CN107483051B

CN107483051B - Precision frequency corrector based on direct digital measurement and processing

Info

Publication number: CN107483051B
Application number: CN201710675629.XA
Authority: CN
Inventors: 白丽娜; 翟鸿启; 刘海东; 谷小倩; 刘蓓玲; 王婷婷; 燕雯; 周渭; 黄李贝; 郭怡萱; 惠月红
Original assignee: Xidian University
Current assignee: Xidian University
Priority date: 2017-08-09
Filing date: 2017-08-09
Publication date: 2020-05-26
Anticipated expiration: 2037-08-09
Also published as: CN107483051A

Abstract

The invention discloses a precise frequency corrector based on direct digital measurement and processing, which is used for solving the problems of aging and self-running of engineering application atomic clocks and precise quartz crystal oscillatorsAdjustment and compensation of frequency accuracy changes. The invention is based on the advanced direct digital measurement principle, namely, in the context of digitization, a two-way ADC measurement structure is adopted, and the technology of restraining the quantization error of an analog-digital converter by means of the clock vernier effect between a sampling clock and an input signal is utilized. Compared with the traditional frequency corrector for adjusting the frequency in a wide range, the invention has the difference that the crystal oscillator is used as a common clock, the set target frequency output is realized through the frequency correction function, the frequency range of the output signal is obviously narrow, such as from 0.1mHz to 10Hz, and the second-level stability of less than 3 x 10 can be realized^‑12And 10 are^‑12A slight frequency correction of magnitude. Compared with the traditional frequency corrector, the corrector is used for continuous correction and has higher cost performance.

Description

Precision frequency corrector based on direct digital measurement and processing

Technical Field

The invention belongs to the technical field of satellite navigation, communication, precise measurement instruments and precise clock sources, and is used for adjusting and compensating aging of engineering application atomic clocks and precise quartz crystal oscillators and self frequency accuracy change.

Background

In order to have a wide frequency adjustment range, a frequency synthesizer often has difficulty in having a fine frequency adjustment step value, and although an atomic clock and a crystal oscillator have excellent performance, aging drift occurs to change a frequency nominal value, so that in order to use the crystal oscillator as a precise frequency source, an instrument or a device is required to finely correct the frequency of an output signal of the atomic clock and the crystal oscillator, thereby maintaining high accuracy of the atomic clock and the crystal oscillator. The prior art has complex structure, high cost and inconvenient use, so a frequency corrector with high precision, wide range and low cost is needed to solve the aging drift problem of atomic clocks and the like.

Disclosure of Invention

The invention discloses a precise frequency corrector based on direct digital measurement and processing, which has the characteristics of high precision, wide range and low cost.

The technical scheme of the invention is as follows: a precise frequency corrector based on direct digital measurement and processing is characterized by comprising a double-path ADC, an FPGA, an MCU, a DAC and a voltage-controlled crystal oscillator VCXO, wherein the MCU is provided with an external USB interface, an external high-stability crystal oscillator is adopted as a common clock signal, and a frequency source input signal f is measured by means of clock vernier effect and quantization error suppression technology₀And the actual output signal f_xThe frequency difference between the two signals is compared with a set frequency difference delta f to realize the set target frequency output, the frequency correction range is from 0.1mHz to 10Hz, and meanwhile, the micro frequency correction with the second-level stability of less than 3 x 10-12 and the 10-12-level can be realized.

The invention adopts a double-path ADC measurement structure, wherein one path is used for collecting a frequency source input signal f₀One path is used for collecting VCXO output signals to form feedback, the two paths of ADCs and the FPGA adopt a unified clock, the MCU obtains frequency counting of reference signals and input signals through the FPGA, so that frequency difference between the reference signals and actual output signals is calculated and compared with set frequency difference, an error signal epsilon is generated to drive the VCXO, the output frequency is kept at a target frequency, and thus a closed loop feedback loop is formed to achieve the purpose of automatic adjustment; the DAC converts the digital signal output by the MCU into an analog signal to drive the crystal oscillator VCXO.

By clock vernier effect is meant the sampling clock signal f_rAnd an input signal f₀Clock vernier effect between, sampling clock signal f in the present invention_rAnd an input signal f₀The method belongs to the relationship that the same frequency has a small frequency difference, and the movement of the sampling clock signal covers the holding and changing states of all quantized values converted by the ADC.

The invention uses quantization error suppression technology in digital measurement, selects a certain fixed zero crossing point as a measurement reference point, greatly suppresses quantization error and ensures high-precision frequency measurement.

Compared with the traditional frequency corrector for adjusting the frequency in a wide range, the frequency corrector has the advantages that the crystal oscillator is used as a common clock, the set target frequency output is realized through the frequency correction function, the frequency range of an output signal is obviously narrow, such as from 0.1mHz to 10Hz, and the second-level stability of less than 3 x 10 can be realized^-12And 10 are^-12A slight frequency correction of magnitude. Compared with the traditional frequency corrector, the corrector is used for continuous correction and has higher cost performance.

Drawings

FIG. 1 is a block diagram of a principle implementation of the present invention.

FIG. 2 is a schematic diagram of a prototype.

Detailed Description

As shown in fig. 1, a precision frequency corrector based on direct digital measurement and processing consists of two parts, namely precision frequency measurement and feedback correction.

The USB interface is further composed of a double-channel ADC, an FPGA, an MCU, a DAC and a voltage-controlled crystal oscillator VCXO, and the MCU is provided with an external USB interface.

The further fine frequency measurement section uses the clock vernier effect. In the digital measurement of the periodic signal, when the clock signal and the measured signal have a small frequency difference Δ f on the basis of the same frequency or a multiple relation, the acquisition point will periodically and sequentially move on the waveform of the measured signal along with the time extension, i.e. the clock vernier effect.

In the invention, the reference clock signal and the control signal to be tested belong to the relationship of same frequency and small deviation, and the movement of the clock signal covers the keeping and changing states of all quantized values converted by the ADC, wherein the keeping and changing states comprise the acquired strict and jump synchronous voltage-time information. Therefore, the clock vernier effect can be utilized, namely, the acquired digital voltage is in a monotonous change relationship, the influence of quantization errors is greatly reduced, and the quantization errors can be eliminated in an ideal state.

The further fine frequency measurement section uses a quantization error suppression technique. Due to quantization errors in the ADC, the sampled data transitions are followed by a "flat region" where the ideal phase coincidence point is "hidden", which is very difficult to capture. The invention selects a certain fixed jump point, namely a zero crossing point, as a gate opening and closing mark. Since the positional relationship of these points to the full coincidence point is fixed, i.e., there is a fixed offset in time, it is understood that the gate moves synchronously forward or backward, but the overall length of the gate does not change. The reference point is used as a measurement reference point, so that the quantization error is greatly inhibited, and the high-precision frequency measurement is ensured.

The further precision frequency measurement part adopts a crystal oscillator with a non-standard frequency as a reference signal. In practice, it is difficult to have a fine step value for frequency adjustment in order to have a wide frequency adjustment range in a conventional frequency synthesizer. Although the crystal oscillator has excellent performance, the aging drift causes the frequency nominal value to change, so in order to use the crystal oscillator as a precise frequency source, an instrument or a device is needed to finely correct the frequency of the output signal of the crystal oscillator, thereby maintaining the high accuracy of the crystal oscillator. The conventional frequency correction uses a standard frequency signal as a reference source, so that the overall performance depends on the performance of the reference source, and the measurement and control principles depend on the standard frequency standard. The working frequency point of the invention is designed aiming at a common precision crystal oscillator, such as 10MHz, but the input frequency standard adopts the crystal oscillator which has deviation with a standard frequency signal as a reference, the standard frequency signal output is realized through an internal measurement and control circuit, and simultaneously, the invention has high regulation precision, so that the output value is stabilized in an error range as small as possible, thereby ensuring the accurate and precise continuous operation of the equipment. Fig. 2 is a schematic diagram of a prototype of the invention.

Further, the feedback correction part adopts a two-way ADC measurement structure, one way is used for collecting a reference signal, the other way is used for collecting a feedback signal output by the VCXO, and the two-way ADC and the FPGA adopt a unified clock. The MCU calculates the deviation between the reference signal and the actual output through the counting value obtained by the FPGA, and compares the deviation with the set deviation to generate an error signal to drive the VCXO so as to keep the output frequency at the expected frequency, thus forming a closed-loop feedback loop and achieving the purpose of automatic adjustment.

Further, the software design of the MCU module is mainly to write the control program of the singlechip by C language. Secondly, the FPGA module is used for obtaining frequency counting of the reference signal and the input signal, strict requirements are required on the clock aspect, and the software design is mainly that Verilog language is used for programming the FPGA module. And finally, converting a voltage signal for driving the voltage-controlled crystal oscillator by the DAC, and converting a digital signal output by the MCU into an analog signal to drive the crystal oscillator.

In the invention, the frequency difference between the crystal oscillator input reference signal with the deviation and the actual output signal is obtained through measurement, and the final output signal is not only narrow in range but also high in second-level stability through the comparison of the frequency difference and the set frequency difference.

Further, a standard frequency output is realized by the frequency correction function, and the frequency range of the output signal is obviously narrow, such as 0.1mHz to 10 Hz. Meanwhile, the output signal is accurate standard frequency (the set frequency difference is zero) or the standard frequency has small deviation (the set frequency difference is not zero but far less than the standard frequency), and the second-level stability of less than 3 x 10 can be realized^-12And 10 are^-12A slight frequency correction of magnitude.

Claims

1. A precise frequency corrector based on direct digital measurement and processing is characterized by comprising a double-path ADC, an FPGA, an MCU, a DAC and a voltage-controlled crystal oscillator VCXO, wherein the MCU is provided with an external USB interface, and an external high-stability crystal oscillator is adopted as a common high-stability crystal oscillatorClock signal, by measuring frequency source input signal f by means of clock vernier effect and quantization error suppression technique₀And the actual output signal f_xThe frequency difference between the two signals is compared with a set frequency difference delta f to realize the set target frequency output, the frequency correction range is from 0.1mHz to 10Hz, and the micro frequency correction with the second-level stability of less than 3 x 10-12 and the 10-12-level can be realized;

the two-way ADC is used for acquiring a frequency source input signal f₀One path is used for collecting VCXO output signals to form feedback, the two paths of ADCs and the FPGA adopt a unified clock, the MCU obtains frequency counting of reference signals and input signals through the FPGA, so that frequency difference between the reference signals and actual output signals is calculated and compared with set frequency difference, an error signal epsilon is generated to drive the VCXO, the output frequency is kept at a target frequency, and thus a closed loop feedback loop is formed to achieve the purpose of automatic adjustment; the DAC converts the digital signal output by the MCU into an analog signal to drive the crystal oscillator VCXO.

2. Precision frequency corrector based on direct digital measurement and processing as claimed in claim 1, characterized in that the clock vernier effect is used to sample the clock signal f_rAnd an input signal f₀Clock vernier effect in between, sampling clock signal f_rAnd an input signal f₀The method belongs to the relationship that the same frequency has a small frequency difference, and the movement of the sampling clock signal covers the holding and changing states of all quantized values converted by the ADC.

3. A precision frequency corrector based on direct digital measurement and processing as claimed in claim 1, characterized in that quantization error suppression techniques are used in the digital measurement, a fixed zero crossing is selected as a reference point for measurement, which substantially suppresses quantization errors and ensures high precision frequency measurement.