Sine Wave from Square wave

If the frequency is fixed you could use a self-designed lowpass or bandpass filter (the necessary order is determined by your distortion requirements: Quality of the sinus signal).
However, a filter that can be tuned together with the varying frequency of the squarewave is harder to realize.

 

How about answering some questions first.

Will the frequency of your square wave be variable?

What is the frequency range?

How quickly will the frequency change? Can you tolerate some distortion while the filter settles to a new frequency?

What is your budget?

 

There are many, many introductory articles accessible through the Internet which can teach you filter design basics. I don`t think that it is possible to do that in a forum like this.

 

If you need only a fixed frequency, why not make an oscillator that produces a sine wave instead? It is easier to convert a sine to a square wave than what you are trying to do.

Bob

 

Or build a cheap sine wave generator from an op-amp and some r's and c's

 

With a switch?

Why don't you tell us what you are trying to do.

Bob