Based on the frequency of adaptive frequency measurement method and its implementation

In electronic measurement, frequency is one of the most basic measurements. More widely used now is equal precision frequency measurement method (multi-cycle synchronous measurement method) This method has high accuracy, measurement accuracy changes with the change in the measured signal characteristics. But this method requires large hardware overhead, and synchronization circuit complexity, could easily lead to false triggering, reliability is not high. In this paper, such as lack of precision frequency measurement method, adaptive method of sub-frequency signal is measured continuously frequency, resulting in reliable gate control signal, simplifying the circuit structure; the same time according to actual needs in the design of the addition of frequency signal pre-processing circuitry, combined with PC104 bus and FPGA technologies, to achieve a wide range of frequency signals, high speed, high precision measurements. A model of the system as part of measurement and calibration equipment has been well applied in practical use.

Based on the frequency of adaptive frequency measurement method and its implementation

1 Equal Precision Frequency Principles and improvements

Equal Precision Frequency Measurement frequency is directly developed from the basis. Equal Precision Frequency is most characteristic of the entire frequency range to achieve the same accuracy, and with the frequency of signal measured size. The basic principles shown in Figure 1.

In the measurement process, the pre-gate signal rising edge of the measurement process starts, but this time on the measured signal count on the reference clock counter 1 and counter 2 counts did not start counting. Pre-gate signal becomes high, the measured signal makes the first rising edge of the actual gate signal into a high effective signal, the counter counts 1 and 2 start. When the preset gate signal into a low-level signals, the counters 1 and 2 and did not immediately stop the count, but to wait until immediately before the rising edge of the subsequent arrival of the measured signal, the actual gate signal goes low invalid signal when to stop counting. If the recorded value of counter 1 counts N1, counter 2 count value N2, base frequency f0, the measured frequency fx, there are fx = f0 · N2/N1.

Equal Precision Frequency methods in addition to the two counters are the measured signal and the reference clock to count, but also need to attach an additional counter to generate preset gate control signal, and because of preset gate control signal, an increase of synchronization circuit complexity. When the measured signal frequency is high, the measured signal's rising edge and falling edge of pre-gate signal may be risk of competition issues, resulting in false triggering, affecting the measurement accuracy, lower reliability of the system.

To solve this problem, improve the future precision frequency measurement principles such as shown in Figure 2.

The work process is divided into two steps precision measurement and precision measurements. Precision measurements, it will be pre-tested sub-frequency signal is set to 2, were divided on their frequency, frequency counter starts after the rising edge of signal 1 on the baseline frequency count, then immediately start the falling edge of the counter 1 on the basis frequency count, then followed by the falling edge to count to stop, according to count the size of the estimated frequency of the measured signal. Precision measurements, in accordance with previous estimates of the frequency and pre-set measuring time, adjust the frequency of pre-measured signal number (such as pre-set measurement time of 1s, estimates of the measured signal frequency 6000Hz, then the adjusted pre-sub- frequency of 1 / (1 / 6000) = 6000), and then repeat on the baseline frequency of the counting process, the completion of the frequency measurements.

Based on the frequency of adaptive frequency measurement method and its implementation

This improvement not only to achieve equal-precision measurement Di basic idea - the measured signal measurement time is an integer cycle, but also the full scan signals of different frequencies, automatically adjusted by the signal prescaler Shu, Zhijieliyong frequency Hou's signal as the control signal. This will count will be measured and sub-frequency signal into one, thus simplifying the circuit structure, reducing the hardware overhead, to avoid false triggering, to improve the reliability of the measurement system to achieve a wide range, such as precision measurement requirements.

2 Error Analysis

Setting the measured signal frequency fx, reference signal frequency f0, of the reference signal frequency count value of N0, precision measuring frequency signal when the measured number of Nx, the other based on the improved accuracy of frequency measurement principle, the measured signal frequency fx = 2 · f0 · Nx/N0.

The relative measurement uncertainty:

Based on the frequency of adaptive frequency measurement method and its implementation

Prescaler number Nx ± 1 error does not exist, so radical in the second frequency is 0; in practice, high stability crystal oscillator, the relative uncertainty of up to 1 × 10 -7; and as continuous on the tested frequency to frequency, so that the gate long enough (eg, 1s), this pulse was filled up to 10 7, N0 the maximum error is ± 1, so the relative uncertainty of N0 can reach 1 × 10 -7. Together, the frequency of the relative uncertainty of measurement results up to 10-7 of magnitude, to achieve high-precision measurements. Meanwhile Duiyu more low frequency signal, N0 the larger the, the relative degree of uncertainty will be smaller, Dan Youyu crystal's relative uncertainties, it is still Zhinengdadao 10-7 Shu Liang Ji, To further Tigaoceliang accuracy , you must use a more precise crystal.

3 Hardware Design

Frequency measurement system, the hardware design shown in Figure 3.

Based on the frequency of adaptive frequency measurement method and its implementation

As a practical frequency and frequency calibration coefficient, measuring the frequency of the signal not only the standard TTL level signal, there are sine signal and the need to isolate the frequency signal, so the frequency measurement coefficients for different types of frequency signal processing (optical isolation , hysteresis comparison, the signal integer, etc.), this will make the frequency measurement system can be widely used.

PC104 embedded processors with small size, high integration, high reliability, low power consumption, easy to carry, etc., can be used as frequency measurement system, controller, complete control and operation frequency measurement task.

Field programmable gate array FPGA as high integration, high speed and high-stability characteristics, which are widely applied to digital logic circuit design. The core of the system frequency measurement is a FPGA chip - Altera Corporation EPF10K10, all the logic modules are realized in the FPGA.

Frequency measurement works as follows: choose a selection from the four-way switch to select the measurement signal as the current channel; measured signal into the frequency module, according to the number of preset frequency to frequency; frequency of signal generated by gate signal generator module gate control signal, when the gate control signal is high effective signal, the reference clock counter module counts were measured; measurement stopped, resulting in a corresponding stop flag and the overflow flag for software to query; when the software queries the measurement stop when reading the counter value, calculated by the frequency value.

4 Software Design

According to the process of measuring frequency can write the corresponding software. Level flow chart of frequency shown in Figure 4.

This design and other precision frequency measurement system, from the principle on the frequency measurement accuracy of conventional methods such as a number of improvements, adaptive frequency method, simplifying the circuit structure and improve system reliability; measurement accuracy up to 10-7 magnitude, measuring range from 0.1Hz to 50MHz, achieve high accuracy, wide measurement range. But in the actual design, by Shi Yong PC104 bus and FPGA chip technologies, Shi Zheng Ge system 具有 small size, low power consumption Bianyu carry the characteristics of application Keyifangpian De Dao field conditions and calibration of the frequency Celiang.