RU2148881C1 - Hydrogen frequency standard - Google Patents

Abstract

FIELD: high-stability signal sources. SUBSTANCE: hydrogen frequency standard has hydrogen producer and crystal oscillator incorporating phase-locked loop (PLL) system operating in response to signal coming from hydrogen producer. Hydrogen producer resonator frequency is tuned to spectral line top using line Q-factor periodic modulation method. Automatic tuning unit of resonator is connected through switching device to output of PLL phase detector. Automatic tuning unit of resonator has series-connected selective filter, analog-to-digital converter, digital synchronous detector, and digital- to-analog converter. Synchronous detector is program-controlled device. Provision is made for discriminating resonator detuning signal from frequency control circuit of crystal oscillator which makes it possible to dispense with auxiliary hydrogen producer in automatic tuning system of resonator. EFFECT: reduced relative frequency instability of frequency standard. 2 cl, 4 dwg

Description

 The invention relates to quantum hydrogen frequency standards and can be used in the development and design of hydrogen frequency standards with automatic tuning of the resonator frequency of a quantum generator.

 A feature of hydrogen frequency standards is the need to tune the frequency of the microwave cavity of a quantum generator to the top of the spectral line of radiation of hydrogen atoms. The instability of the frequency of the microwave cavity is the main reason for the instability of the frequency of the standard over long time intervals (more than 1 day).

 Devices that automatically adjust the resonator of a quantum generator in hydrogen frequency standards are known (“Quartz and quantum frequency measures.” Edited by B.I. Makarenko, USSR Ministry of Defense, 1989, pp. 369–377).

 To tune the resonator frequency to the top of the line of emission of hydrogen atoms, the line Q-switching method is used by changing the intensity of the atomic beam injected into the microwave cavity or by applying an inhomogeneous magnetic field to the interaction region of the atoms with the field of the microwave cavity.

 With a change in the quality factor of the line in the hydrogen generator, a frequency pull-up effect occurs, which, when the frequency of the microwave cavity is detuned relative to the hydrogen line, leads to a change in the generation frequency.

 Periodic modulation of the quality factor of the line leads to modulation of the frequency of the tunable generator. The criterion for tuning the generator is the disappearance of frequency modulation.

To increase the accuracy of tuning the microwave cavity, two hydrogen generators are usually used (one is customizable, the second is reference) and a device for comparing their frequencies (frequency comparator). The tuning accuracy is determined by the efficiency of modulation of the Q-factor of the radiation line and the error in measuring the difference frequency and reaches 5 • 10 ^-14 - 5 • 10 ^-15 (according to the output frequency of the hydrogen standard).

 The best results are obtained using a digital resonator automatic tuning system (ANR). The basis of her work is the considered modulation method. When tuning the microwave cavity, the system compares the frequencies of the tunable hydrogen generator at two values of the quality factor of the spectral line, which depends, for example, on the beam intensity. Depending on the magnitude and sign of the obtained frequency difference, the AHR system adjusts the frequency of the microwave cavity by changing the bias voltage of the varicap, which controls the frequency of the resonator.

 Similarly, the AHR system works in the hydrogen standard for frequency and time of type Ch1-75, taken as the closest analogue of the invention (prospectus "Standard for frequency and time hydrogen" Ch1-75; "Quartz and quantum frequency measures. Edited by B.I. Makarenko . Ministry of Defense of the USSR, 1989, p. 369 - 377). When tuning the resonator, the Q-factor of the spectral line of the hydrogen generator is modulated. For 100 s, the reverse counter counts the beat period of two signals in the forward direction at one Q-factor of the spectral line and for 100 s in the reverse direction at another Q-factor. The difference in the counting results in the forward and reverse directions is proportional to the detuning of the frequency of the resonator of the hydrogen generator from the top of the spectral line. Information about the magnitude and sign of the detuning is converted by a digital-to-analog converter into voltage, which is supplied to a varicap of a hydrogen generator, adjusting its frequency.

 The disadvantage of the above hydrogen frequency standard is that it is impossible to tune the resonator without an auxiliary hydrogen generator.

Hydrogen generators are known for the adjustment of which an additional hydrogen generator is not required ("Standards of frequency and time based on quantum generators and discriminators." Edited by B.P. Fateev. M., Sov. Radio, 1978, p. 202) . The operation of the hydrogen generator tuning system in them is based on modulation by the varicap of the frequency of the tune of the resonator (and not the quality factor of the spectral line). The modulation frequency is ~ 30 Hz. The AHP system emits an amplitude modulation signal that is fed to a synchronous detector. An error signal from a synchronous detector is used to correct the resonator frequency if it differs from the frequency of the spectral line. However, this frequency standard has great frequency instability (6 • 10 ^-13 from 100 s to 1 day), the generator frequency may differ from the frequency of the spectral line by ~ 5 · 10 ^-12 , there is a dependence of the tuning frequency on the characteristics of varicaps, etc. .

 The technical problem, which is achieved by the invention, is the creation of a hydrogen frequency standard with low relative frequency instability, which allows it to be used as a source of highly stable signals for time-frequency measurements and for working in standard measuring instruments, as well as to exclude an auxiliary hydrogen generator from the composition of its AHR system , i.e. reduce the cost of the standard.

 The essence of the technical solution lies in the fact that in the hydrogen frequency standard, which includes a hydrogen generator, a crystal oscillator and a phase locked loop (PLL) of a crystal oscillator according to a hydrogen generator signal, and uses the method of periodic Q-switching of the Q-line to adjust the frequency of the microwave resonator to the top of the spectral line , the frequency detuning signal of the microwave resonator of the hydrogen generator is extracted from the frequency control circuit of the crystal oscillator in the PLL system.

 To do this, in the hydrogen frequency standard, containing a crystal oscillator, a frequency synthesizer and a phase detector connected to the ring, a frequency converter connected to the second input of the phase detector, one of whose inputs is connected to a hydrogen generator and the other input is connected to the output of the crystal oscillator, automatic tuning unit the resonator of the hydrogen generator, one output of which is connected to the resonator, and the other output is connected to the modulator of the hydrogen generator, and the input of the automatic resonance tuning unit torus connected to the output of the phase detector, the crystal oscillator connected to the frequency converter through the frequency multiplier, and a phase detector connected via a key device for automatic tuning of the resonator unit comprising in series a selective filter, an analog-digital converter, a digital synchronous detector and a digital to analog converter.

 In FIG. 1 is a structural diagram of the device, FIG. Figure 2 shows diagrams explaining the operation of the device APP unit.

 The device contains a quantum hydrogen generator 1 connected to the first input of the frequency converter 2, the output of which is connected to one of the inputs of the phase generator 3, the output of which is connected to the input of the crystal oscillator 4, connected to the tunable frequency synthesizer 5. The output of the synthesizer 5 is connected to another phase input detector 3. A crystal oscillator 4 is connected to the second input of the frequency converter 2 through a frequency multiplier 6. At the output of the phase detector 3, an AHP 7 unit is also connected through a key device 8, one whose output is connected to the varicap of the microwave resonator of the hydrogen generator 1, and the second output is connected to the input of the Q-factor of the line of the hydrogen generator. Block AHP 7 contains serially connected: a selective filter with an amplifier 9, an analog-to-digital converter (ADC) 10, a digital synchronous detector 11, and a digital-to-analog converter 12.

 The device operates as follows.

 Frequency converter 2 provides a frequency transfer of 1.42 GHz of the signal generated by hydrogen generator 1 to a frequency of 405 kHz of phase detector 3. As a reference signal for converter 2, a 5 MHz signal of crystal oscillator 4, passed through frequency multiplier 6, is used.

 A tunable frequency synthesizer 5 is used to generate a comparison signal supplied to the phase detector 3. Synthesizer 5 provides a discrete electronic tuning of the output frequencies of the hydrogen standard. The signal from the output of the phase detector 3 is used to control the frequency of the 5 MHz crystal oscillator 6. From the same circuit, a signal is extracted that is used to adjust the frequency of the microwave resonator to the top of the spectral line of hydrogen emission.

 The frequency tuning of the microwave cavity is as follows.

 To adjust the frequency of the microwave resonator in a hydrogen generator, a cyclic (with a period of ≈ 20 s) Q-switching of the quality factor of the spectral line of emission of hydrogen atoms is carried out. Q-switching is carried out in two ways: by changing the intensity of the beam of excited hydrogen atoms injected into the microwave cavity or by applying an inhomogeneous magnetic field to the storage flask of the microwave cavity.

 A change in the frequency of generation of the hydrogen generator 1 caused by the Q-factor of the spectral line of hydrogen atoms is detected by the phase-locked loop system (blocks 2, 6, 3, 5) of the crystal oscillator 4, and a periodic signal synchronous with the Q-factor of the line is suppressed in the crystal frequency control circuit. This signal is fed to the AHP unit 7, in which a control voltage is generated to adjust the varicap of the microwave resonator of the hydrogen generator 1. The signal from the output of the phase detector 3 through the key device 8 (locked for the duration of the transition process that appears in the PLL at the time of switching the Q factor of the line) and a selective amplifier filter 9, tuned to the frequency of the Q-switching of the line, is fed to the ADC 10. The digitized resonance detuning signal is fed to a digital synchronous detector 11, in which synchronous detection signal from the accumulation results (500 - 4000 N measurement cycles, depending on the magnitude of the frequency detuning of the resonator). The process of generating the control voltage is illustrated in FIG. 2.

Накопление результатов производится в течение N_max = 500 - 4000 циклов (Рабинер Л. Гоулд Б. Теория и применение цифровой обработки сигналов. М.: Мир, 1978):
E_N+1 = E_N(1 - 1/N_max) + E_цикл.As can be seen from FIG. 2, voltage measurements from the output of the phase detector are made at the end of each half-cycle of Q-switching. Within 4.4 s with an interval of 110 ms, 40 voltage values are determined. To improve accuracy, each of these values is determined by statistical processing of 20 consecutive measurements taken over about 1 ms. Static processing is reduced to calculating a "robust" estimate of the average value of measurements V _{Q max} (V _{Q min} ), which consists in discarding measurements that go beyond the ± 1.5 standard deviation. The resulting values are summarized. The error signal (a value proportional to the detuning of the resonator) is calculated by the formula:

The accumulation of results is carried out during N _max = 500 - 4000 cycles (Rabiner L. Gould B. Theory and application of digital signal processing. M .: Mir, 1978):
E _{N + 1} = E _N (1 - 1 / N _max ) + E _cycle .

After the accumulation time, the code applied to the DAC 12 and, therefore, the resonator tuning frequency are corrected:
DAC = DAC - E / K,
where K is the coefficient determined by the slope of the resonator tuning.

 The flow chart of the operation of block 11 is shown in FIG. 3a, b.

The described process of accumulating and processing the error signal and controlling the resonator frequency allows one to obtain high stability of the frequency of the hydrogen standard (no more than 2 • 10 ^-15 per day) if there is only one hydrogen generator in the structure of the hydrogen standard.

 The circuit design of blocks 1 to 6 is similar to the implementation of the corresponding blocks of the hydrogen frequency standard type Ch1-75.

 The digital elements used in this standard are widely known (Veniaminov V.N. et al. Microcircuits and their application. M: Radio and communications, 1989).

 So in block 8, microchips of type 590KN1 are used, in block 9 - of type 544UD1, in block 10 - of type AD7851, in block 12 - of type AD 7243.

 The implementation of block 11 is also known, for example, from radar technology, which uses the scheme of separation of known signals from noise. It is more efficient to execute block 11 programmatically using a computer, for example, of the Pentium type.

 Industrial applicability.

 The proposed quantum hydrogen frequency standard can be used as a source of highly stable signals for time-frequency measurements and for working in standard, reference and working measuring instruments.

 The standard can be used both independently and as part of automated measuring systems.

 It is possible to remotely control the standard and diagnose its status via Internet communication lines.

 The main areas of application of the standard are: accurate time storage systems, radio navigation, radio astronomy and scientific research.

Claims (2)

 1. A hydrogen frequency standard, comprising a quartz oscillator, a frequency synthesizer and a phase detector connected to a ring, a frequency converter connected to a second input of the phase detector, a hydrogen generator connected to one of its inputs, and a resonator automatic tuning unit connected to the output of the crystal oscillator hydrogen generator, one output of which is connected to the resonator, the other output is with a modulator of the hydrogen generator, and the input is connected to the output of the phase detector, characterized in that a crystal oscillator connected to the frequency converter through the frequency multiplier, and a phase detector connected via a key device for automatic tuning of the resonator unit comprising a series-connected selective filters, analog-to-digital converter, a digital synchronous detector and a digital to analog converter.  2. The hydrogen frequency standard according to claim 1, characterized in that the synchronous detector is made software.

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