A rubidium atomic clock digital servo system and method based on second-order integration

Abstract

The present invention discloses a rubidium atomic clock digital servo system and method based on second-order integration. The system includes a single-chip microcomputer, an integration unit, a crystal oscillator unit, a frequency synthesis unit and an optical detection unit. The single-chip microcomputer is used to generate a modulation signal to modulate the microwave frequency output by the frequency synthesis unit; the optical detection unit is used to output an amplitude-modulated optical detection signal after the microwave frequency is modulated, and use the single-chip microcomputer to perform digital synchronous detection, and output a frequency error signal after digital synchronous detection; the integration unit is used to perform integral calculation and summation on the frequency error signal output by the single-chip microcomputer to obtain a control voltage sequence, and output it to the crystal oscillator unit after DAC digital-to-analog conversion, so as to achieve the purpose of controlling the output phase of the crystal oscillator unit and the phase of the transition spectrum line of the rubidium atomic clock to deviate. The present invention adopts a digital servo system circuit with small temperature drift, no parameter influence, can improve stability, and can improve the timekeeping accuracy of the rubidium atomic clock.

Description

Technical Field

[0001] The present invention relates to the technical field of signal frequency control, and in particular to a rubidium atomic clock digital servo system and method based on second-order integration. Background Art

[0002] The rubidium clock is also called the rubidium atomic clock. It consists of a rubidium quantum part and a voltage-controlled crystal oscillator. The frequency of the voltage-controlled crystal oscillator is multiplied and synthesized, and sent to the quantum system to compare with the rubidium atomic transition frequency. The error signal is sent back to the voltage-controlled crystal oscillator, and its frequency is adjusted to lock it to the frequency corresponding to the unique energy level transition of the rubidium atom. The short-term stability of the rubidium atomic frequency standard can reach up to 10 -12 The accuracy is ±5×10 -11 .

[0003] The rubidium frequency standard does not require a vacuum system, a deflection magnet, or an atomic beam, so it is small in size, small in mass, has a short warm-up time, and is cheap, but has poor accuracy and relatively large frequency drift, and can only be used as a secondary standard. The rubidium frequency standard can be quickly disciplined and externally synchronized through GPS to overcome the drift of the rubidium oscillator itself, and can be regarded as a basic synchronous clock unit. Through improvements in design and process, the reliability and mass production of the product are also guaranteed, and the conditions for industrialization are now met. It can be expected that this high-performance miniaturized rubidium clock with external-second discipline will be used in harsh environments such as unmanned operation, which will greatly expand the application field of rubidium clocks.

[0004] The rubidium atomic clock is mainly composed of several modules, including single-chip microcomputer circuit, servo circuit, microwave frequency multiplication circuit, frequency modulation, and frequency multiplication integrated circuit. The rubidium frequency standard is a passive atomic frequency, which uses the transition between the ground state hyperfine energy levels, and the corresponding transition frequency is 6834.682614MHz. The atomic transition acts as a frequency discrimination for the microwave signal and generates an error signal. The frequency of the crystal oscillator is locked to the atomic transition frequency through the phase-locked loop servo, so that the excitation signal frequency is locked to the atomic transition frequency, achieving a highly stable and accurate output frequency of the crystal oscillator.

[0005] Rubidium atomic clocks are usually used in high-performance timekeeping devices. The principle block diagram of the rubidium atomic clock is shown in Figure 1, where the servo system loop of the rubidium atomic clock includes a crystal oscillator, a frequency synthesis system, a physical part and a servo system. The crystal oscillator frequency (usually 10MHz) is multiplied to 6.835GHz by the frequency synthesis system, and a light detection signal is obtained under the action of the physical part. The frequency error signal is obtained through the servo system, and feedback control is performed to lock the crystal oscillator frequency with the transition spectrum of the rubidium atomic clock. The disciplined timing loop uses TDC (time to digital converter) to measure the difference between the output phase of the rubidium atomic clock and the reference PPS (polynomial phase signal) phase, and fine-tunes the output frequency of the rubidium atomic clock through feedback from the disciplined system. The temperature coefficient of the transition spectrum of the rubidium atomic clock is usually within 1E-12 / ℃. However, the actual output of the rubidium atomic clock uses a crystal oscillator as a local oscillator. The temperature coefficient of the crystal oscillator can reach 1E-10 / ℃. ​​When the traditional analog PID (proportional-integral-differential) control system is used to implement the rubidium clock servo system, the rapid frequency change of the crystal oscillator will cause the output phase of the crystal oscillator to deviate from the phase of the transition spectrum line of the rubidium atomic clock (the integral of the transition frequency), thereby affecting the frequency stability of the rubidium atomic clock and the phase accuracy during timing and punctuality. Summary of the invention

[0006] The first purpose of the present invention is to provide a rubidium atomic clock digital servo system based on second-order integration to solve the technical problem that when the rubidium clock servo system is realized by a traditional analog PID (proportional-integral-differential) control system, the rapid frequency change of the crystal oscillator will cause the output phase of the crystal oscillator to deviate from the phase of the transition spectrum line of the rubidium atomic clock (the integral of the transition frequency), thereby affecting the frequency stability of the rubidium atomic clock and the phase accuracy in the timing and punctuality process.

[0007] The first objective of the present invention is achieved by adopting the following technical scheme: a rubidium atomic clock digital servo system based on second-order integration, comprising a rubidium atomic clock, a single-chip microcomputer, an integration unit, a crystal oscillator unit, a frequency synthesis unit and an optical detection unit, wherein the single-chip microcomputer is used to generate a modulation signal to modulate the microwave frequency output by the frequency synthesis unit; the optical detection unit is used to output an amplitude-modulated optical detection signal after the microwave frequency is modulated, the optical detection signal is converted by ADC analog-to-digital conversion, and digital synchronous detection is performed by the single-chip microcomputer, and a frequency error signal is output after digital synchronous detection; the integration unit is used to integrate and calculate the frequency error signal output by the single-chip microcomputer and sum it to obtain a control voltage sequence, and the control voltage sequence is output to the crystal oscillator unit after DAC digital-to-analog conversion, so as to achieve the purpose of controlling the deviation between the output phase of the crystal oscillator unit and the phase of the transition spectrum line of the rubidium atomic clock.

[0008] Furthermore, the single chip microcomputer includes a synchronous detection module and a low-pass filter module. The synchronous detection module is used to perform digital synchronous detection on the modulation signal and the optical detection signal. The digital synchronous detection output obtains a frequency error signal after passing through the low-pass filter module.

[0009] Furthermore, the low-pass filtering module adopts a cascaded integral comb filter.

[0010] Furthermore, the modulation signal is a square wave signal, and the square wave signal performs square wave modulation on the microwave frequency output by the frequency synthesis unit.

[0011] Furthermore, the optical detection unit is a physical part of the rubidium atomic clock, and after the physical part of the rubidium atomic clock is modulated at the microwave frequency, it outputs an amplitude-modulated optical detection signal of the same frequency.

[0012] Furthermore, the integration unit includes a first-order integration module and a second-order integration module, which respectively perform first-order integration and second-order integration on the frequency error signal, and sum the first-order integration and second-order integration results to obtain a control voltage sequence.

[0013] The second purpose of the present invention is to provide a digital servo method for a rubidium atomic clock based on second-order integration, so as to solve the technical problem that when a rubidium clock servo system is realized by a traditional analog PID (proportional-integral-differential) control system, the rapid frequency change of the crystal oscillator will cause a deviation between the output phase of the crystal oscillator and the phase (integral of the transition frequency) of the transition spectrum line of the rubidium atomic clock, thereby affecting the frequency stability of the rubidium atomic clock and the phase accuracy in the timing and punctuality process.

[0014] The second object of the present invention is achieved by the following technical means: a rubidium atomic clock digital servo method based on second-order integration, comprising the following steps:

[0015] Generate a modulation signal by means of a single chip microcomputer, and output the modulation signal to a frequency synthesis unit, so as to modulate the microwave frequency output by the frequency synthesis unit;

[0016] After the microwave frequency is modulated, the optical detection unit outputs an amplitude-modulated optical detection signal of the same frequency. After the optical detection signal is converted into digital by ADC, it is digitally synchronously detected with the modulation signal by using a single-chip microcomputer, and a frequency error signal is output;

[0017] The frequency error signal output by the single-chip microcomputer is integrated and summed by the integration unit to obtain a control voltage sequence, which is output to the crystal oscillator unit after DAC digital-to-analog conversion, so as to achieve the purpose of controlling the deviation between the output phase of the crystal oscillator unit and the phase of the transition spectrum line of the rubidium atomic clock.

[0018] Furthermore, the integrating unit integrally calculates and sums the frequency error signal output by the single-chip microcomputer, including the following sub-steps:

[0019] Perform first-order integration on the frequency error signal;

[0020] Perform a second-order integration on the frequency error signal;

[0021] The first-order integration result and the second-order integration result are summed to obtain a control voltage sequence.

[0022] The beneficial effects of the present invention are as follows: the temperature drift of the digital servo system circuit is small, there is no parameter influence, the stability can be improved, and the timekeeping accuracy of the rubidium atomic clock can be improved; and a modular design can be assembled to form a complex algorithm suitable for various situations. In addition, the reliability of the rubidium atomic clock can be significantly improved and the cost can be reduced. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on the structures shown in these drawings without paying creative work.

[0024] Figure 1 This is the principle block diagram of the traditional rubidium atomic clock;

[0025] Figure 2 This is a principle block diagram of the present invention;

[0026] Figure 3 It is a flow chart of the present invention;

[0027] Figure 4 This is a closed-loop amplitude-frequency characteristic diagram of the present invention;

[0028] Figure 5 This is the closed-loop amplitude-frequency characteristic diagram of the traditional rubidium clock servo system;

[0029] Figure 6 This is the phase measurement result diagram;

[0030] Figure 7 This is a frequency stability result diagram of the present invention;

[0031] Figure 8 This is the frequency stability result diagram of the traditional rubidium clock servo system. DETAILED DESCRIPTION

[0032] In order to make the purpose, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Generally, the components of the embodiments of the present invention described and shown in the drawings here can be arranged and designed in various different configurations.

[0033] It should be noted that similar reference numerals and letters denote similar items in the following drawings, and therefore, once an item is defined in one drawing, further definition and explanation thereof is not required in subsequent drawings.

[0034] Some embodiments of the present invention are described in detail below in conjunction with the accompanying drawings. In the absence of conflict, the following embodiments and features in the embodiments can be combined with each other.

[0035] Embodiment 1:

[0036] See also Figure 2 A rubidium atomic clock digital servo system based on second-order integration includes a rubidium atomic clock, a single-chip microcomputer, an integration unit, a crystal oscillator unit, a frequency synthesis unit and an optical detection unit. The single-chip microcomputer is used to generate a modulation signal to modulate the microwave frequency output by the frequency synthesis unit; the optical detection unit is used to output an amplitude-modulated optical detection signal after the microwave frequency is modulated, and the optical detection signal is converted by ADC analog-to-digital conversion and then digitally synchronously detected by the single-chip microcomputer, and a frequency error signal is output after digital synchronous detection; the integration unit is used to integrate and sum the frequency error signal output by the single-chip microcomputer to obtain a control voltage sequence, and the control voltage sequence is output to the crystal oscillator unit after DAC digital-to-analog conversion, so as to achieve the purpose of controlling the deviation between the output phase of the crystal oscillator unit and the phase of the transition spectrum line of the rubidium atomic clock.

[0037] In this embodiment, the single chip microcomputer includes a synchronous detection module and a low-pass filter module. The synchronous detection module is used to perform digital synchronous detection on the modulation signal and the optical detection signal. The digital synchronous detection output obtains a frequency error signal after passing through the low-pass filter module.

[0038] In this embodiment, the low-pass filter module adopts CIC filtering (cascaded integrator comb filter), the extraction factor is 3600, the number of differential link delays is 1, and the number of integrators is 1, so that the frequency of the frequency error signal obtained is 1 Hz.

[0039] In this embodiment, the modulation signal is a square wave signal, and the square wave signal performs square wave modulation on the microwave frequency output by the frequency synthesis unit. Further, first, the initial value of the crystal oscillator unit voltage control is set to a state close to resonance. At this time, the single chip microcomputer is at f s =3.6kHz sampling frequency generates and outputs frequency f m =90Hz square wave signal S n , the square wave signal S n Connect to the frequency synthesis unit to perform square wave modulation on the microwave frequency output.

[0040] In this embodiment, the optical detection unit is the physical part of the rubidium atomic clock. After the microwave frequency is modulated, the physical part of the rubidium atomic clock outputs an amplitude-modulated optical detection signal D(t) of the same frequency. The optical detection signal D(t) is transmitted through f s =3.6kHz sampling frequency ADC is used to sample the optical detection signal sequence D n Then use the microcontroller and the modulation signal to perform digital synchronous detection to get the output I n =D n S n-d , where d is the time delay. The error signal can be maximized by adjusting d in the open-loop state to obtain the optimal value of d. The synchronous detection output after digital low-pass filtering (CIC filtering) is the frequency error signal sequence E m .

[0041] In this embodiment, the integration unit includes a first-order integration module and a second-order integration module, and the frequency error signal is respectively integrated in the first order and in the second order through the first-order integration module and the second-order integration module, and the first-order integration and the second-order integration results are summed to obtain a control voltage sequence, wherein the first-order integration transfer function is: The second-order integral transfer function is: The control voltage sequence C is obtained by summing the first-order integral result and the second-order integral. m , the crystal oscillator frequency is controlled through the DAC output.

[0042] The frequency discrimination efficiency (ratio of error signal to relative frequency deviation) of the rubidium atomic clock can be measured in advance as The voltage-controlled electrical regulation rate of the crystal oscillator (the ratio of the relative frequency deviation of the crystal oscillator to the voltage-controlled voltage) is K i Can be set to K ii Can be set to Where B is the loop bandwidth, in this case B = 0.1Hz. It can be seen that the coefficient of the first-order integration is large, providing rapid control of the crystal frequency; the second-order integration can reduce the additional phase difference caused by the rapid change of the crystal frequency, making the output phase of the crystal basically consistent with the frequency integral of the transition spectrum of the rubidium atomic clock.

[0043] For further information, see Figure 4 , Figure 5 The closed-loop amplitude-frequency characteristic diagram of the present invention and the closed-loop amplitude-frequency characteristic diagram of the traditional rubidium clock servo system are respectively given, wherein the dotted line is the closed-loop noise transfer function, and the solid line is the closed-loop transfer function. The gain characteristic of the present invention at the low-frequency end is 40dB / dec, which is greatly improved compared with the 20dB / dec of the traditional PID system, and the random walk frequency noise (frequency power spectrum density S) of the crystal oscillator under the influence of temperature can be reduced. y ∝f-2 ) is reduced to white phase noise (frequency power spectral density S y ∝f 2 ) to ensure that the output phase of the rubidium atomic clock is consistent with the integral of the rubidium atomic spectral line frequency.

[0044] For further information, see Figure 6 , where the dotted line is the servo system provided by the present invention, and the solid line is the traditional PID servo system. In the taming-keeping test of the rubidium atomic clock, the test process includes taming for 24 hours and then keeping time for 24 hours, where the PPS input is provided by the UT4B0 receiver. The results show that: under the GNSS control taming state, the timing phase accuracy (i.e., phase standard deviation) is improved from 1.9ns to 0.4ns, and the 24-hour keeping time phase accuracy is improved from 127ns to 42ns.

[0045] For further information, see Figure 7 , Figure 8 , respectively give the frequency stability result diagram of the present invention and the frequency stability result diagram of the traditional rubidium clock servo system, the results show that after adopting the servo system provided by the present invention, the 100s and 1000s frequency stability are improved.

[0046] See also Figure 3 The present application also provides a rubidium atomic clock digital servo method based on second-order integration, the method comprising the following steps:

[0047] Generate a modulation signal by means of a single chip microcomputer, and output the modulation signal to a frequency synthesis unit, so as to modulate the microwave frequency output by the frequency synthesis unit;

[0048] After the microwave frequency is modulated, the optical detection unit outputs an amplitude-modulated optical detection signal of the same frequency. After the optical detection signal is converted into digital by ADC, it is digitally synchronously detected with the modulation signal by using a single-chip microcomputer, and a frequency error signal is output;

[0049] The frequency error signal output by the single-chip microcomputer is integrated and summed by the integration unit to obtain a control voltage sequence, which is output to the crystal oscillator unit after DAC digital-to-analog conversion, so as to achieve the purpose of controlling the deviation between the output phase of the crystal oscillator unit and the phase of the transition spectrum line of the rubidium atomic clock.

[0050] In this embodiment, the step of integrating and summing the frequency error signal output by the single chip microcomputer through the integration unit includes the following sub-steps:

[0051] Perform first-order integration on the frequency error signal;

[0052] Perform a second-order integration on the frequency error signal;

[0053] The first-order integration result and the second-order integration result are summed to obtain a control voltage sequence.

[0054] In this embodiment, before the single-chip microcomputer generates a modulation signal, first, the voltage control initial value of the crystal oscillator unit needs to be set to a state close to resonance. At this time, the single-chip microcomputer then generates a modulation signal at f s =3.6kHz sampling frequency generates and outputs frequency f m =90Hz square wave signal S n , the square wave signal S n Connect to the frequency synthesis unit to perform square wave modulation on its output microwave frequency. After the microwave frequency is modulated, the physical part of the rubidium atomic clock (photodetection unit) will output an amplitude modulated photodetection signal D(t) of the same frequency. s =3.6kHz sampling frequency ADC is used to sample the optical detection signal sequence D n . Use the microcontroller and the modulation signal to perform digital synchronous detection to obtain the output I n =D n S n-d d is the time delay. The error signal can be maximized by adjusting d in the open loop state to obtain the optimal value of d. The synchronous detection output is the frequency error signal sequence E after digital low-pass filtering. m The low-pass filter uses CIC filtering (cascaded integrator comb filter), with a decimation factor of 3600, a differential delay of 1, and an integrator of 1. The frequency of the error signal obtained is 1Hz. The error signal is subjected to first-order integration (transfer function ) and the second-order integral (transfer function ) and then sum to get the control voltage sequence C m The crystal frequency is controlled by the DAC output. The frequency discrimination efficiency (ratio of error signal to relative frequency deviation) of the rubidium atomic clock can be measured in advance as The voltage-controlled electrical regulation rate of the crystal oscillator (the ratio of the relative frequency deviation of the crystal oscillator to the voltage-controlled voltage) is K i Can be set to K ii Can be set to Where B is the loop bandwidth, in this case B = 0.1Hz. It can be seen that the coefficient of the first-order integration is large, providing rapid control of the crystal frequency; the second-order integration can reduce the additional phase difference caused by the rapid change of the crystal frequency, making the output phase of the crystal basically consistent with the frequency integral of the transition spectrum of the rubidium atomic clock.

[0055] The digital servo system circuit of the present invention has a small temperature drift and is not affected by parameters, so the stability can be improved and the timekeeping accuracy of the rubidium atomic clock can be improved. The present invention can also be modularly designed and assembled to form complex algorithms suitable for various situations. In addition, the reliability of the rubidium atomic clock can be significantly improved and the cost can be reduced.

[0056] It should be noted that, for the above embodiments, for the sake of simplicity, they are all described as a series of action combinations, but those skilled in the art should be aware that the present application is not limited by the order of the actions described, because according to the present application, some steps can be performed in other orders or simultaneously. Secondly, those skilled in the art should also be aware that the embodiments described in the specification belong to preferred embodiments, and the actions involved are not necessarily required by the present application.

[0057] The above embodiments describe the basic principles and main features of the present invention and the advantages of the present invention. Those skilled in the art should understand that the present invention is not limited by the above embodiments, and the above embodiments and descriptions are only for explaining the principles of the present invention. Without departing from the spirit and scope of the present invention, the changes and modifications made by those skilled in the art shall be within the scope of protection of the appended claims of the present invention without departing from the spirit and scope of the present invention.

Claims

1. A rubidium atomic clock digital servo system based on second-order integration, comprising a rubidium atomic clock, characterized in that: It comprises a single-chip microcomputer, an integration unit, a crystal oscillator unit, a frequency synthesis unit and a light detection unit. The single-chip microcomputer is used to generate a modulation signal to modulate the microwave frequency output by the frequency synthesis unit; the light detection unit is used to output an amplitude-modulated light detection signal after the microwave frequency is modulated, the light detection signal is converted by ADC analog-to-digital conversion, and then digital synchronous detection is performed by the single-chip microcomputer, and a frequency error signal is output after digital synchronous detection; the integration unit is used to integrate and calculate the frequency error signal output by the single-chip microcomputer and sum it to obtain a control voltage sequence, and the control voltage sequence is output to the crystal oscillator unit after DAC digital-to-analog conversion, so as to achieve the purpose of controlling the deviation between the output phase of the crystal oscillator unit and the phase of the transition spectrum line of the rubidium atomic clock; The integration unit includes a first-order integration module and a second-order integration module, and the frequency error signal is respectively integrated in the first order and in the second order through the first-order integration module and the second-order integration module, and the first-order integration and the second-order integration results are summed to obtain a control voltage sequence, wherein the first-order integration transfer function is: The second-order integral transfer function is: in, K d is the frequency discrimination efficiency of the rubidium atomic clock, K c is the voltage-controlled electrical regulation rate of the crystal oscillator, and B is the loop bandwidth.

2. A rubidium atomic clock digital servo system based on second-order integration as claimed in claim 1, characterized in that: The single chip microcomputer comprises a synchronous detection module and a low pass filter module. The synchronous detection module is used to perform digital synchronous detection on the modulation signal and the optical detection signal. The digital synchronous detection output obtains a frequency error signal after passing through the low pass filter module.

3. A rubidium atomic clock digital servo system based on second-order integration as claimed in claim 2, characterized in that: The low-pass filtering module adopts a cascaded integral comb filter.

4. A rubidium atomic clock digital servo system based on second-order integration as claimed in claim 1, characterized in that: The modulation signal is a square wave signal, and the square wave signal performs square wave modulation on the microwave frequency output by the frequency synthesis unit.

5. A rubidium atomic clock digital servo system based on second-order integration as claimed in claim 1, characterized in that: The optical detection unit is the physical part of the rubidium atomic clock. After the microwave frequency is modulated, the physical part of the rubidium atomic clock outputs an amplitude-modulated optical detection signal of the same frequency.

6. A rubidium atomic clock digital servo method based on second-order integration, using a rubidium atomic clock digital servo system based on second-order integration as claimed in any one of claims 1 to 5, characterized in that: The steps include: Generate a modulation signal by means of a single chip microcomputer, and output the modulation signal to a frequency synthesis unit, so as to modulate the microwave frequency output by the frequency synthesis unit; After the microwave frequency is modulated, the optical detection unit outputs an amplitude-modulated optical detection signal of the same frequency. After the optical detection signal is converted by ADC analog-to-digital conversion, the single-chip microcomputer and the modulated signal are used for digital synchronous detection, and a frequency error signal is output; the frequency error signal output by the single-chip microcomputer is integrated and summed by the integration unit to obtain a control voltage sequence, which is output to the crystal oscillator unit after DAC digital-to-analog conversion, so as to achieve the purpose of controlling the deviation between the output phase of the crystal oscillator unit and the phase of the transition spectrum line of the rubidium atomic clock.

7. A rubidium atomic clock digital servo method based on second-order integration as claimed in claim 6, characterized in that: The integrating unit integrally calculates and sums the frequency error signal output by the single-chip microcomputer, and includes the following sub-steps: Perform first-order integration on the frequency error signal; Perform a second-order integration on the frequency error signal; The first-order integration result and the second-order integration result are summed to obtain a control voltage sequence.

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