thats interesting, so maybe a 555 generating 5 khz and a chaing of op amps with LC circuits ? that have such a high gain that the output will still be square ?If your 5khz is very rich in harmonics you can use an amp tuned to 50khz to pick off the harmonic you want. If this is too weak you use a chain of harmonic amps. Example: amplify 10khz, square it up and then amplify 50khz. It's an old technique not needed these days.