ODP: thru zero VCO questions

[Martin Czech]

:::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.
:::

You want to supress a sideband at 1MHz with LC-filters?
Forget it. It will be difficult enough to have the quartz
osc. stable enough, they usually spec to 100ppm (Drift, aging).

HAM people get a (very bad) sideband rejection with mechanical filters
@ 455 kHz (something like -30dB), that's good for radio, but
not for a frequency modulation substitute.

There must be a reason why people use complicated polyphase
or phase difference networks in order to throw
away the unwanted sideband.

And there is certainly a reason why J.H. did it all in base band.

The higher you modulate, the higher are the filter requirements,
and also frequency stability requirements, differences of large values.

m.c.