Design of a wide range and high stability crystal oscillator frequency stability test system

0 Introduction
High-stability quartz crystal resonator (referred to as high-stability crystal oscillator) is an important device widely used in electronic information products such as communications, electronic countermeasures, digital radio stations, computers, etc. The indicators of high-stability crystal oscillator directly affect the reliability of the product, so how to detect its performance is very important.
The representative measuring instrument is the frequency stability test system (error multiplier + multi-way switch). Its principle is to multiply the frequency fluctuation △f of the measured frequency source, and then use the frequency counter to measure the frequency to calculate the accuracy, aging rate, daily fluctuation and other indicators. In the design of the frequency stability test system, the signal source is an important component. Its function is to generate a high-performance output frequency (1~100 MHz) settable clock signal, mix it with the signal of the measured crystal oscillator, and the output difference is within the range of the frequency multiplication loop. At present, it is widely used in the direct digital frequency synthesis (DDS) technology of signal source design, with the advantages of wide output frequency range, high resolution, low phase noise, and easy implementation; secondly, for high-resolution frequency counters, under the same conditions, the frequency resolution is one bit higher, and the multiplication loop can be reduced by one. After comparison and analysis, we chose DDS technology as the core, and used AVR microcontroller and CPLD as the control system, which can control, calculate and process in a timely manner, and connect with the upper microcomputer to complete multi-channel detection, analysis, drawing of state diagrams and other tasks.

1 Basic principle of frequency measurement by frequency error multiplication method
Let Fr be the frequency of the standard clock (rubidium atomic clock), and the measured frequency Fx = Fr + △f. Through the first stage multiplication, we get Fr + m△f, through the second stage multiplication, we get Fr + m2△f..., through n-stage multiplication, we have:

generally m = 10, n = 1, 2, 3, 4, due to the influence of the background noise of the frequency multiplier, the highest n = 5 levels. The maximum frequency error multiplication is 10-5.
From formula (1), it can be seen that the frequency stability test system only measures the measured signal close to the output frequency of the standard clock. Therefore, it is necessary to develop a new generation of high-performance test systems to detect the quality of high-stability crystal oscillator production. [page]
2 Design of test system
2.1 Signal source design
DDS is a new frequency synthesis technology developed after direct frequency synthesis and indirect frequency synthesis. It is driven by the system clock pulse to read the pre-stored waveform data, and the required signal is obtained after the D/A digital-to-analog converter and low-pass filtering (ninth-order elliptical filter) output. The output of different frequencies is achieved by changing the content of the frequency control word. DDS technology has high frequency and phase resolution, a wide output frequency range, continuous phase of the output signal, and low phase noise.

ADI's AD9852 can be selected as the core part of the standard clock, and the rubidium atomic clock provides its working clock. It is controlled by AVR (ATmega128) microcontroller and MAXⅡ (EPT240) programmable device, realizing a signal source with adjustable frequency and phase and an output frequency range of 1 to 100 MHz. Its block diagram is shown in Figure 1.

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3 Design of frequency stability test system
3.1 Direct measurement with high-resolution frequency counter
It is well known that averaging the measured values ​​in repetitive time interval (frequency) measurements is a simple way to improve the resolution. The idea is simple: repeatedly measure the same interval and assume that there is a certain degree of independence between the measurements. Then, the interval can be estimated using the averaged measured value, which can statistically reduce the quantization error of ±1 word in each measurement. When measuring time interval (frequency) in this way, it should be noted that:
(1) Non-average caused by coherence: When the time interval repetition rate is coherent with the clock pulse frequency, or the time interval rate is a multiple of the clock pulse, then the phase of each measured time interval relative to the clock pulse is the same. Therefore, all measurements will read exactly the same value, and no statistical averaging will be produced. In this case, the quantization error of one million measurements is no different from that of a single measurement.
(2) Random phase modulation of the time base: The coherence between the time interval pulse sequence and the time base clock pulse sequence can be destroyed by introducing random phase modulation, so that meaningful time interval average measurement can be made regardless of the time interval rate. The phase modulated signal is derived from the noise generated by the Zener diode, as shown in Figure 3.

First, measure the frequency of the measured signal, or input the frequency of the measured signal, and give the control word Fr=Fx-△f according to the preset calculation formula, and send it to AD9852. After follower amplification, filtering by the ninth-order elliptical filter, and 50 Ω matching coaxial line output, the point frequency is given to the frequency multiplier, providing a standard signal for the error multiplication unit to complete the measurement of the frequency stability system. The calculation of frequency stability has the Allan variance formula:

Where: fi1, fi2 are two frequency values ​​measured for a group respectively; m is the number of sampling groups, which is usually 100.

4 Conclusion
Frequency stability is an important indicator to measure the performance of high-stability crystal oscillators. With the development of communication technology, the requirements for its signal quality have also increased. Since the high-stability clock frequency required by the communication carrier is non-standard, its clock signal needs to be measured and analyzed. Using the new generation of frequency stability test system "Wide Range High-stability Crystal Oscillator Frequency Stability Test System", you can quickly understand the performance of high-stability crystal oscillators and improve the quality of communication products.