A square wave to sine wave converter can be built using 6 passive components, namely capacitors and three resistors. Using these three capacitors and three resistors, a 3 stage RC network can be built that takes a square wave as an input and a sine wave as an output. A simple single-stage RC network circuit is shown below:
In the above circuit, a single-stage RC filter is shown where a single resistor and a single capacitor is used. The above circuit is pretty simple. The capacitor gets charged depending on the status of the square wave. If the square wave in the input is in a high position, the capacitor will get charged, and if the square wave is in a low position, the capacitor gets discharged.
A varying signal wave such as a square wave has a frequency, and depending on this frequency, the output of the circuits gets changed. Due to this behavior of the circuit, the RC filter is called an RC integrator circuit. An RC integrator circuit changes the signal output depending on the frequency and could change the square wave to a triangular wave or triangular wave to a sine wave.
The circuit needs a stable, precisely symmetrical supply voltage since the control is derived directly from this via resistor R2. It also needs a symmetrical triangular voltage at a level of 350 mV p-p at pin 3 of IC1. The output in those conditions is a sinusoidal voltage at a level of 2.85 V r.m.s. Provided the circuit is constructed carefully and P1 is set correctly, the distortion does not exceed 1.2%. The figure deteriorates rapidly when the supply voltage is not accurate or precisely symmetrical.
And that’s pretty much it for constructing and fine-tuning this square-to-sine wave converter circuit.
If we already have a square wave, we can build a sine wave generator circuit with a few simple components.