuit and a sequential circuit.

The thermometer uses quantum voltage pseudo-noise as a reference source, and uses four measurement channels, which can simultaneously measure the source to be measured and the quantum voltage noise source, and the measurement integration time required to achieve the same statistical uncertainty will be half of the integration time of the dual-channel noise thermometer . Compared with the two-channel noise thermometer, the new four-channel noise thermometer can effectively reduce the integration time required for measurement and reduce the system deviation, and will play a huge role in the research of thermodynamic temperature and Boltzmann constant measurement.

Description of drawings

Fig. 1 schematic diagram of four-channel noise thermometer;

Fig. 2 single-channel data acquisition circuit block diagram;

The overall block diagram of the four-channel data acquisition circuit of Fig. 3;

The principle diagram of the sequential circuit of Fig. 4;

Figure 5 Communication block diagram between the analog-to-digital conversion circuit and the computer

detailed description

Embodiments of the present invention will be described in detail below with reference to the accompanying drawings.

A schematic diagram of a four-channel noise thermometer is shown in FIG. 1 , which includes a noise source to be measured 10 , a reference quantum voltage pseudo-noise source 11 , a switch conversion circuit 12 , a four-way amplification filter circuit 13 and a data acquisition and processing part 14 . Among them, in the amplification and filtering circuit 13, Preamp represents a preamplification circuit, Buffer1 is a first-stage buffer circuit, and Buffer2 is a second-stage buffer circuit, and each amplification and filtering circuit uses a passive low-pass filter 1 and a filter 2 , preferably both filters are 11th-order Butterworth low-pass filters, and the cut-off frequency of each filter is 800kHz. Two high-order low-pass filters can quickly attenuate the high-frequency signal aliased by the ADC, and ensure that the aliased signal is at least 120dB lower than the signal within the measurement bandwidth (10-800kHz).

It can be seen from FIG. 1 that there are two differential outputs at both ends of the reference quantum voltage pseudo-noise source 11 and the noise source 10 to be tested, and the four outputs of the two noise sources are respectively input to the rear four amplification and filtering circuits 13 through the switch conversion circuit. Each two amplification filter circuits 13 form a group of measurement channels, and the two groups of measurement channels alternately measure the reference quantum voltage pseudo-noise source and the noise source to be measured. For two sets of measurement channels, the principle of temperature measurement is the same, and one set of measurement channels will be analyzed below.

Assuming that the quantum voltage pseudonoise power spectral density calculated by the quantum voltage synthesis program is By measuring the power ratio of the noise source under test and the reference quantum voltage pseudo-noise source The temperature T to be measured can be directly calculated, and the calculation formula is shown in (1):

$\mathrm{<span\; class="notranslate">\; T</span>}<span\; class="notranslate">\; =</span>\frac{\stackrel{<span\; class="notranslate">\; \&OverBar;</span>}{{\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; T</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}}{\stackrel{<span\; class="notranslate">\; \&OverBar;</span>}{{\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; r</span>}<span\; class="notranslate">\; ,</span>\mathrm{<span\; class="notranslate">\; T</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}}\frac{\stackrel{<span\; class="notranslate">\; \&OverBar;</span>}{{\mathrm{<span\; class="notranslate">\; v</span>}}_{\mathrm{<span\; class="notranslate">\; rcal</span>}<span\; class="notranslate">\; ,</span>\mathrm{<span\; class="notranslate">\; T</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}}{\mathrm{<span\; class="notranslate">\; 4</span>}\mathrm{<span\; class="notranslate">\; kR</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; T</span>}<span\; class="notranslate">\; )</span>}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; )</span>$

In the formula, T is the temperature to be measured, and are the measured power of the noise source to be tested and the quantum voltage pseudo-noise source, respectively. By selecting the appropriate length of the transmission line, matching the transfer function of the two noise sources, so that the power ratio of the two noise sources Matched within the measurement frequency bandwidth.

As shown in FIG. 1 , the hardware circuit of the four-channel noise thermometer includes a four-input-four-output switching circuit 12 , an amplification and filtering circuit 13 and a data acquisition circuit 15 . Wherein, the switch conversion circuit 12 includes a relay switching circuit and a switch control circuit, and the data acquisition circuit 15 includes an analog-to-digital conversion circuit, a sequential circuit and a data processing circuit.

The data acquisition circuit 15 is one of the most important components of the four-channel noise thermometer, and each acquisition channel of the four-channel noise thermometer adopts a data acquisition circuit with the same circuit structure, as shown in the block diagram of the single-channel data acquisition circuit shown in Figure 2. Including: analog-to-digital conversion circuit, sequential circuit and data processing circuit. In order to prevent the digital high-frequency circuit from interfering with the analog-to-digital conversion circuit, both the timing circuit and the data processing circuit exchange signals with the analog-to-digital conversion circuit through optical fibers. The clock signal, trigger signal and data output in the analog-to-digital conversion circuit are all connected to the high-frequency sequential circuit and data processing circuit through the optical fiber transceiver to realize photoelectric isolation. The data processing circuit converts the serial data obtained by analog-to-digital conversion of four channels into four channels of 16bits parallel data through the Field Programmable Logic Gate Array (FPGA), and inputs them to the 16 I/O acquisition channels of PCI-6541 in sequence superior.

As shown in Figure 3, it is the overall block diagram of the four-channel data acquisition circuit 15, the analog-to-digital conversion chip used in the data acquisition circuit 15 of the four-channel noise thermometer is a charge redistribution successive approximation type (ADI) company's production of Analog Devices Inc. (ADI) SAR) architecture chip AD7626. The chip has no pipeline or pipeline delay, and can obtain higher sampling rate and conversion bits, and is currently widely used in occasions that require high sampling rate and high resolution. AD7626 adopts a typical SAR architecture with 16-bit resolution and 10MSPS throughput, which can ensure no missing codes during conversion. Under the condition of input frequency fin=20kHz, the signal-to-noise ratio (SNR) and signal-to-noise ratio (SINAD) can reach 91dB, and the spurious-free dynamic range (SFDR) can reach 105dB. The integral nonlinearity (iNL) is ±0.45LSB, the differential nonlinearity (DNL) is ±0.35LSB, the common-mode rejection ratio (CMRR) is 68dB at the input frequency fin=1MHz, and the power consumption is only 136mW.

AD7626 converts the differential input voltage when the rising edge of the trigger signal arrives, and converts the output serial low-voltage differential signal (LVDS). LVDS can effectively increase the data transmission rate, and the differential input can eliminate common-mode signals caused by external interference. Because the amplifying circuit is a single-ended output, a driving circuit is required between the AD7626 and the amplifying circuit to perform single-ended to differential conversion. Single-ended to differential selection uses ADA4932-1 and peripheral resistors to form a single-ended to differential circuit. This solution has high requirements on the accuracy and matching of peripheral resistors, and is suitable for driving high-frequency signal input, and has no requirement on the polarity of analog signal input. Since the thermal noise signal is white noise, and the signal amplitude fluctuates around zero volts, it is bipolar, so this scheme is chosen to build the drive circuit.

AD7626 output has two interface modes to choose from the clock mode (Self-Clocked mode). Three LVDS pins (CNV±, CLK± and D±) are required. Each ADC output conversion data word has a 010 sequence in front of it. According to the position of 010, the appropriate 16-bit conversion data is extracted.

The timing circuit 16 is responsible for generating five trigger signals and clock signals, four signals are input to four analog-to-digital conversion circuits, and one signal is input to the data processing circuit. The timing circuit is implemented by FPGA, because the analog-to-digital conversion circuit requires a 50MHz clock signal and a 2MHz trigger signal, so the FPGA first multiplies the input 10MHz signal through the internal phase-locked loop PLL1, and then divides the frequency to output five clocks and trigger signals. The 10MHz reference signal input by the FPGA is provided by the signal generator Agilent33250A. The 33250A reference clock is phase-locked with the 10MHz output of the rubidium clock. The frequency of the rubidium clock is finally traced to GPS. The frequency stability of the rubidium clock is 2×10 ^-12 . In order to prevent frequency drift in the working process, the frequency equipment in the system is phase-locked with the reference rubidium clock. The schematic diagram of the sequential circuit is shown in Figure 4, where cnvpADi: AD7626 trigger signal; clkADi: AD7626 clock signal, i=1, 2, 3, 4; FPGA cnvp: data processing circuit trigger signal; Strobe: data processing circuit clock signal .

Figure 5 is a block diagram of the communication between the analog-to-digital conversion circuit and the computer: the high-speed digital I/O card and FPGA are used to realize data transmission, and the four-way serial data output by the analog-to-digital conversion circuit is first transmitted to the FPGA via optical fiber, and converted in parallel by the FPGA Output to the digital I/O card, and finally read the data collected in the memory of the digital I/O card directly through the computer.

The high-speed digital I/O card adopts the PCI card PCI-6541 produced by NI Company. NI PCI-6541 is a 50MHz digital waveform generator/analyzer used to connect digital electronic products. The board has 32 channels, each channel The direction of the channel is controllable, and its high-capacity onboard memory has the functions of triggering and pattern programming. Different sizes of memory capacity can be selected according to needs. The maximum clock frequency can reach 50MHz. The preferred PCI-6541 card memory capacity is 64MB. The clock and trigger signal input by PCI-6541 are provided by the FPGA core board of the data processing circuit. The frequency of the input clock signal is 8MHz, and the frequency of the trigger signal is 2MHz. In 1s, PCI-6541 can collect 8M data.

The FPGA data processing circuit is responsible for converting the serial data of the four analog-to-digital conversion circuits into parallel data output, and providing the clock and trigger signals required by the digital I/O card. The serial-to-parallel conversion is realized by the FPGA program, and the number of data bits output in parallel is 16 bits, and the 16-bits data is collected by channels 0-15 of the PCI-6541 card. The data processing circuit is connected with PCI-6541 through a 1m cable.

The four-input-four-output switching conversion circuit of the present invention is realized by using FPGA and magnetic latching relay. The FPGA control circuit is preferably a Cyclone II core development circuit, and the FPGA model is preferably EP2C5T144C8N. Compared with the analog switch, the magnetic latching relay does not need an external power supply, and the switching is simple and has little influence on the signal. The relay is preferably a single-coil magnetic latching relay TQ2-L-4.5V, which has the characteristic of being able to hold when power is off. In order to reduce the influence of the digital control circuit on the relay switching circuit, the control circuit and the switching circuit are placed in two different shielding boxes. The control circuit receives computer serial port commands through optical fiber, and after the serial port commands are processed by FPGA, 32 control signals are generated to control 16 relays to switch. In order to reduce digital circuit interference and battery consumption, the power supply of the control circuit is turned off during system measurement, and it is turned on only when switching. The power supply shutdown of the control circuit is also controlled through the computer serial port and optical fiber.

In the four-channel noise thermometer of the present invention, a quantum voltage waveform with a frequency range from DC to 10MHz is synthesized, the fundamental frequency f ₀ =300Hz, and the frequency interval is 600Hz, the waveform only includes odd harmonic components, and the bandwidth range is extended to 10MHz. Although the bandwidth of pseudo noise is different from that of Johnson noise, the impact of frequency signals other than 10MHz on the signal in the measurement frequency band in Johnson noise is much smaller than 1×10-6. Moreover, the base frequency of the synthesized waveform is 300 Hz, and the frequency of the aliased signal does not coincide with the frequency of the synthesized signal during sampling measurement, and does not directly affect the amplitude of the synthesized signal, but is introduced in the form of power addition, so it is different from when measuring white noise The effect of the aliased signal is consistent, so that no systematic error is introduced when calculating the power ratio of the two noise sources.

The four-channel noise thermometer of the present invention adopts the hardware design of a data acquisition circuit and a switch conversion circuit. The data acquisition circuit includes an analog-to-digital conversion circuit, a sequential circuit and a data processing circuit, wherein the analog-to-digital conversion circuit passes through a high-speed digital I/O card PCI-6541 communicate with the computer. The system uses quantum voltage pseudo-noise as a reference source, and uses four measurement channels, which can simultaneously measure the source to be measured and the quantum voltage noise source, and the measurement integration time required to achieve the same statistical uncertainty will be half of the integration time of the dual-channel noise thermometer .

It can be understood that although the present invention has been disclosed above with preferred embodiments, the above embodiments are not intended to limit the present invention. For any person skilled in the art, without departing from the scope of the technical solution of the present invention, the technical content disclosed above can be used to make many possible changes and modifications to the technical solution of the present invention, or be modified to be equivalent to equivalent changes. Example. Therefore, any simple modifications, equivalent changes and modifications made to the above embodiments according to the technical essence of the present invention, which do not deviate from the technical solution of the present invention, still fall within the protection scope of the technical solution of the present invention.

Claims (2)

1. a kind of four channel noise thermometers with quantum voltage as a reference, it comprises: noise source to be measured, reference quantum voltage pseudo-noise source, four input-four connected with described noise source to be measured and reference quantum voltage pseudo-noise source Output switch conversion circuit, the four-input-four-output switch conversion circuit includes a relay switching circuit and a switch control circuit, the switch control circuit and the relay switching circuit are respectively arranged in two different shielding boxes, the switch control circuit The power supply is in an off state during system measurement, and is only turned on when switching, and the power supply shutdown of the switch control circuit is controlled through serial ports and optical fibers; Amplification and filtering circuit, each amplification and filtering circuit includes a preamplification circuit, a first-stage buffer circuit, a second-stage buffer circuit and two high-order low-pass filters, which can quickly attenuate ADC aliasing The high-frequency signal that comes back; the four-way amplification filter circuit is connected to the four-way acquisition channel of the data acquisition and processing part; it is characterized in that: there are two differential outputs at both ends of the reference quantum voltage pseudo-noise source and the noise source to be measured , the four outputs of the reference quantum voltage pseudo-noise source and the noise source to be measured are respectively input to the following four amplification and filter circuits through the switch conversion circuit, and four measurement channels are used to simultaneously measure the noise source to be measured and the reference quantum voltage pseudo-noise source . 2. The four-channel noise thermometer according to claim 1, characterized in that: the data acquisition and processing part comprises a data acquisition circuit and a sequential circuit.