Behold, Pete KE9OA signalled from keyed 4-1000A filament:

Right now, I
am working on a low noise VCO design, with a tracking filter at the
output.

Hi Pete,

Hadn't you mentions DDS?

Anyway, what about a Colpitts running at 1/2 Fo, then buffering on the
second harmonic. This was a trick used by pre-SS ham's to get a nice,
clean VFO out.

Another, I forget the name, is a Colpitts with the emitter tuned to 1/2
Fo, obtaining a fairly pure spectral output.

Just an idea

--
Gregg
*It's probably useful, even if it can't be SPICE'd*
http://geek.scorpiorising.ca