Right, that's something other than simple FM applied to two starting
signals; and it sounds like a good way to do it.
Well, actually, that was a brain fart on my part! That SHOULD have
been a circuit to simply add 100Hz to one, not to generate one that
goes up while the other goes down. Sigh. Replace the 6.1kHz with
0.1kHz, but then be careful that you filter out the difference
frequency, and do the filtering in a way that preserves the phase
relationships: in other words, make sure that you delay both the
3.0kHz and 3.1kHz paths by the same amount, including the filtering.
It would likely be easier to generate, say, 100kHz FM and mix that
down to 3.0kHz and 3.1kHz, passing each through a low-order filter
that has essentially the same delay for both of the outputs.
The problem if the delay is unequal: imagine a modulation frequency
of, say, 200Hz, and a delay in the two paths that differs by 1/400 of
a second. The the modulations will be out of phase: one will go up
while the other goes down in frequency, and the difference is
modulated. That's extreme, but to have the difference exactly
constant, you need to have equal delays.
Cheers,
Tom