RU2741476C1 - Method for automatic adjustment of resonator of hydrogen generator - Google Patents

Abstract

FIELD: quantum frequency standards.SUBSTANCE: invention relates to quantum frequency standards and can be used in hydrogen frequency standards. The aim of the invention is to increase the accuracy of stabilizing the frequency of the resonator of a hydrogen generator (HG) thereby reducing the instability of the frequency of HG at small and large time intervals of 10 s to 10 days.EFFECT: the use of the proposed method for automatic adjustment of a resonator, which by means of linear transformations during the formation of a sounding signal without frequency modulation and during the processing thereof implemented by means of high-speed computing technology enables increasing the allowed power of the sounding signal, increasing the signal/noise ratio of the AAR system and thereby improving the accuracy of stabilization of the resonant frequency of the HG resonator and thus reducing the instability of the HSF frequency. The instability of the frequency of the hydrogen standard frequency at intervals of 1-10 days can be reduced from two to five times compared to analogues.1 cl, 4 dwg

Description

The invention relates to the field of quantum frequency standards and can be used in the development and production of hydrogen frequency standards (HSP).

In HSP, the frequencies of the output signals (5, 10, 100 MHz) are rigidly tied by a phase-locked loop (PLL) to the generation frequency of the hydrogen generator (HG). The main destabilizing factor affecting the SH frequency is the detuning of the microwave resonator; therefore, the HSP uses an automatic resonator tuning system (AHR), which makes it possible to stabilize the natural frequency of the resonator, which changes due to the influence of aging, climatic and mechanical influences. The relative change in the second-harmonic generation frequency is expressed by the formula:

where: Q _p = 5⋅10 ⁴ - Q-factor of the SH resonator;

Q _bg = 1⋅10 ⁹ - Q-factor of the emission line of hydrogen atoms;

- относительное изменение частоты генерации ВГ;

- the relative change in the generation frequency of the SH;

- относительное изменение частоты резонатора.

- the relative change in the resonator frequency.

From formula (1) it can be seen that to ensure the relative instability of the SH 1⋅10 ^-15, it is necessary to maintain the resonant frequency of the resonator F _p = 1420405.7 ... MHz with a relative error of no more than 2⋅10 ^-11 , that is, no more than 0.03 Hz ... Maintaining a given natural frequency of the SHF resonator with such a high accuracy is provided by the ANR system.

In autonomous HSPs, AHR systems with an amplitude tuning criterion are used, which do not require the use of an external highly stable signal. The essence of the amplitude tuning method consists in frequency modulation of the SH resonator or a special probing signal (SS), as a result of which amplitude modulation (AM) appears on the generation signal or the SS signal, according to the magnitude and phase of which one can judge the degree of resonator detuning. When the resonator is precisely tuned to the frequency of the probing signal, AM is absent. This is the amplitude tuning criterion according to which the ANR tracking system with the help of a varicap maintains the resonant frequency of the resonator. The potential accuracy of maintaining a given resonator frequency of such AHR systems is determined by the signal-to-noise ratio of the useful amplitude-modulated AHR signal, according to which, over a time determined by the time constant of the control system, a resonator frequency control signal is generated.

The invention is illustrated by drawings. FIG. 1 shows the AHR method with modulation of the natural frequency of the resonator; in fig. 2 shows the AHR method with an external FM sounding signal; in fig. 3 shows a functional diagram of the system using the method of automatic tuning of the resonator using the FM probe signal; in fig. 4 shows a functional diagram of a system using the proposed method for automatic tuning of a resonator with an unmodulated probe signal of two harmonics.

, от частоты генерации

в сигнале ВГ появляется амплитудная модуляция. Причем глубина AM зависит от степени расстройки резонатора, а фаза от знака отстройки частоты резонатора

относительно

. При

AM отсутствует. После усиления и преобразования к промежуточной частоте сигнал ВГ детектируется амплитудным детектором. Затем из сигнала огибающей AM с помощью синхронного детектора получают сигнал ошибки АНР, который используется следящей системой для стабилизации частоты резонатора.The known method AHR with frequency modulation of the resonator VH [1, 2], using the amplitude tuning criterion (Fig. 1). Frequency modulation (FM) of the natural frequency of the resonator is performed by a coupling loop using a varicap or a switching diode. In this case, in the case of detuning of the average resonator frequency

, on the generation frequency

amplitude modulation appears in the SH signal. Moreover, the depth AM depends on the degree of resonator detuning, and the phase on the sign of the resonator frequency detuning

relatively

... When

AM is missing. After amplification and conversion to an intermediate frequency, the SH signal is detected by an amplitude detector. Then, an AHP error signal is obtained from the AM envelope signal using a synchronous detector, which is used by the tracking system to stabilize the resonator frequency.

The AHR method with modulation of the natural frequency of the resonator is simple; therefore, it is widely used in HSP, however, its capabilities are limited by a small signal-to-noise ratio determined by the SH generation signal, which is also a probing signal for AHR. Therefore, HSPs with this AHR method have insufficiently high metrological characteristics, especially at short time intervals (up to 1000 s).

потенциальные возможности по точности настройки резонатора, ввиду того, что мощность ЗС может быть значительно больше сигнала генерации ВГ. Поэтому ВСЧ с таким способом АНР, обладающим

отношением сигнал/шум, имеют более высокие метрологические характеристики. Однако такой способ АНР более сложный, так как требуют высокой точности поддержания значений частот ЗС и высокой степени одинаковости амплитуд при ЧМ модуляции, что может быть реализовано с помощью цифровых синтезаторов. Прошедший через резонатор ЗС, в котором появляется AM, то есть сигнал АНР, требует цифровой обработки.There is also known a method of AHR with an external frequency modulated (FM) probe signal introduced into the resonator [3], obtained from the output signal of the HSP (Fig. 2), which has

potential opportunities for the accuracy of tuning the resonator, since the power of the ZP can be much higher than the SH generation signal. Therefore, HSP with this AHR method, which has

signal-to-noise ratio, have higher metrological characteristics. However, this AHR method is more complicated, since it requires a high accuracy of maintaining the values of the SZ frequencies and a high degree of uniformity of amplitudes during FM modulation, which can be implemented using digital synthesizers. The SZ passing through the resonator, in which AM appears, that is, the ANR signal, requires digital processing.

An implementation of the AHR method with a frequency-shift keyed signal with continuous phase (FM) is shown in FIG. 3. The formation of the ES and the processing of the AHR signal is performed in the automatic frequency control unit (AFC) 10.

The probe signal is formed from the signal of the auxiliary crystal oscillator f ₀ = 142 MHz 18, tied by the PLL 16 system to the frequency of the output signal 100 MHz VSP, and the signal of the digital synthesizer 25. Digital synthesizer, from the clock signal of 50 MHz, obtained by dividing the frequency of the 100 MHz signal by two by a frequency divider 11, generates an FM signal with a suppressed carrier frequency f ₀ ≈ 405.7 kHz, a frequency deviation ΔF _m ≈ 15 kHz (approximately equal to the half-width of the passband of the SH resonator) and a modulation frequency F _m set by a frequency divider 19 by 2 ²² and equal to F _m ≈ 11.9 Hz. That is (405.7 ± ΔF _m ) kHz. All parameters of the ES frequency are tied to the 100 MHz signal. The amplitude of the digital synthesizer output signal does not depend on the generated frequencies, since they are set by digital codes. Further, the signals from the auxiliary crystal oscillator f ₀ = 142 MHz and the digital synthesizer (405.7 ± ΔF _m ) kHz are combined in the adder 24 and fed to the mixing coupling loop 4 located in the resonator 2 of the hydrogen generator 1. In the microwave resonator, the probe signal (1420405 , 7 ± ΔF _m ) kHz is formed from the tenth harmonic of the 142 MHz signal and the FM signal (405.7 ± ΔF _m ) kHz of the digital synthesizer. In this case, the power of the ES is quite sufficient for the operation of the ANR system.

The ZS passed through the resonator (the ANR signal) is removed from the communication loop 5, amplified by the microwave amplifier 6, converted in the mixer 7 to a frequency of 20.4 MHz using the heterodyne frequency 1400 MHz from the frequency multiplier 8 by multiplying the output frequency of the HSP 100 MHz by 14, amplified amplifier 9 and fed to the LLLT unit. In this case, the AM value of the AHR signal depends on the degree of detuning of the SH resonator with respect to the central frequency of the SW. AM is absent when fine tuned.

The AHP signal of 20.4 MHz is converted in the mixer 13 to an intermediate frequency of 4.6 MHz using the 25 MHz heterodyne frequency, obtained by dividing the 50 MHz frequency by a frequency divider 12. Then this signal is fed to the input of the synchronous detector 14, which is used with the reference signal frequency of 4.6 MHz crystal oscillator 15, restoring the carrier, selects the AHR signal at a frequency of ΔF _m ≈ 15 kHz. Then it is amplified by a selective amplifier 17 with a narrow bandwidth and digitized using an analog-to-digital converter (ADC) 21 with a clock frequency of 12.5 MHz obtained by dividing by four frequencies 50 MHz with a frequency divider 20. The samples from the ADC are fed to the signal processor 26. On Based on these data, the processor calculates the amplitude and phase of the AHP 15 kHz signal for each half-period of the modulation frequency F _m = H, 9 Hz.

Synchronous detection makes it possible to obtain a pure AM AHP signal at a modulation frequency of 15 kHz from the FM-AM signal of AHP 4.6 MHz. To do this, using the PLL system, you can set the frequency and phase of the crystal oscillator to 4.6 MHz, at which there will be no PM and FM in the output signal of the synchronous detector. For this, the signal processor program implements the function of a frequency-phase detector. Based on the phase values calculated by the processor, using the FM and PM minimization criteria, an error signal is calculated, which is then integrated. Then, using a digital-to-analog converter (DAC) 22, it generates a voltage that controls the frequency and phase of the CH 4.6 MHz, at which there are no FM and PM in the ANR 15 kHz signal, which makes it possible to effectively isolate the AM signal of the ANR using narrowband amplifier 17.

The amplitude values calculated by the processor are used to determine the difference for each modulation period, which is then integrated. Then, with the help of the DAC 23, a control voltage is generated, which is fed to the loop of the varicap 3 of the resonator and ensures the stabilization of the set value of its resonant frequency.

The disadvantage of this AHR method is that the formation of a frequency-modulated ES and synchronous detection of the AHP FM signal occurs by nonlinear transformations. At the same time, the frequency-modulated ES contains a wide spectrum of harmonics that are multiples of 2F _m . Harmonics in the vicinity of the SH signal affect the oscillation frequency, causing an increase in frequency instability. The noise of the AHR signal limits the accuracy of the carrier phase reconstruction used for synchronous detection, which degrades the accuracy of maintaining the resonator frequency and, accordingly, increases the instability of the SH frequency. In addition, at the moments of frequency switching during the frequency modulation of the GS, a ringing occurs in the resonator, which creates interference at the generation frequency of the SH, which affects the system of coupling the frequency of the CG output signal of the VSP to the SH signal, reducing the reliability of synchronization up to its breakdown. All this limits the power of the ES at a level of 10-15 dB relative to the generation signal.

The technical objective of the invention is to improve the accuracy of stabilizing the frequency of the SH resonator by increasing the permissible power of the probing signal, increasing the signal-to-noise ratio of the AHR system, as well as by improving the stability of the ES formation and processing the AHR signal, leading to a decrease in the SH frequency instability at small and large intervals time from 10 s to 10 days.

The technical result of the proposed solution to the AHP method of a hydrogen generator consists in reducing the frequency instability of the HSP by using linear transformations both in the formation of the ES without frequency modulation and in the processing of the AHR signal, which can be implemented using digital signal processing.

In the proposed solution, the ZS is formed by adding sequences of codes from two digital synthesizers that generate codes of signals of the same amplitude with frequencies (f-ΔF) kHz and (f + ΔF) kHz, where f = 405.7 kHz, from which an analog a signal consisting of two harmonics of exactly the same amplitude. Moreover, the values of frequencies and the equality of their amplitudes do not depend on time, environmental parameters and supply voltage, since they are formed from a stable output signal of the frequency standard by direct digital synthesis using the same DAC. Further, this signal with the help of the tenth harmonic n = 10 of the signal f ₀ = 142 MHz, satisfying the condition: f _SH = nf ₀ + f = 10⋅142000 + 405.7 = 1420405.7 kHz, is transferred to the SH frequency, forming a probe in the resonator a signal consisting of two harmonics (1420405.7-ΔF) kHz and (1420405.7 + ΔF) kHz, where ΔF is approximately equal to the half-width of the resonance curve of the SH resonator (about 15 kHz). In this case, parasitic signals that affect the generation frequency are not formed near the SH signal. Under the influence of the amplitude-frequency characteristic of the SH resonator, the amplitude difference between these two harmonics appears in the probe signal, which depends on the degree of resonator detuning. With a fine tuning of the resonator, the amplitudes of the harmonics become the same. That is, the amplitude criterion for tuning the resonator is used. The AHP signal taken from the resonator is amplified, converted to an intermediate frequency f _IF = 4.6 MHz, then digitized using an ADC and further processed according to a specially developed algorithm. By means of digital signal processing, both harmonics of the GS are filtered, their amplitudes are determined, the amplitude difference is calculated, which is then integrated, and on the basis of these data, a voltage is generated using a DAC, which is supplied to the VH resonator varicap to stabilize its resonant frequency. The formation of the ES by direct digital synthesis and its digital filtering after passing through the resonator are linear operations that do not create additional interference and noise, which makes it possible to increase the power of the ES to 25-30 dB relative to the generation signal, thereby increasing the accuracy of maintaining the resonant frequency of the SH resonator and , respectively, to reduce the frequency instability of the HSP. The maximum permissible power of the ES is limited mainly by the nonlinearity of the receiving-amplifying path of the ANR system.

The implementation of the proposed method AHP is shown in Fig. 4. The probing signal is formed from the signal of the auxiliary crystal oscillator f ₀ = 142 MHz 18, tied by the PLL 16 system to the frequency of the output signal 100 MHz VSP, and the signals of two digital synthesizers 30 and 31. Digital synthesizers from the 100 MHz clock signal form a sequence of codes of two frequencies (405.7-ΔF) kHz and (405.7 + ΔF) kHz of the same amplitude, which are then added in the code adder 32. Next, the DAC 33 generates an analog sounding signal from the total code, consisting of two harmonics of the same amplitude. Then they are combined with the KG signal 142 MHz in the adder 24 and fed to the mixing coupling loop 4, located in the resonator 2 of the hydrogen generator 1. In the microwave resonator, a probe signal is isolated, consisting of two harmonics (1420405.7-ΔF) kHz and (1420405, 7 + ΔF) kHz, formed from the tenth harmonic of the 142 MHz signal and the (405.7-ΔF) kHz and (405.7 + ΔF) kHz signals. The AHR signal taken from the communication loop 5 of the resonator is amplified by the microwave amplifier 6, converted in the mixer 7 to a frequency of 20.4 MHz using the heterodyne frequency of 1400 MHz, obtained from the frequency multiplier 8 by multiplying the output frequency of the HSC 100 MHz by 14, amplified by the amplifier 9 and supplied into the BLL block.

In the BLL, the AHP signal of 20.4 MHz is converted in the mixer 13 to an intermediate frequency f _IF = 4.6 MHz using the 25 MHz heterodyne frequency from the frequency divider 12 by dividing the 50 MHz frequency obtained from the clock pulse generator 27 by two. Further, the AHP signal 4.6 MHz is digitized by the ADC 28, the samples from which are fed to the programmable logic integrated circuit (FPGA) 29. In the FPGA, using digital filtering, the components of the AHP signal (4.6-ΔF) MHz and (4.6 + ΔF) MHz are extracted, which are transmitted to the signal processor 34. The signal processor calculates their amplitudes, determines the error signal from the amplitude difference and, on the basis of these data, calculates the control signal in real time, from which the voltage supplied to the varicap loop 3 of the VG resonator 1 is then generated using the DAC 35 to stabilize its resonant frequency.

This achieves the technical result of the proposed method for automatic tuning of the resonator, which, due to the use of linear transformations in the formation of a sounding signal without frequency modulation and during its processing, realized with the help of digital signal processing, makes it possible to increase the permissible power of the sounding signal and thereby increase the accuracy of stabilization of the resonant frequency of the resonator VH and, accordingly, reduce the frequency instability of VSP.

At time intervals of 1-10 days, the frequency instability of the HSP can be reduced by at least 2 times compared to the ANR method with a FM sounding signal, and up to 5 times compared to the ANR method with modulation of the natural frequency of the resonator.

Literature

1. Gaigerov B.A., Elkin G.A. Automatic tuning of the hydrogen frequency standard. Measuring technology. 1968, No. 6, p. 90.

2. Gaigerov BA, Rusin FS, Polnikov NI, Sysoev VP, Ovchinnikov SN Transportable quantum clocks based on a hydrogen generator with a small-sized resonator. Measuring technology. 1989, no. 4, p. 17.

3. B.A. Gaigerov, V.P. Sysoev, Yu.S. Samokhvalov. Active transportable hydrogen frequency standard with digital AHR system. Research in the metrology of time and space: Proceedings of VNIIFTRI. Issue 50 (142). M2005. S. 57-70.

Claims (1)

A method for automatic tuning of the resonator AHR of a hydrogen generator with an amplitude tuning criterion, in which the probe signal is formed from the output frequency of the standard by adding sequences of codes from two digital synthesizers that generate signals of the same amplitude with frequencies (f-ΔF) and (f + ΔF), where ΔF is equal to the half-width of the passband of the hydrogen generator resonator, from which, using a digital-to-analog converter, an analog signal is generated from two components, which, adding up with the signal of the auxiliary crystal oscillator f ₀ , satisfying the condition: f _VH = nf ₀ + f or f _VH = nf ₀ -f , where f _VH = 1420405.7 ... kHz is the generation frequency of the hydrogen generator, n is the number of the harmonic of the frequency of the auxiliary crystal oscillator linked by the PLL circuit to the output frequency of the standard, is fed to the diode of the resonator mixing loop, with the help of which from the n-th harmonic of the signal f ₀ and signals (f-ΔF) and (f + ΔF), a probe signal is generated in the resonator of two components at the frequency of the hydrogen generator (1420405.7-ΔF) kHz and (1420405.7 + ΔF) kHz, which, passing through the resonator, is removed from the coupling loop (AHP signal), amplified and converted to the intermediate frequency f _IF using signals of heterodyne frequencies formed from the output frequency of the standard, then digitized by an analog-to-digital converter, the samples from which are fed to a programmable logic integrated circuit, where, using digital signal processing methods, the components of the AHR signal (f _IF -∆F) and (f _IF + ∆F ), which are fed further to the signal processor, which calculates their amplitudes, determines the error signal from the amplitude difference and, on the basis of these data, calculates the control signal in real time, from which the voltage is then supplied to the varicap loop of the hydrogen generator resonator using a digital-to-analog converter. stabilization of its resonant frequency.

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