A perfectly symmetrical square wave doesn't contain any even harmonics.
For best results when using this technique to generate even harmonics,
the square wave needs to have a longer "on" period than "off" period or
vice-versa.

Roy Lewallen, W7EL

Paul Burridge wrote:

Feed the square output to a coil+capacitor in parallel and you'll get
a sine wave. Tune the this 'tank' circuit for harmonics of the
funamental and you can double, triple, quadruple and so on the
original square wave's frequency.