JPH0318775B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a hydrogen maser frequency standard, and more particularly to an automatic tuning system for a hydrogen maser that automatically adjusts the resonant frequency to match the hydrogen maser oscillation frequency with the resonant frequency of hydrogen atoms. It is something.

Although the frequency accuracy of hydrogen maser frequency standards is inferior to that of cesium (Cs) atomic beam frequency standards,
Its frequency stability is the best among currently used frequency standards. Therefore, it has become an essential device as a highly stable frequency signal source for VLBI (Very Long Baseline Interferometer) and tracking of deep space satellites.

In the cutting-edge technology field mentioned above, the higher the frequency stability of the hydrogen maser frequency standard (hereinafter simply referred to as hydrogen maser), the higher the measurement accuracy.
There is a strong demand for improved frequency stability.

Furthermore, the accuracy of the output frequency of the hydrogen maser standard is also important, and therefore the hydrogen maser resonance frequency must be adjusted.

Below, an overview of such a hydrogen maser, an automatic tuning system, and frequency stability issues will be explained.

Figure 1 schematically shows the main body of the hydrogen maser, where 1 is a discharge tube to which hydrogen molecules are supplied, 2 is a high-frequency oscillator for discharging to dissociate the hydrogen molecules in the discharge tube to form hydrogen atoms, 3 is a level selection magnet for selecting atoms with a high energy level from dissociated hydrogen atoms, 4 is a beam shutter for adjusting the hydrogen beam flow rate, and 5 is for accumulating the injected hydrogen atoms. A hydrogen storage sphere whose inner surface is coated with Teflon, 6 a cavity resonator (system), 7 a resonator cylinder, 8 a resonator support constant temperature bath equipped with a temperature control heater 8a on the outer periphery, and 9 a static magnetic field 10 is a magnetic shield for shielding the influence of an external magnetic field, and 11 is a vacuum bell gear. Note that 12 is a post for finely adjusting the resonance frequency, 13 is a loop for taking out the oscillation output within the cavity resonator 6, and 14 is an ion pump.

In the hydrogen beam system of the hydrogen maser with such a structure, the level selection magnet 3 selects the energy levels of hydrogen atoms such as F=0, m _F =0, and F=1.
The hydrogen atom at m _F = -1 diverges, F = 1, m _F
=0, the atoms at m _F =+1 are focused in the hydrogen storage sphere 5.

The hydrogen atoms injected into the hydrogen storage sphere 5 repeatedly collide with the Teflon membrane wall on the inner surface of the hydrogen storage sphere 5, and are
It remains in the hydrogen storage sphere 5 for nearly a second, and the cavity resonator 6
Receives electromagnetic wave excitation within.

The energy level used as the standard frequency is the frequency that transitions from the F = 1, m _F = 0 state to F = 0, m _F = 0, and is approximately 1420405752 Hz (standard frequency f ₀ ).
It is.

Now, if the resonant frequency fc of the cavity resonator 6 is adjusted to the vicinity of this standard frequency f ₀ , the atoms at F = 1 and m _F = 0 in the hydrogen storage sphere 5 will be in the cavity resonator 6. It is excited by an electromagnetic wave close to f ₀ and transitions to the lower energy level F = 0, m _F = 0.
The hydrogen maser causes self-oscillation at the maser oscillation frequency fm due to the electromagnetic waves it radiates at this time. The output of this maser oscillation frequency fm is taken out from the loop 13.

By the way, if the resonant frequency fc of the cavity resonator 6 and the standard frequency _f0 of hydrogen atoms are even slightly different,
The maser oscillation frequency fm is shifted from the standard frequency _f0 .

It is known that the frequency relationship of this shift is fm−f ₀ ={Qc/Ql}(fc−f ₀ ) (1).

Here, Qc is the Q value of the cavity resonator, and Ql is the Q value of the resonance line (assuming the resonance spectrum frequency band is Δf, it is expressed as Ql=fm/Δf).

Therefore, as (fc−f ₀ ) increases, (fm−
f ₀ ) i.e. maser oscillation frequency fm and standard frequency f ₀
The deviation of
Changes in Ql due to changes in the number of collisions between hydrogen atoms)
This also impairs frequency stability.

For example, when Qc/Ql=5×10 ^-5 , in order to adjust fm−f ₀ /f ₀ to about 1×10 ⁻¹² , fc−f ₀ should be set to about 30
Hz or less, and the resonant frequency fc must be set extremely strictly.

Therefore, measures are taken to prevent fluctuations in the resonant frequency fc by using crystal glass, etc., which has a small temperature change in the resonant frequency Fc, and by using the resonator cylinder 7 under highly accurate temperature control. However, an automatic tuning device is initially required to match the resonant frequency fc of the cavity resonator 6 with the standard frequency _f0 of the maser.

FIG. 2 shows an outline of an automatic tuning device for driving the resonant frequency of the cavity resonator 6 within such a strict frequency range, and 20 indicates the quantum system of the hydrogen maser shown in FIG. 21 is a varactor diode that finely adjusts the resonant frequency fc of the cavity resonator 6.

Further, 22 opens and closes the beam shutter 4;
A drive circuit for increasing and decreasing the amount of atomic beam injected into the hydrogen storage sphere 5 is shown.

23 surrounded by a dashed line is the maser oscillation frequency fm
PLL to extract oscillation output phase-synchronized with
(Phase Looked Loop) circuit, which includes a crystal oscillator 24 whose oscillation frequency is controlled by a DC voltage, a frequency multiplier 25, a frequency converter (Mixer) 26,
It is formed from an intermediate frequency amplifier 27, a phase detector 28, a frequency synthesizer 29, a DC amplifier 30, and the like.

Further, 31 surrounded by a dashed line indicates an automatic tuning control system, and this automatic tuning control system 31 includes a reference oscillator 32 that supplies a reference signal, a frequency mixer 3
3. Beat period measuring device 34, sequence control section 3
5. The drive circuit 22 and the control signal generating section 36 supplying control signals to the varactor diode 21.

Note that as the reference signal source 32, another maser frequency standard, a rubidium or cesium atomic frequency standard, or an ultra-highly stable crystal oscillator is used.

Next, an overview of the operation in this figure will be explained.

The maser oscillation frequency fm in the quantum system 20 is
When oscillating at 1420405752Hz, PLL circuit 2
3, the oscillation frequency of the crystal oscillator 24 is set to
If it is 10MHz, the frequency multiplier 25 will have 140
From the frequency converter 26 by multiplying the
A difference signal of fm−1400MHz=20405.752KHz can be extracted. Therefore, the frequency synthesizer 29 generates 20405.752K based on the oscillation output of the crystal oscillator 24.
By forming a Hz signal and controlling the output of the phase detector 28 to be 0, the oscillation frequency (10 MHz) of the crystal oscillator 26 will be phase-locked to the maser oscillation frequency fm. (Actual phase detection is
(done at 405.752KHz or 5.752KHz).

On the other hand, the automatic tuning control system 31 controls the maser oscillation frequency fm to be the standard frequency f ₀ , that is, in the equation (1), f ₀ -fc=0.

The principle of this control method is based on the pressure quenching method.

In other words, if f ₀ = fc in the above equation (1), fm = f ₀ holds true even when Qc/Ql is changed. It is known that Qc (Q value of resonance line) changes by increasing the amount of hydrogen beam and changing the relaxation rate due to collisions between hydrogen atoms.
When switching between Hi (high) and L ₀ (low), the resonant frequency fc is adjusted by the varactor diode 21 so that the maser oscillation frequency fm does not change, so that fm=f ₀ .

In FIG. 2, the output frequency of a frequency converter 26 or intermediate frequency amplifier 27 and the output frequency of a reference oscillator 32 consisting of a crystal oscillator or other hydrogen maser frequency standard are input to a frequency mixer 33, and the difference between them is input to a frequency mixer 33. The signal frequency is counted by a beat period meter 34.

The beat period measuring device 34 is composed of, for example, an addition/subtraction (up/down) counter, and performs addition and subtraction in synchronization with the Hi/L ₀ switching signal of the beam shutter 4 output from the sequence controller 35.

Therefore, the amount of hydrogen beam is now increasing (Hi)
Maser oscillation frequency fm _H and reference oscillator 3 when
The difference fm _H −fr between the two output frequencies fr is added by the beat period measuring device 34, and then the amount of hydrogen beam decreases (L ₀ ).
Maser oscillation frequency fm _L and reference oscillator 3
By subtracting the difference in fr of the two output frequencies fm _L - fr from the value counted by the beat period measuring device 34, we get fm _H
−fm _L = △fm is the difference signal (period count difference △fm′)
Detected as . Therefore, if the capacitance value of the varactor diode 21 is varied by a control signal corresponding to this difference signal Δfm, and the resonant frequency fc of the cavity resonator 6 is controlled to approach the standard frequency _f0 , the hydrogen beam Gradually, with the quantity Hi, L ₀ as one cycle,
The maser oscillation frequency fm converges to the standard frequency _f0 .
In other words, by bringing the resonant frequency fc of the cavity resonator 6 closer to the standard frequency _f0 , the maser oscillation frequency
fm can be made to approximately match the standard frequency _f0 .

Note that the hydrogen beam amount was switched between Hi and L ₀ by opening and closing the beam shutter 4, but for example, by applying a static magnetic field to the hydrogen discharge tube 1 and changing the strength of this static magnetic field, the discharge intensity can be changed. The amount of hydrogen atom beam may be increased or decreased accordingly.

However, such an automatic tuning device has the following problems.

(1) With the automatic tuning system beat period measuring device 34, the period count number for Hi or L ₀ of the hydrogen beam amount is approximately 10 digits, but the period count difference △fm' between Hi and L ₀ is extremely small. The convergence time constant of the resonator control process is small (about 5 digits, for example), and requires several hours even when using a hydrogen maser frequency standard as the reference oscillator, and more than a day when using an ultra-high stability crystal oscillator. It is.
Therefore, if the control sequence is even slightly distorted due to flicker noise etc. in the automatic tuning process, unexpected large Hi, L ₀
There is a problem in that a cycle count difference of is output, the resonant frequency fc jumps significantly, the maser oscillation frequency fm is greatly disturbed, and it takes a long time for the resonant frequency fc to return to its normal state.

This invention was made in order to eliminate such drawbacks in automatic tuning devices, and compares the difference signal detected from the beat period measuring device with a predetermined error discrimination setting value to determine whether the difference signal is appropriate. The present invention provides an automatic maser tuning system in which tuning control of a cavity resonator is smoothly performed by providing a function to perform this processing and a control circuit that performs the processing.

An embodiment of the automatic maser tuning system of the present invention will be described below with reference to FIG.

In this figure, block diagrams having the same functions as those in FIG. 2 are designated by the same reference numerals. 35A represents a control circuit composed of a microprocessor, etc., 35B represents a memory section, and 35C represents an alarm device. An error setting value Δe (count number Δe') is inputted to the control circuit 35A from the outside using a keyboard or the like.

As described above, the automatic tuning device of the present invention is provided with a control circuit 35A having a judgment function and a memory section 35B in place of the mechanical sequence control, so that according to the program from the control circuit 35A,
The hydrogen beam amount is alternately switched between Hi and L ₀ , and the difference signal △fm of the maser oscillation frequency fm at that time is detected by a beat period measuring device (beat count number △
fm′), then the difference signal △fm(number of counts △
fm') and the error setting value Δe (count number Δe'). and the difference signal △fm (number of counts △fm′)
If it is smaller than the error setting value △e (number of counts △e'), it is stored in the memory section 35B, and the capacitance of the varactor diode 21 is changed by a control signal corresponding to the difference signal △fm (number of counts △fm'). Make the value variable.

However, if the difference signal Δfm output from the beat period measuring device 34 is larger than the error setting value Δe, this measured value is discarded and the latest difference signal Δf(M) (count The number △fm'(M)) is output.

Therefore, even when a difference signal △fm (count number △fm′) with an abnormal value is output due to the influence of flicker noise, this value will adjust the resonant frequency fc.
It is possible to solve the problem that the signal jumps to a point outside the tuning process and greatly disturbs the convergence state.

It goes without saying that the control circuit 35A, memory section 35B, beat period measuring device 34, control signal generating section 36, etc. can be formed as one microcomputer.

FIG. 4 shows a flowchart of the automatic tuning system of the present invention. In this flowchart, if the difference signal △fm (number of counts △fm') larger than the error setting value △e (number of counts △e') continues _N0 times or more, the alarm device 35C is activated and an abnormality occurs. A program that displays the occurrence of is also included.

Therefore, when the alarm device is activated during the tuning process, it is considered that some kind of failure or abnormal condition has occurred, and the device should be re-inspected at this point.

As explained above, in the hydrogen maser automatic tuning system of the present invention, since the control circuit of the automatic tuning device is programmed with the judgment function and its processing function for difference signal data, the resonance frequency that is likely to occur in the automatic tuning process is This has the great effect of being able to prevent abnormal jump phenomena and shortening the convergence time of the tuning process.

[Brief explanation of the drawing]

Figure 1 is a schematic diagram to explain the basic principle of a hydrogen maser, Figure 2 is a block diagram to explain the PLL circuit locked at the maser oscillation frequency and the automatic tuning device circuit, and Figure 3. 4 is a block diagram of an automatic tuning device showing an embodiment of the present invention, and FIG. 4 shows a difference signal (△
This is a flowchart showing the judgment and processing of fm). In the figure, 1 is a discharge tube, 2 is a high-frequency oscillator, 3 is a level selection magnet, 4 is a beam shutter, 5 is a hydrogen storage bulb, 6 is a cavity resonator (system), 21 is a varactor diode, and 32 is a reference oscillator. , 33 is a frequency mixer, 34 is a beat period measuring device, 35A is a control circuit, 35B is a memory section, 35C is an alarm device, and 36 is a control signal generator.

Claims (1)

[Scope of Claims] 1. means for alternating the flow rate of effective hydrogen atoms focused on the hydrogen storage sphere of the hydrogen maser over two levels at predetermined time intervals; a hydrogen maser resonator system including the hydrogen storage sphere that oscillates at maser oscillation frequencies (fm _H , fm _L ); and a detection means for detecting a difference signal (Δfm) between the two maser oscillation frequencies (fm _H , fm _L ). and; storage means for storing the difference signal (Δfm) obtained by the detection means; and comparing the difference signal (Δfm) stored in the storage means with a preset error setting value (Δe). , when Δfm<Δe, the value of the difference signal (Δfm) stored in the storage means is selected as is, and when Δfm>Δe, the value of the difference signal (Δfm) stored in the storage means immediately before is selected. determining means for selecting and outputting a value of a difference signal; controlling the difference signal (Δfm) of the maser oscillation frequency of the hydrogen maser resonator system to approach 0 as time passes by the output signal of the determining means; and a signal output system that receives a signal from the hydrogen maser resonator system and outputs a standard frequency. 2. A patent claim characterized in that the means for changing between the two levels controls the flow rate of hydrogen atoms by means of changing the strength of a hydrogen discharge tube and a static magnetic field applied to the hydrogen discharge tube. An automatic tuning system for a hydrogen maser according to item 1. 3. The determining means includes a counting circuit, and outputs an alarm signal when the counting circuit reaches a value obtained by counting the difference signal (Δfm)>error set value (Δe) N times in a row. An automatic tuning system for a hydrogen maser according to claim 1 or 2.