Column: Circuit drill


Frequency performance of a zero- crossing detection circuit


By Sulaiman Algharbi Alsayed, Managing Director, Smart PCB Solutions T


he zero-crossing detection circuit is a comparator circuit that indicates when the input signal waveform crosses the zero point. Tis simple circuit is extremely


useful when tracking input signal changes from positive to negative, and vice versa. Tis circuit also converts a sinusoidal waveform into a square one at the zero- point crossing. Te point is a time marker, widely used in timer and frequency- counter circuits.


Setup Here we use a simple zero-crossing detection circuit consisting of a single op- amp as the main component, connected as a comparator; see Figure 1. Figure 2 shows the output voltage at the


input signal’s zero-crossing points. In a single wave, there are two zero


crossings: one at half cycle, and the second at the end of a full cycle. Hence, throughout this article we will use the terms “half cycle” and “full cycle” zero crossing. Te circuit shows increase or decrease


in output voltage with each zero crossing; however, its response time to show the output is what we will examine here. So, the question is whether the zero-crossing detection circuit response changes with the input signal frequency; i.e., can such


12 October 2022 www.electronicsworld.co.uk


Can such a circuit be used with a wide range of frequencies, or does it have substantial operating frequency limitations?


a circuit be used with a wide range of frequencies, or does it have substantial operating frequency limitations? In this setup, input sine waves were


applied to the circuit, with frequencies starting at 1Hz; the half-cycle and full- cycle zero-crossing response times were measured at its output, and plotted. Te sine-wave frequencies varied between 10Hz, 100Hz, 1MHz and 10MHz; Figure 3 shows the response times for each. Two new values were introduced to


make the logged results more meaningful: “Response 1” and “Response 2”. Response 1 is calculated by dividing the half-cycle zero-crossing response time by the input signal’s period, and Response 2 is calculated


by dividing the full-cycle zero-crossing response time by the same period. Tis division makes more sense since we can’t compare the circuit’s response times when the input signal frequency and signal period decrease. So, to make the collected values more representative and meaningful, we introduce Responses 1 and 2, allowing us to calculate the response time percentage in reference to the input’s period, and hence determine if the circuit maintains a constant performance whilst being subjected to a wide range of input frequencies.


Results Table 1 shows the findings of the experiment. By plotting the values of Responses


1 and 2 against the input signal frequencies, we generate the trends shown in Figure 3. It can be seen that the zero-crossing detection circuit has a fairly stable response when the input signal frequency is at 100Hz or below, yet is unstable at frequencies higher than 100Hz. Te finding is valuable to electronic


circuit designers, since this is an important limitation to consider when designing circuits. At frequencies approaching 1kHz, the circuit’s response will be greatly delayed.


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