There are dedicated oscillators that produce a sine wave, but as I mentioned before, a sine wave is an integral part of any square wave. We can get to it by removing higher spectral components. We will do it by stringing RC filters one after the other.
All of the RC filters we will be using a resistance of 1 kOhm and a capacitance of 10 nF, which is the same as the ones we used in our oscillator. It’s worth noting that the corner frequency of such a filter is around 16 kHz, which is approximately half the frequency of the oscillator. This will result in some signal attenuation but will also result in a faster removal of higher spectral components. In the first oscillogram, the yellow trace represents the oscillator’s output, and the green trace represents the filtered output using the same RC filter as in the oscillator. After that, I’m confident you will be able to identify which waveform corresponds to which filter by comparing their shapes.
After first RC:
After second RC:
After third RC:
After going through three stages of filtering, our initial square wave began to resemble a sine wave. However, there are still multiple higher-order components present in the signal’s spectrum. Nonetheless, the fact that the next highest component is attenuated by more than 20 dB in comparison to the first one is significant. A 20 dB attenuation is more than 20 times. It’s important to note that the resulting sine wave is significantly smaller in amplitude than the original square wave. Therefore, an amplifier is necessary to adjust the amplitude. Unfortunately, there is no way around this requirement in this particular oscillator design.