High-precision time interval measuring instrument based on GNSS and TDC-GPX2

Abstract

The application discloses a kind of high-precision time interval measuring instrument based on GNSS and TDC-GPX2, including thermostatic crystal oscillator, level conversion module, clock buffer module, GNSS receiver module, time interval measurement module, main control module and digital-analog conversion module.The application combines the technology of taming thermostatic crystal oscillator, uses the 1PPS signal of GNSS receiver output to tame thermostatic crystal oscillator, designs the outlier algorithm module based on Allan variance to improve stability in taming process, according to the Kalman filter state equation of thermostatic crystal oscillator output frequency established, reasonable filter parameters can be obtained, so that Kalman filter algorithm module can effectively reduce the measurement error caused by 1PPS signal jitter, thermostatic crystal oscillator can output high-precision clock signal under long-term taming, and the high-precision frequency signal output by thermostatic crystal oscillator is used as reference clock signal by time interval measurement module, so as to improve measurement accuracy.The time interval measuring instrument of the application has the characteristics of simplicity, low cost and high precision.

Description

Technical Field

[0001] This invention belongs to the field of time and frequency measurement technology, and in particular relates to a high-precision time interval measuring instrument based on GNSS and TDC-GPX2. Background Technology

[0002] Time is a fundamental physical quantity that underpins the operation of the entire economy and society. Time interval measurement is widely used in fields such as space science, laser ranging, time and frequency transmission, and medicine.

[0003] Currently, commonly used time interval measurement instruments on the market include SR620, 53230A, and YN5636, etc. While these devices offer high accuracy, they are expensive and require an additional frequency reference to achieve high-precision measurements. TDC-GPX2 is a dedicated time-to-digital converter chip that offers low cost and high accuracy. Currently, in most time interval measurement instrument designs based on TDC-GPX2, ordinary crystal oscillators are used as the reference clock. However, clock jitter is significant, affecting measurement accuracy, and ordinary crystal oscillators are prone to aging, with their accuracy continuously decreasing over prolonged use.

[0004] Based on the current situation, in order to optimize existing technologies, this invention combines the technology of a disciplined temperature-controlled crystal oscillator to design a multi-channel, simple, low-cost, and high-precision time interval measuring instrument that integrates a high-precision reference clock source. Summary of the Invention

[0005] To address the issues of existing high-precision time interval measurement instruments being expensive, requiring additional frequency references, and the inability to guarantee measurement accuracy in TDC-GPX2-based designs using ordinary crystal oscillators as reference clocks, this invention provides a high-precision time interval measurement instrument based on GNSS and TDC-GPX2. It utilizes the 1PPS output from the GNSS receiver to discipline a local temperature-controlled crystal oscillator, thereby locking the output frequency of the temperature-controlled crystal oscillator and improving its accuracy and stability. This provides a high-precision reference signal for the time interval measurement module, enhancing measurement accuracy.

[0006] To address the aforementioned technical problems, this invention proposes a high-precision time interval measuring instrument based on GNSS and TDC-GPX2, comprising:

[0007] A temperature-controlled crystal oscillator for outputting a 10MHz square wave CMOS level signal;

[0008] A level conversion module for receiving 10MHz square wave CMOS level signals and converting them into 10MHz square wave LVCMOS level signals;

[0009] A clock buffer module used to buffer the 10MHz signal output from the level conversion module and divide it into multiple outputs;

[0010] GNSS receiver module for outputting 1PPS signal and NMEA-0183 data format message information;

[0011] This is a measurement module used to measure the 1PPS output of the GNSS receiver module and the time interval of the physical event under test, using the 10MHz signal output by the clock buffer module as a reference clock.

[0012] The main control module is used to receive and extract message information from the GNSS receiver module, process peripheral interrupt signals from the time interval measurement module, receive and process measurement result data from the time interval measurement module, and output the control signal for the thermostatic crystal oscillator.

[0013] A digital-to-analog converter module used to convert the control signal output by the main control module into a voltage quantity and output it to the voltage control terminal of the thermostatic crystal oscillator.

[0014] Furthermore, the clock buffer module is a CDCLVC1104 clock buffer, which outputs four 10MHz signals. Pins Y0 and Y1 are electrically connected to the SMA interface for external output, and pin Y2 is electrically connected to the time interval measurement module to provide a reference clock.

[0015] Furthermore, the time interval measurement module is a TDC-GPX2 time digital chip, including 4 channels:

[0016] Measurement channel 1 is electrically connected to the GNSS receiver module for inputting 1PPS signal measurements;

[0017] Measurement channels 2 to 4 are electrically connected to the SMA2, SMA3, and SMA4 interfaces, respectively, for user measurements.

[0018] Furthermore, when the TDC-GPX2 time digital chip completes each measurement, the coarse and fine count results are cached in the internal FIFO, and then an interrupt signal is generated. The interrupt signal is electrically connected to the main control module to prompt the main control module to receive data.

[0019] Furthermore, the main control module is an STM32F407ZGT6 single-chip microcontroller, which integrates four modules: computation time, outlier removal, Kalman filtering, and PID control.

[0020] Furthermore, the STM32F407ZGT6 microcontroller integrates USART, I2C, and SPI hardware peripherals, and is externally connected to an LCD12864 liquid crystal screen.

[0021] Furthermore, its working modes include two types: taming mode and maintaining mode.

[0022] 1) By parsing the message information output by the GNSS receiver module, the number of satellites currently received is obtained. When the number of satellites received reaches the required level, the 1PPS output by the GNSS receiver module is highly accurate and stable, and it enters the discipline mode. The software process in the discipline mode is as follows: the main control module receives the measurement results from the time interval measurement module, calculates the time data, removes outliers from the time data and performs Kalman filtering, calculates the time data into frequency data, calculates the deviation between the current frequency and the nominal frequency of the temperature-controlled crystal oscillator, and finally the PID controller calculates the current control quantity.

[0023] 2) When the number of received satellites does not meet the requirements, the 1PPS signal accuracy is low and the error is large. In this case, the hold mode is activated. In the hold mode, the center voltage of the output thermostatic crystal oscillator is used to control the thermostatic crystal oscillator.

[0024] Compared with the prior art, the advantages of the present invention are as follows:

[0025] This invention combines the technology of taming a cryogenic crystal oscillator. It uses a 1PPS signal output from a GNSS receiver to tame the cryogenic crystal oscillator and designs an outlier algorithm module based on Allan variance to improve stability during the taming process. Based on the established Kalman filter state equation for the output frequency of the cryogenic crystal oscillator, reasonable filter parameters can be obtained, enabling the Kalman filter algorithm module to effectively reduce measurement errors caused by 1PPS signal jitter. Under long-term taming, the cryogenic crystal oscillator can output a high-precision clock signal. The time interval measurement module uses the high-precision frequency signal output by the cryogenic crystal oscillator as a reference clock signal, thereby improving measurement accuracy. The time interval measuring instrument of this invention is simple, low-cost, and highly accurate. Attached Figure Description

[0026] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments recorded in the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0027] Figure 1 This is a flowchart of the technical route of the present invention;

[0028] Figure 2 This is a flowchart of the software process of this invention. Detailed Implementation

[0029] To enable those skilled in the art to better understand the technical solution of the present invention, the present invention will be described in detail below with reference to the accompanying drawings. The description in this part is only exemplary and explanatory, and should not be used to limit the scope of protection of the present invention in any way.

[0030] Please refer to Figures 1 to 2 This invention provides a high-precision time interval measuring instrument based on GNSS and TDC-GPX2, comprising a temperature-controlled crystal oscillator, a level conversion module, a clock buffer module, a GNSS receiver module, a time interval measuring module, a main control module, and a digital-to-analog conversion module, wherein:

[0031] like Figure 1 According to the signal flow direction, the temperature-controlled crystal oscillator adopts the AOCJY3-A-10MHz crystal oscillator to output a 10MHz square wave CMOS level signal, which is electrically connected to the level conversion module and the digital-to-analog conversion module respectively.

[0032] The level conversion module is an LSF0101 level conversion chip, which is used to receive a 10MHz square wave CMOS level signal and convert it into a 10MHz square wave LVCMOS level signal. Its input port is electrically connected to the temperature-controlled crystal oscillator and its output port is electrically connected to the clock buffer module.

[0033] The clock buffer module is a CDCLVC1104 clock buffer, used to buffer the 10MHz signal output from the level conversion module and divide it into multiple outputs. Specifically, the clock buffer outputs four 10MHz signals:

[0034] The first Y0 circuit is connected to the SMA interface for external output, providing a 10MHz clock source.

[0035] The second Y1 circuit is connected to the SMA interface for external output, providing a 10MHz clock source.

[0036] The third Y2 signal is electrically connected to the time interval measurement module as its reference clock.

[0037] The 1PPS output (one pulse per second) port of the GNSS receiver module is electrically connected to the time interval measurement module. The message information output port (NEMA-0183 format) outputs via serial protocol and is electrically connected to the main control module. Specifically, the GNSS receiver module of this invention is a domestically developed UT4B0 full-system high-frequency high-precision timing board. It outputs messages in multiple formats. This invention configures the message information output format to NEMA-0183. The message information is transmitted via serial communication protocol and is electrically connected to the main control module. The main control module extracts the UTC time, latitude and longitude, and number of received satellites from the message information.

[0038] The decision to use a 1PPS-controlled temperature-controlled crystal oscillator depends on whether the number of satellites detected is greater than four. When the number of satellites detected is less than four, the accuracy of the 1PPS output by this module decreases. In this case, the center voltage of the temperature-controlled crystal oscillator is used to control the temperature-controlled crystal oscillator. At the same time, the "current instrument failed to lock onto satellite" message is output through the RS232 serial port and the LCD12864 liquid crystal. This GNSS receiver outputs a total of one 1PPS signal (one pulse per second), and this 1PPS signal is synchronized with the atomic clock of the satellite. Therefore, this 1PPS signal is used as a standard reference signal and is electrically connected to the time interval measurement module.

[0039] The time interval measurement module is used to measure 1PPS output by the GNSS receiver module and the physical event under test, using the 10MHz signal output by the clock buffer module as a reference clock. It is designed based on the TDC-GPX2 time digital chip, which has a measurement accuracy of 20ps. Internally, it uses a combination of coarse and fine counting to perform digital interpolation to measure the time of adjacent edges of the measured signal. The coarse counting uses the frequency of the clock signal input to the reference port, but there will be a quantization error of one reference clock cycle. The fine counting utilizes the gate delay technology of digital circuits to perform a more precise counting of the quantization error of the coarse counting. It includes four independent channels:

[0040] Measurement channel 1 is electrically connected to the GNSS receiver module for inputting 1PPS signal measurements;

[0041] Measurement channels 2 to 4 are electrically connected to the SMA2, SMA3, and SMA4 interfaces respectively for user measurements.

[0042] The third Y2 signal of the clock buffer module is electrically connected to the reference clock port of the TDC-GPX2;

[0043] When the 1PPS signal rises, the TDC-GPX2 completes a measurement and stores the values ​​of the coarse and fine counters in the internal FIFO. At the same time, the interrupt port of the TDC-GPX2 generates an interrupt signal to prompt the main control module to receive data. Both the coarse and fine counters have a 24-bit data width, and the measurement results are transmitted via a four-wire SPI communication protocol.

[0044] The digital-to-analog converter (DAC) is designed based on TI's DAC1220E high-precision DAC chip. This invention uses the chip's 20-bit operating mode, with an output voltage range of 0-5V. The DAC module is electrically connected to the main control module via a three-wire SPI communication protocol to receive control voltage data, convert it into an analog voltage, and then connect it to the control voltage port of a temperature-controlled crystal oscillator (TCC) through its voltage output port to regulate the TCC's output frequency. The DAC1220E chip requires a 2.5V reference voltage at its reference voltage port; this instrument uses Analog Devices' ADR4525BRZ voltage reference chip to provide this voltage reference.

[0045] The main control module is designed based on STMicroelectronics' STM32F407ZGT6 microcontroller. This microcontroller is 32-bit with a main frequency of 168MHz. The chip integrates hardware peripherals such as USART, I2C, and SPI. As the main controller of the measuring instrument, the microcontroller's functions include receiving and extracting message information from the GNSS receiver, processing peripheral interrupt signals of the time interval measurement module, receiving measurement result data from the four measurement channels of the time interval measurement module, calculating and processing the measurement result data, transmitting data with the digital-to-analog conversion module, and integrating four modules: time calculation, outlier removal, Kalman filtering, and PID control.

[0046] The data received by the microcontroller from the time interval measurement module is in timestamp format. After receiving the data, the microcontroller sums the values ​​of the received coarse and fine counters to calculate the time, which is then input into the outlier removal module and the Kalman filter module to obtain filtered time data. Let the time data obtained at time t0 be d0 and the time data obtained at time t1 be d1, then the time interval is time1 = d1 - d0. The 1PPS signal output by the GNSS receiver is used as the standard second pulse, that is, the time interval between two consecutive rising edges of this signal is 1 second. Each time the 1PPS rises, the time interval measurement module buffers the results of the coarse and fine counters internally. Therefore, if the frequency output by the oven-controlled crystal oscillator is absolutely accurate, the difference between the results of two consecutive measurements by the time interval measurement module will be 1 second. Therefore, by subtracting the value of time1 from 1 second, the difference between the oven-controlled crystal oscillator and the nominal frequency can be calculated. This difference is then input into the PID control algorithm to calculate the current control quantity.

[0047] High-precision time measurement and filtering are crucial for obtaining high-precision time interval data, which is essential for taming the temperature-controlled crystal oscillator. The 1PPS signal output during satellite time synchronization will generate random jitter. The jitter noise mainly comes from the noise caused by the satellite signal propagation process and the receiver's own noise, which is mainly white noise.

[0048] Kalman filtering can be used to reduce measurement errors caused by 1PPS signal jitter. However, even when the GNSS receiver is locked, it can sometimes produce significant errors, known as outliers. Outliers cause oscillations in the Kalman filter, affecting stability during the training process and increasing the frequency deviation of the oven-controlled crystal oscillator output. The commonly used 3σ criterion is ineffective when faced with consecutive outliers and may sometimes cause the filter to diverge. Therefore, an outlier identification and removal algorithm based on Allan variance plus threshold constraint was designed. This algorithm leverages the high short-term stability of the oven-controlled crystal oscillator output frequency, meaning that the second-level Allan variance of the oven-controlled crystal oscillator is less prone to abrupt changes.

[0049] Let's say we're at a certain time, t0, and the data collected at that time is denoted as r0. First, we update and cache the data collected at time t0 and the 19 samples prior to time t0 using a sliding method in the array buf.

[0050] buf = {r -19 ,L,r -1 ,r0}

[0051] The formula for calculating Allan variance is:

[0052]

[0053] Since the main controller is a microcontroller, the data precision is limited. Therefore, the constant term in the above formula is discarded, i.e.

[0054]

[0055] Based on the above formula, calculate the Allan variance of the 20 data points in the buf array, and let the Allan variance result be allan_f.

[0056] Let the threshold be tre0. Then the discriminant function is: |allan_f-b0|<=tre0; b0 is the second-level Allan variance parameter provided in the isothermal crystal oscillator datasheet.

[0057] If an outlier occurs at a certain moment, the calculated allan_f will increase, and the discriminant function will not meet the condition. The least squares method is used to fit the data of the first 19 moments of the buf array, and the predicted value is calculated using the mathematical relationship of the fit. Let the predicted value be e0.

[0058] At this point, replace the current measurement data r0 with the data e0 estimated by the least squares method, and then use e0 to update the array buf.

[0059] As mentioned above, to reduce random errors in the satellite timing process, Kalman filtering is selected to process the time data of measurement channel 1. The ideal formula for the deviation between the instantaneous frequency of the isothermal crystal oscillator and the standard frequency is:

[0060]

[0061] x0 is the initial frequency deviation, a is the fixed frequency drift of the temperature-controlled crystal oscillator, D is the frequency drift rate (aging rate), and ε(t) is the noise, which follows a Gaussian distribution. Suppose a discrete time sampling sequence t0, t1L t n-1 ,t n L, then its sampling period T s =t n+1 -t n =M(s), where n = 0, 1, 2, ... Let x(n) is T s The average value of the relative frequency offset over a given time period.

[0062]

[0063] in Since ε(n) follows a Gaussian distribution with variance σ 2 / M. Taking x(n) as the state variable in the Kalman filter model, then

[0064] x(n)=x(n-1)+aT s +2Dt n-1 +ε(n)-ε(n-1)

[0065] make but

[0066]

[0067] Satisfies Gaussian distribution The values ​​of σ and a are determined by the performance of the temperature-controlled crystal oscillator. Given a sampling period of T... s In T s The average relative frequency offset over a time period is x(n), and each sampling period T... s The time interval measurement module can obtain a measured value of the output frequency of a thermostatic crystal oscillator, corresponding to T. s The measured value Z(n) of the average frequency over a time period is

[0068]

[0069] in This section addresses the measurement noise introduced when measuring the frequency of a cryogenic crystal oscillator using 1PPS. Given the system's sampling period of 1 second and the slow aging process of the cryogenic crystal oscillator (typically measured in years), 2Dt is used. n-1 Discarding the above, the Kalman filter state equation for the constructed isothermal crystal oscillator is:

[0070] Equations of state:

[0071] Measurement equation:

[0072] The recursive formula for the Kalman filter equation is:

[0073] State prediction in one step:

[0074] One-step prediction mean square error:

[0075] State estimation:

[0076] Filter gain:

[0077] Estimate mean square error: P n =(IK n H n )P n / n-1

[0078] Given an initial value Under the condition of P0, based on the measured value Z at time k n The state estimate at time n can then be calculated recursively. Among them, P n It is the minimum mean square error matrix, Φ n,n-1 It is a one-step transition array, Γ n-1 It is a system noise-driven array. It is a measurement array, Q (n-1) yes The variance matrix, K n R is the Kalman filter gain coefficient at time n. n yes The variance matrix is ​​given. The taming system in this paper collects one-dimensional time-series signals. Based on the actual system, let...

[0079] To make the filter equation more accurately predict the recursion, Q needs to be determined. (n-1) and R n Value. According to analysis, the short-term stability of the oven-controlled crystal oscillator is relatively high, and its frequency does not change abruptly. However, the 1PPS output of the GNSS receiver includes jitter. Therefore, the state equation for predicting the output frequency of the oven-controlled crystal oscillator is... The measurement equation of the time interval measurement module Small, therefore R n It should be greater than Q (n-1) Based on this, the two hyperparameters Q were determined experimentally. (n-1) The value is 0.0001, R n The value is 1.0.

[0080] The adjustable voltage of the temperature-controlled crystal oscillator (TCS) voltage control port and the output of the digital-to-analog converter (DAC) module have certain ranges, which requires the final output of the PID control algorithm to be within a certain range. Therefore, based on incremental PID, a limit-based weakening integral PID is adopted.

[0081] like Figure 2 The diagram shows the software flowchart of the time interval measuring instrument. The software design of this instrument includes the initialization of the serial port, SPI, GPIO, system tick timer, basic timer, and interrupts by the microcontroller (STM32F407ZGT6).

[0082] A delay is applied during power-on to allow the thermostatic crystal oscillator to warm up. If TDC-GPX2 initialization fails, it will attempt initialization again.

[0083] After initialization is complete, the microcontroller receives the message information and extracts the time, latitude and longitude, and number of satellites from the message information.

[0084] Afterwards, the system waits for the time interval measurement module to take measurements. Once data is available in the time interval measurement module's FIFO, an interrupt is triggered. Upon detecting the interrupt, the microcontroller receives the values ​​from the coarse and fine counters of the time interval measurement module. It then calculates the received results into time and time interval. If the received data comes from measurement channels 2 to 4, the result is output using the RS232 serial protocol and displayed on the LCD12864. If the result comes from channel 1, it is buffered.

[0085] If the microcontroller detects that the number of satellites received in the message information output by the GNSS receiver does not meet the requirements, it will directly enter the hold mode. In the hold mode, the center voltage data is output to the digital-to-analog converter module, and then the temperature-controlled crystal oscillator is controlled. At this time, the RS232 serial port and the LCD12864 liquid crystal output the message "Current instrument failed to lock satellite".

[0086] If the message information is normal, the time data of measurement channel 1 in the microcontroller's cache will be considered valid and the microcontroller will enter the discipline mode.

[0087] The discipline mode is as follows: The GNSS receiver requires approximately 20 minutes to stabilize after power-on. If the GNSS receiver is not stable, the output 1PPS is not accurate enough. To ensure the discipline process quickly reaches stability, it first checks if the instrument has been powered on for 20 minutes. If not, the result from the time interval measurement module is directly input to the Kalman filter algorithm module. If the time interval measurement module is stable, the result is first input to the outlier removal algorithm module, then the outlier removal result is input to the Kalman filter algorithm module. Next, the current frequency is calculated from the time data, and the deviation between the current frequency and the nominal frequency of the temperature-controlled crystal oscillator (10MHz) is calculated. This deviation is input to the PID control algorithm module to calculate the control input, and finally, the control input is input to the digital-to-analog converter module.

[0088] The time interval measuring instrument designed by combining the controlled-temperature crystal oscillator technology with the TDC-GPX2 chip has relatively low cost, high measurement accuracy, and certain advantages in measurement method, which can provide a certain guarantee for the measurement accuracy of the system in different environments.

[0089] Working principle:

[0090] The thermostatic crystal oscillator outputs a 10MHz signal, which is then converted to a level by a level conversion module. The signal is then output to a clock buffer module to buffer multiple 10MHz signals. One of the 10MHz signals from the clock buffer module is input to the reference clock port of the time interval measurement module. The time interval measurement module uses this 10MHz signal as a reference clock to measure the 1PPS signal input to measurement channel 1. The main control module reads the result from the time interval measurement module, processes the result using an outlier elimination algorithm, and then performs a Kalman filter algorithm to reduce measurement errors caused by jitter in the 1PPS signal. It then calculates the frequency deviation between the thermostatic crystal oscillator and its nominal frequency at the current moment, inputs the deviation to a PID control algorithm to calculate control information, and finally inputs the control information to a digital-to-analog converter module. This converted voltage is then input to the voltage control terminal of the thermostatic crystal oscillator to regulate its output frequency. When the satellite signal is lost, the control information from before the signal loss is used to control the thermostatic crystal oscillator.

[0091] The present invention can obtain a high-precision 10MHz signal through the above solution, which solves the problem of insufficient measurement accuracy of the time interval measurement module due to the inaccuracy of the reference clock, thereby improving the measurement accuracy of the time interval measurement module.

[0092] The above description, in conjunction with specific preferred embodiments, provides a further detailed explanation of the present invention. It should not be construed that the specific implementation of the present invention is limited to these descriptions. For those skilled in the art, various simple deductions or substitutions can be made without departing from the concept of the present invention, and all such modifications and substitutions should be considered within the scope of protection of the present invention.

Claims

1. A high-precision time interval measuring instrument based on GNSS and TDC-GPX2, characterized in that, include: A temperature-controlled crystal oscillator for outputting a 10MHz square wave CMOS level signal; A level conversion module for receiving 10MHz square wave CMOS level signals and converting them into 10MHz square wave LVCMOS level signals; A clock buffer module used to buffer the 10MHz signal output from the level conversion module and divide it into multiple outputs; GNSS receiver module for outputting 1PPS signal and NMEA-0183 data format message information; This module is used to measure the 1PPS output of the GNSS receiver module and the time interval of the physical event under test, using the 10MHz signal output by the clock buffer module as a reference clock. The main control module is used to receive and extract message information from the GNSS receiver module, process peripheral interrupt signals from the time interval measurement module, receive and process measurement result data from the time interval measurement module, and output the control signal for the thermostatic crystal oscillator. A digital-to-analog converter module used to convert the control signal output by the main control module into a voltage quantity and output it to the voltage control terminal of the thermostatic crystal oscillator.

2. The high-precision time interval measuring instrument based on GNSS and TDC-GPX2 according to claim 1, characterized in that, The clock buffer module is a CDCLVC1104 clock buffer, which outputs four 10MHz signals. Pins Y0 and Y1 are electrically connected to the SMA interface for external output, and pin Y2 is electrically connected to the time interval measurement module to provide a reference clock.

3. The high-precision time interval measuring instrument based on GNSS and TDC-GPX2 according to claim 1, characterized in that, The time interval measurement module is a TDC-GPX2 time digital chip, including 4 channels: Measurement channel 1 is electrically connected to the GNSS receiver module for inputting and measuring 1PPS signals; Measurement channels 2 to 4 are electrically connected to the SMA2, SMA3, and SMA4 interfaces, respectively, for user measurements.

4. The high-precision time interval measuring instrument based on GNSS and TDC-GPX2 according to claim 3, characterized in that, When the TDC-GPX2 time digital chip completes each measurement, the coarse and fine count results are cached in the internal FIFO, and then an interrupt signal is generated. The interrupt signal is electrically connected to the main control module to prompt the main control module to receive data.

5. The high-precision time interval measuring instrument based on GNSS and TDC-GPX2 according to claim 1, characterized in that, The main control module is an STM32F407ZGT6 single-chip microcontroller, which integrates four modules: computation time, outlier removal, Kalman filtering, and PID control.

6. The high-precision time interval measuring instrument based on GNSS and TDC-GPX2 according to claim 5, characterized in that, The STM32F407ZGT6 microcontroller also integrates USART, I2C, and SPI hardware peripherals, and is connected to an external LCD12864 liquid crystal screen.

7. The high-precision time interval measuring instrument based on GNSS and TDC-GPX2 according to claim 1, characterized in that, Its working modes include two types: taming mode and maintaining mode. 1) By parsing the message information output by the GNSS receiver module, the number of satellites currently received is obtained. When the number of satellites received reaches the required level, the 1PPS output by the GNSS receiver module is highly accurate and stable, and it enters the disciplined mode. The software process in the disciplined mode is as follows: the main control module receives the measurement results from the time interval measurement module, calculates the time data, removes outliers from the time data and performs Kalman filtering, calculates the time data into frequency data, calculates the deviation between the current frequency and the nominal frequency of the temperature-controlled crystal oscillator, and finally the PID controller calculates the current control quantity. 2) When the number of received satellites does not meet the requirements, the 1PPS signal accuracy is low and the error is large. In this case, the hold mode is activated. In the hold mode, the center voltage of the output thermostatic crystal oscillator is used to control the thermostatic crystal oscillator.

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