1. Introduction
Frequency measurement is one of the most basic and important method in electronic measurements. Frequency signals are strong anti-interference, easy to transmit, and can be measured with higher precision. Therefore, the study of frequency measurement method is important in the practical engineering applications. The commonly used frequency measurement methods are direct frequency measurement, direct period measurement, and equal precision frequency measurement, etc. Direct frequency measurement is the method that counts the pulse number N within time t, then calculates the pulse number per unit time, i.e., the frequency of measured signal. Direct period measurement is the method that first measures the period T of the measured signal, then calculates the signal frequency by f = 1/T. However, these two methods will produce ±1 period error of the measured signal, and therefore have some limitations in practical applications. According to the measurement principle, it is very easy to find that the frequency measurement method is suitable for high-frequency signal, and period measurement method is suitable for low-frequency signal, however, both can not take into account the requirement of equal precision measurement of high and low frequencies [1]. The maximum advantage of equal precision frequency measurement method compared to other frequency measurement methods is that it can achieve the equal precision in the entire frequency range, and have nothing to do with the size of the signal frequency.
2. Principle of Equal Precision Frequency Measurement
The equal precision frequency measurement is also known as multi-cycle synchronous frequency measurement method. Its most prominent feature is that the actual gate time is not a fixed value, but a value related with the measured signal, it is exactly an integer multiple of the measured signal period. Within the time allowed counting, we count pulse numbers of the standard signal and the measured signal at the same time, then derive the frequency of the measured signal through mathematical formula. As the gate time is an integer multiple of the measured signal period, this eliminates the ±1 period error of the measured signal, however, it will produce the ±1 period error of the standard signal [2]. The schematic diagram of equal precision measurement is shown in Figure 1.
First, the control circuit gives out the gate signal, but the counter does not start counting until the rising edge of the measured signal comes. Then, the two counters start counting the pulse numbers of the measured signal and the standard signal respectively, and end counting until the falling edge of the measured signal, thus complete one measurement process. The counter’s opening and ending is completely synchronized with the measured signal.
Figure 1 shows that the actual gate time is not a fixed value, it is exactly an integer multiple of the measured signal period, and it is synchronized with the measured signal. This eliminates the ±1 period error of the measured signal. From the principle of frequency measure