A C-field intensity control and servo method for a small cesium clock

Abstract

The present invention discloses a C-field intensity control and servo method for a small cesium clock, comprising the following steps: 1) performing ultra-low bandwidth filtering on the C-field current of the small cesium clock to eliminate stray noise on the C-field current; 2) measuring the frequency difference between the Ramsey transition signals of two energy level transitions that undergo Zeeman frequency shift in cesium atoms, and calculating the C-field intensity based on the frequency difference; 3) calculating the Second-order Zeeman frequency shift of the clock frequency of the small cesium clock based on the C-field intensity; adjusting the output frequency of the small cesium clock based on the Second-order Zeeman frequency shift to compensate for the frequency shift change of the clock frequency of the small cesium clock. The present invention realizes the precise control, measurement, and servo process of the magnetic field intensity. Since the present invention does not require adjusting the C-field current, the low response rate has no effect on the system.

Description

Technical Field

[0001] The invention belongs to the field of microwave frequency standards, and in particular relates to a C field intensity control and servo method for a small cesium clock. Background Art

[0002] Atomic clocks rely on an atomic frequency reference to output an accurate and stable reference frequency. Generally, atomic clocks use a magnetic field to distinguish the energy levels of the atoms' magnetic sub-levels. In small cesium clocks, the magnetic field used to separate the energy levels of the cesium atoms' magnetic sub-levels is called the C field.

[0003] The C field of the small cesium clock is generated by a C field coil wound on the microwave interaction region in the cesium beam tube, such as Figure 1 When the C-field coil is energized, a magnetic field perpendicular to the microwave cavity plane is formed in the microwave interaction region. Generally, the C-field current is tens of mA and the C-field intensity is tens of mGs. Under the action of the C-field, the cesium atoms in the microwave interaction region undergo Zeeman energy level splitting, and the Zeeman frequency (e.g. Figure 2 As shown, characterizing m F = 0 Ramsey and m F = +1 Ramsey frequency (called the Zeeman frequency) is tens of kHz. The magnetic field strength and direction in the microwave interaction region affect the output frequency of the small cesium clock, causing a second-order Zeeman frequency shift. Therefore, when the magnetic field strength or direction changes, the small cesium clock requires precise control and servo control of the C field strength to maintain a constant output frequency.

[0004] During the research on the small cesium clock, the following two points were discovered:

[0005] 1. In an indoor environment without a special magnetic shielding design, the ambient magnetic field is composed of the Earth's magnetic field and the magnetic field of surrounding equipment. The size and direction of these magnetic fields will change. Without precise control and servo, the output frequency performance of the small cesium clock will deteriorate;

[0006] 2. The C field strength servo needs to use the actual cesium atoms as a reference, otherwise the servo cannot be accurate;

[0007] Therefore, the common C field strength servo method for small cesium clocks is to adjust the C field current using the Zeeman frequency as a reference. The problems with this C field strength servo method are:

[0008] 1. The Zeeman frequency includes the first-order and second-order effects of the magnetic field on cesium atoms, and the calculation of the corresponding magnetic field strength is more complicated;

[0009] 2. The relationship between C field current and C field strength is limited by the manual winding error of the coil and is not an ideal linear relationship;

[0010] 3. In order to make the C-field current noise low enough, the C-field current is filtered with ultra-low bandwidth, which reduces the response rate of the C-field current regulation and makes it impossible to servo the C-field current quickly and accurately. Summary of the Invention

[0011] The present invention overcomes the shortcomings of the traditional C-field servo method of a small cesium clock, provides a C-field intensity control and servo method for a small cesium clock, and realizes the precise control, measurement and servo process of the magnetic field intensity.

[0012] The technical solution adopted by the present invention can be summarized as follows:

[0013] 1. Ultra-low bandwidth filtering is performed on the C field current of the small cesium clock (the bandwidth of this system is controlled to 10mHz) to eliminate stray noise on the C field current. The present invention uses an ultra-low bandwidth control circuit to control the C field current Ic of the small cesium clock. The ultra-low bandwidth control circuit includes a field effect transistor, a comparator, a loop filter, and a sampling resistor. One input terminal of the comparator inputs a set voltage, and the other input terminal inputs the voltage on the sampling resistor. The output terminal of the comparator is connected to the gate of the field effect transistor through the loop filter to control the on and off of the field effect transistor. The field effect transistor operates in a linear operating region. When the voltage on the sampling resistor is higher than the set voltage, the output of the comparator controls the field effect transistor to reduce the C field current Ic. When the voltage on the sampling resistor is lower than the set voltage, the output of the comparator controls the field effect transistor to increase the C field current Ic, thereby maintaining the voltage on the sampling resistor equal to the set voltage.

[0014] 2. Measure the frequency difference between the Ramsey transition signals of two energy levels in the cesium atom where the Zeeman frequency shift occurs, and calculate the C field strength felt by the current cesium atom; generally use m F = ±1, m F = ±2 or m F = ±3. Since their Second-order Zeeman frequency shifts are the same, the difference can eliminate the influence of the Second-order Zeeman frequency shift, making the calculation simpler. F =±1 is more convenient, and ±1 generally has better signal quality.

[0015] 3. Using the obtained C field strength, calculate the Second-order Zeeman frequency shift of the small cesium clock and adjust the output frequency of the small cesium clock to compensate for the change in frequency shift;

[0016] 4. Repeat steps 2 and 3 every 2 minutes to maintain the output frequency of the small cesium clock unchanged.

[0017] The technical solution of the present invention is:

[0018] A C-field intensity control and servo method for a small cesium clock is characterized by comprising an ultra-low bandwidth control circuit for the C-field current, a method for measuring and calculating the C-field intensity, and a method for compensating the clock frequency by calculating the Second-order Zeeman frequency shift.

[0019] The present invention includes the ultra-low bandwidth control circuit of the C field current, which is characterized by a circuit based on the voltage sampling negative feedback of the amplifier, that is, a closed-loop feedback circuit using a field effect tube MOSFET, an operational amplifier, a loop filter and a sampling resistor R ( Figure 1 The lower part is an example: operational amplifier U is used as a comparator. One input terminal inputs the set voltage, and the other input terminal inputs the voltage on the sampling resistor. The difference between the two inputs is amplified and output. The output of the comparator passes through a loop filter (to prevent loop oscillation) and is input to the gate of the field-effect transistor to control the on and off of the field-effect transistor. The field-effect transistor operates in the linear operating area. When the voltage on the sampling resistor is higher than the set voltage, the comparator output causes the field-effect transistor to be properly turned off, reducing the current flow rate. When the sampling voltage is lower than the set voltage, the comparator output causes the field-effect transistor to be properly turned on, increasing the current flow rate, and ultimately maintaining the sampling voltage and the set voltage equal).

[0020] Furthermore, an ultra-low bandwidth (10 MHz) filter is connected to the set voltage terminal of the circuit to reduce noise of the set voltage.

[0021] The present invention includes a method for measuring and calculating the C field strength B ( Figure 2 The main physical principle is that the seven groups of π transition lines generated by cesium atoms in the microwave interaction region when stimulated by microwaves will undergo Zeeman frequency shift and Second-order Zeeman frequency shift under the action of a magnetic field. The formulas for calculating the frequency shift are:

[0022] Zeeman frequency shift = 7.0083 × m F B

[0023]

[0024] Furthermore, the magnitude of the Zeeman frequency shift is related to the magnon energy level m F Related, m F The sign of m determines the direction of frequency shift. F The larger the absolute value of m, the larger the Zeeman frequency shift; the Second-order Zeeman frequency shift is only related to m FThe absolute value of m F = +1 and m F = -1 Ramsey transition vertices have a simple linear relationship with the magnetic field strength;

[0025] Furthermore, the microwave frequency source is set to a scanning mode, and the scanning center frequency of the microwave frequency is set at m F = +1 Ramsey central peak spectrum line, and scan the microwave frequency within a certain range (200Hz), m can be measured F = +1 Ramsey central peak spectrum line, the horizontal axis is microwave frequency, the vertical axis is voltage;

[0026] Furthermore, the measurement results are linearly fitted to accurately find m F = +1 Ramsey's central transition peak corresponds to the microwave frequency P +1 ;

[0027] Furthermore, the microwave frequency source is set to the scanning mode, and the scanning center frequency of the microwave frequency is set at m F = -1 Ramsey's central peak spectrum line, and scan the microwave frequency within a certain range to measure m F = -1 Ramsey central peak spectrum; linear fitting is performed on the measurement results to obtain m F = -1 Ramsey center transition peak corresponding to the microwave frequency P -1 ;

[0028] Furthermore, through the microwave frequency P +1 and microwave frequency P -1 The frequency difference between them is calculated in the microcontroller to obtain the C field strength B felt by the current atom, that is,

[0029]

[0030] The present invention includes a method for compensating clock frequency by calculating the Second-order Zeeman frequency shift (see Figure 3 The process shown). F = 0 is a clock transition, which is only affected by the Second-order Zeeman. Its formula is:

[0031] The second-order Zeeman frequency shift of the clock transition is F = 427.44 × B 2

[0032] Furthermore, the Second-order Zeeman frequency shift F of the clock frequency can be calculated based on the above formula and the C field strength B;

[0033] Furthermore, by subtracting the current Second-order Zeeman frequency shift value F from the theoretical value F0 of the microwave frequency of the small cesium clock, the Second-order Zeeman frequency shift of the clock frequency of the small cesium clock can be compensated.

[0034] Furthermore, when the small cesium clock is operating normally, the C field strength is calculated and compensated every certain period of time (2 minutes).

[0035] The present invention has the following advantages:

[0036] (1) The C field current can be filtered using ultra-low bandwidth, and the ripple noise can be effectively suppressed;

[0037] (2) Through m F = +1 and m F = -1 Ramsey spectrum line central peak apex microwave frequency measurement: 1) avoid the influence of the second-order Zeeman frequency shift; 2) under the same conditions, the resolution accuracy of the C field intensity is twice that of measuring the Zeeman frequency;

[0038] (3) Accurately calculate the intensity of the Second-order Zeeman frequency shift in the clock frequency and compensate it directly at the clock frequency, thus avoiding the disadvantage of slow response of the C field current.

[0039] The present invention controls the bandwidth to 10 MHz. Noise is evenly distributed across the entire frequency domain (white noise). Therefore, the smaller the filter bandwidth, the less noise can pass through the filter, and more noise can be removed. The problem with a small bandwidth is a low response rate; in other words, there's a trade-off between noise removal and response rate. Since the present invention doesn't require adjustment of the C-field current, a low response rate has no impact on the system. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] Figure 1 It is a schematic diagram of closed-loop control of C field and C field current.

[0041] Figure 2 This is a schematic diagram of the transition of cesium atoms and clock frequency compensation under the action of C field.

[0042] Figure 3 This is a flow chart of the C-field servo method of a small cesium clock. DETAILED DESCRIPTION

[0043] The present invention will be further described below with reference to the accompanying drawings and through specific implementations. However, the examples in the accompanying drawings do not constitute any limitation to the present invention.

[0044] like Figure 1As shown, a key component of the cesium beam tube is the microwave cavity, approximately 17 cm long. Its outer surface is axially slotted with wires wound around it, forming the C-field current coil. The wires, with 12 turns and a total resistance of no more than 3Ω, extend beyond the cesium beam tube and connect to the C-field control circuit.

[0045] The C field control current circuit consists of a metal-oxide semiconductor field effect tube MOSFET (IRF520 can be selected), an operational amplifier U (OP177 can be selected), a high-precision low-temperature drift resistor R (200Ω can be selected, <5ppm / ℃) and a power supply V cc , set the voltage and filter, loop filter, etc., the C field coil is connected to Figure 1 Position shown.

[0046] Among them, set the voltage value V s The resistance R of the high-precision low-temperature drift resistor determines the current intensity I passing through the C field coil. c for:

[0047]

[0048] Generally, it is about tens of mA. When the loop filter bandwidth is appropriate, the coil current intensity I c The stability depends on the resistance R and the set voltage value V s stability.

[0049] For high-precision, low-temperature drift resistor R, select a resistor with a rating of 5ppm / °C or higher; set the voltage value V s The signal is filtered through an ultra-low bandwidth (10 MHz, 1 MHz = 0.001 Hz) filter to remove noise and maintain high stability.

[0050] When a highly stable C-field current passes through the conductor, a stable magnetic field can be formed in a direction perpendicular to the plane of the microwave cavity, which is generally about tens of mGs (mGs is milligauss, 1mGs=0.001Gs).

[0051] like Figure 2 As shown, it shows the m sampled from the cesium atomic clock F =-1, m F =0 and m F =+1 three adjacent Ramsey signals, the horizontal axis is microwave frequency, and the vertical axis is signal strength.

[0052] Combine Figure 3 As shown, the servo process is controlled by the microcontroller. F = +1 and m F=-1Ramsey's central transition peak, scan the microwave frequency, and use linear fitting to obtain the microwave frequency corresponding to its peak, that is, P +1 and P -1 .

[0053] In the MCU program, according to P +1 and P -1 , the C field strength B and the Second-order Zeeman frequency shift value F are calculated.

[0054] The frequency output unit of the cesium atomic clock is controlled by a single-chip microcomputer, which compensates the frequency output signal according to the Second-order Zeeman frequency shift value F, thereby achieving the effect of servo C field.

[0055] Finally, it should be pointed out that the present invention is not limited to small cesium clocks. Atomic clocks such as rubidium clocks that use magnetic fields to separate magnetic energy levels to extract transition frequencies and require long-term servoing of the magnetic field can all be improved through the present invention.

Claims

1. A method for controlling and servoing the C field intensity of a small cesium clock, comprising the following steps: 1) Perform ultra-low bandwidth filtering on the C field current of the small cesium clock to eliminate stray noise on the C field current; 2) measuring the frequency difference between the Ramsey transition signals of the two energy level transitions that have the Zeeman frequency shift in the cesium atom, and calculating the C field strength according to the frequency difference; wherein the two energy level transitions are the m F = ±1 energy level transition; the method for measuring and calculating the C field strength is: 21) Set the microwave frequency source to scanning mode, and set the scanning center frequency of the microwave frequency to m F = +1 Ramsey's central peak spectrum line, and scan the microwave frequency within a certain range to measure m F = +1 Ramsey's central peak spectrum line; 22) Perform linear fitting on the measurement results of step 21) to find m F = +1 Ramsey's central transition peak corresponds to the microwave frequency P +1 ; 23) Set the microwave frequency source to scanning mode, and set the scanning center frequency of the microwave frequency to m F = -1 Ramsey's central peak spectrum line, and scan the microwave frequency within a certain range to measure m F = -1 Ramsey's central peak spectrum; 24) Perform linear fitting on the measurement results of step 23) to find m F = -1 Ramsey center transition peak corresponding to the microwave frequency P -1 ; 25) According to the formula Calculate and obtain the C field strength B; 3) Calculating the Second-order Zeeman frequency shift of the clock frequency of the small cesium clock according to the C field strength; adjusting the output frequency of the small cesium clock according to the Second-order Zeeman frequency shift to compensate for the frequency shift of the clock frequency of the small cesium clock.

2. The method according to claim 1, characterized in that An ultra-low bandwidth control circuit is used to control the C field current Ic of a small cesium clock. The ultra-low bandwidth control circuit includes a field effect transistor, a comparator, a loop filter, and a sampling resistor. A set voltage is input to one input terminal of the comparator, and the voltage on the sampling resistor is input to the other input terminal. The output terminal of the comparator is connected to the gate of the field effect transistor through the loop filter to control the on and off of the field effect transistor. The field effect transistor operates in a linear operating region. When the voltage on the sampling resistor is higher than the set voltage, the output of the comparator controls the field effect transistor to reduce the C field current Ic. When the voltage on the sampling resistor is lower than the set voltage, the output of the comparator controls the field effect transistor to increase the C field current Ic, thereby maintaining the voltage on the sampling resistor equal to the set voltage.

3. The method according to claim 2, characterized in that An ultra-low bandwidth filter is used to perform ultra-low bandwidth filtering on the C field current of the small cesium clock; and the filtered voltage is input into the comparator as the set voltage.

4. The method according to claim 2 or 3, characterized in that The comparator is an operational amplifier.

5. The method according to claim 1, wherein The Second-order Zeeman frequency shift F = 427.44 × B 2 .

6. The method according to claim 1, wherein The current Second-order Zeeman frequency shift value F is subtracted from the theoretical value F0 of the microwave frequency of the small cesium clock to achieve compensation for the Second-order Zeeman frequency shift of the clock frequency of the small cesium clock.

7. The method according to claim 1, characterized in that Steps 1) to 3) are performed regularly to maintain the clock frequency of the small cesium clock unchanged.

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