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.

Thank you Paul! This is a good hint. Can you tell me in which book did you
learn it? I would really like to get into this stuff.

Regards
Tom