How to Convert 7 Hz Square Wave to Sine Wave Using Filters or Circuits?

Good morning

I have a square wave at about 7 hz. Is it possible to transform it into a sine wave or similar?
Thank you
John

The easiest way is to use a tuned circuit (if the frequency is constant). You would need fairly large inductance and capacitance values, and you would get some loss, so you may be able to include the tuned circuit in an op-amp filter circuit. You call also use active filters (op amps again) or digital filters (which are good but way outside my areas of expertise). A lot depends (as above) on whether the frequency is constant, on the amplitude of your 7Hz signal, the impedances required, and the purity of the resultant sine wave that is required.
Bear in mind that a square wave consists of the fundamental frequency at an amplitude of 1, 3x the frequency at an amplitude of 1/3, 5 x the frequency at and amplitude of 1/5, etc...etc. so it is a matter of filtering out the higher harmonics that are present in the square wave.

There is a now obsolete IC called TCA580 - a gyrator - that is great at simulating large inductors for tuned circuits, it would be ideal for this. I can send you more info if it is of interest.

Use 7Hz lowpass filter, OpAmp based may be better.

First thing that comes to mind is to use an active filter, to attenuate the harmonics of the 7-Hz square wave, & leave only the fundamental. A 3rd-order Butterworth low-pass filter might do the job nicely. You'd need an opamp, 3 capacitors, & 3 resistors, and would roll off the harmonics at the rate of about -18db/octave. The nice thing about active filters is that no inductor$ are required.

But first, there are some things we should know that could affect the filter design:
1. Does the ~7 Hz square wave change frequency, or is it a steady 7 Hz square wave? If it changes frequency, how much does it change?
2. Id phase shift be a problem?
3. How clean does the sine wave need to be? How much do you need to attenuate the square-wave's harmonics? Or, is there a total harmonic distortion specification?
4. What is the application, if you can mention it?