ODP: thru zero VCO questions

[terry michaels]

Message text written by Roman Sowa
>And how can one make stable VCO running so high?
I mean if we want to tune it at, say, 50Hz (afer downconversion)
and have reasonable stability, it must run at 20050Hz, and no 0.1Hz
less. It's almost crystal stability...
Of course, such thing would be used only for deep modulation,
but how can we tune it anyhow?

BTW, better to put 'braking point at 40kHz (LO=40kHz, VCO=20..60kHz,
IF=20kHz..0..20kHz) which makes stability problem bigger.

Am I missing something?

Roman<

Hi Roman:

How about this:  Run the VCO in the normal frequency range, mix the output
with a crystal controlled oscillator at, say, 1 MHz, then go through a
bandpass filter, 20 KHz wide, centered at 1.01 MHz, then mix again with a
crystal controlled oscillator to mix down to the audio band, then low pass
filter, 20 KHz.  The two cyrstal oscillators should be at slightly
different frequencies, maybe 10 or 20 KHz apart.  That way the resulting
output can pass through zero and into the "negative frequency" (reversed
phase) range.  The 1.01 MHz filter can be done with LC components.  The
stability of the crystal oscillators should be no problem, so the stability
of the whole thing should end up the same as the first VCO.

Terry Michaels