Design of Multi-function Counter Based on Single Chip Microcomputer and FPGA

1 Introduction

Frequency, period and phase are the three major elements of AC signals. In general, the analysis of AC signals requires the study of their frequency and phase, while the period can be directly calculated from the frequency. The requirements for the accuracy of frequency and phase measurement of sinusoidal signals are constantly increasing, and with the development of electronic technology, the measurement methods are still being improved and perfected. The frequency measurement method of direct frequency measurement was adopted earlier. In order to ensure the test accuracy, the period measurement method was generally used for low-frequency signals, while the frequency measurement method was used for high-frequency signals, which was very inconvenient to measure. The phase measurement initially used the method of measuring a cycle parameter of the signal. The accuracy of this method is suitable for low frequencies, while the error becomes larger at high frequencies. The multifunctional counter uses an equal-precision measurement method to measure the signal frequency, and uses a counting phase measurement method based on a single-chip microcomputer and FPGA to complete the precise phase measurement, and can display the frequency, period and phase difference of the current signal in real time on the LCD display. The counter integrates the measurement of the frequency and phase of the sinusoidal signal, with high accuracy and strong practicality.

2 Design demonstration

2.1 Frequency Measurement

The measurement time of the equal-precision measurement method is set manually, and the opening and closing of the gate is controlled by the rising edge of the measured signal. The measurement accuracy is independent of the measured signal frequency, so the measurement accuracy can be guaranteed to remain unchanged in the entire measurement frequency band. The count of the measured signal is synchronized, and there is an error of ±1 for the reference signal. As long as the count is large enough, it can meet the high-precision requirements.

2.2 Phase Measurement

The phase difference-time measurement method is to send the two shaped square waves into the FPGA, detect the rising edges of the two signals respectively, and count the crystal oscillator between the rising edges of the two signals through the internal counter of the FPGA. In the low frequency band, the output of the RC filter circuit fluctuates greatly. This phase measurement uses high-frequency counting pulses, and the phase is less affected by the signal frequency, which can achieve higher measurement accuracy.

3 System Hardware Circuit Design

The hardware circuit design of this system is composed of peak detection sampling, shaping comparison, broadband channel amplification, frequency measurement, phase measurement, display and other modules. The low-frequency comparator LM311 has a good shaping effect on signals from 1 Hz to 2 MHz, and the high-frequency comparator TL3116 has a good shaping effect on signals above 200 kHz. In order to achieve the frequency measurement of 1 Hz to 10 MHz signals, the system uses 1 as 0.01~5 V signal, which should be selected after peak detection and A/D conversion, and then the analog switch channel is selected for program-controlled amplification, and the measurement is performed after shaping, and finally the measurement result is sent to the display module. Figure 1 is the overall block diagram of the system.

3.1 Programmable amplifier circuit

The program-controlled amplification is divided into three stages. The small signal of 0.01-50 mV is amplified 100 times, the small signal of 50 mV-1 V is amplified 10 times, and the signal of 1-5 V is not amplified. The 8-way analog switch MAX308 is selected. In order to collect and realize the millivolt level signal, a broadband amplifier circuit must be used for amplification. Therefore, the OPA637 broadband operational amplifier of TI is used to realize the amplification of Gain="11" and Gain"=120. Figure 2 shows the amplifier circuit of OPA637 with a gain of 11 times. The amplifier circuit with a gain of 120 can be realized by cascading 2 stages of OPA637.

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3.2 Zero-Crossing Comparison Circuit

The input signal is sent to LM311 for hysteresis comparison, which can effectively eliminate edge glitches and realize low-frequency signal shaping. TL3116 is a high-frequency comparator. The input signal is sent to TL3116 for hysteresis comparison to obtain a more ideal high-frequency square wave shaping signal. Therefore, the shaping circuit is designed in two stages when measuring the frequency. The shaping circuit shapes the input periodic signal into a square wave of the same frequency and inputs it into FPGA for frequency measurement. Figure 3 shows the LM311 hysteresis comparison circuit, and the TL3116 external circuit is the same as it.

4 System Software Design

The system software design is divided into two parts: frequency measurement and phase measurement. When measuring frequency, the signal is peak detected, A/D sampled and sent to FPGA, and the analog switch is selected to program-controlled amplification of signals of different amplitude segments. The amplified signals are shaped by two comparators respectively, and then sent to FPGA for counting respectively. When high frequency, the count value shaped by the high frequency comparator is used, and when low frequency, the count value shaped by the low frequency comparator is used to accurately measure the signal frequency. For phase measurement, the signal shaped by the low frequency comparator is directly sent to FPGA for counting. The program flow is shown in Figure 4.

5 Conclusion

The multifunctional counter for measuring the frequency, period and phase difference of sinusoidal signals realizes accurate frequency measurement of sinusoidal signals with a frequency of 1Hz to 10 MHz and an amplitude of 0.01 to 5 Vrms. Its accuracy reaches 10-6Hz. At the same time, the counter design also realizes accurate phase measurement of sinusoidal signals with a frequency of 10 Hz to 100 kHz and an amplitude of 0.5 to 5 Vrms, with an accuracy of 1°, and can display the frequency, period and phase difference of the current signal in real time on the LCD display. The system is simple to operate, highly modularized, highly accurate, and has a friendly display interface. It has strong feasibility and practicality and has good market prospects.