Column: Circuit drill
The stable performance range of a simple frequency-to- voltage converter circuit
By Sulaiman Algharbi Alsayed, Managing Director, Smart PCB Solutions T
he frequency-to- voltage converter has several uses: for example, in tachometer measurements it is used to report the speed of a
motor, and in electrical relay protection devices it stops the power to connected loads if a change in frequency is detected along the power line. In most cases there are many
electronic components in a frequency- to-voltage converter, making the
circuit complex. However, here we present a simple converter that transforms frequency into voltage; see Figure 1. This a circuit that is frequently used in simple frequency meters. By setting up this experiment, we
are trying to determine what input frequency range allows this basic frequency-to-voltage circuit to provide reliable results in measurement. We will also determine if it performs well at all frequencies.
Setting up the experiment With a function generator we input a 1Vac signal into the circuit. Te frequency of the signal is varied between 1Hz and 1kHz. Te output voltage’s root- mean-square (RMS) value is measured with a voltmeter with each input signal frequency value. We can then determine the connection between the output signal’s RMS voltage level and the input signal’s frequency by dividing the output value by the input value, giving us the relationship between the two values. Te outcome is expressed as a ratio, indicating the relationship between the two numbers, which we then record as a graph. For the experiment we assumed the
following: • Te input signal amplitude is kept constant at 1Vac throughout the experiment.
• All circuit component values were leſt unchanged.
Figure 1: Simple frequency-to-voltage converter circuit
• Te input signal frequency was varied, from 1Hz to 1kHz.
• Te circuit output was not connected to any load. Te output/input ratio was displayed,
and so were the RMS voltage values of the output signal and the ratio between the collected values; see Figure 2.
Figure 2: The relation between the input signal frequency and the output signal RMS voltage at various input signal frequencies
08 April 2023
www.electronicsworld.co.uk
Results From the curve in Figure 2 we can see that the simple frequency-to-voltage converter produces a high-precision signal for input frequencies below 40Hz. Between 40Hz and 110Hz, the accuracy deteriorates slightly, but is still acceptable. However, above 110Hz, the accuracy degrades significantly, making the circuit unsuitable for measurements of frequencies beyond that level, which requires a more complex circuit.
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