[sdiy] Through-zero FM oscillator

[jhaible]

> I reckon I can make a through zero linear FM oscillator, without going to
> extremes....
>
> Make a 20KHz sine oscillator which has quadrature outputs

> OK, so I haven't built it yet, but surely this will do the job?

Certainly. This is what I referred to as the BFO method early
in this thread. You don't even need quadrature stuff.
One fixed oscillator, one variable oscillator, one ring modulator,
and a fixed filter to get rid of the unwanted sideband.

-> Bode Frequency shifter

Now your enemies are mistracking & drift (you're evaluating the small
difference between two large numbers ...) and locking (soft sync).

But I wonder if locking is really a problem here: For FS applications,
you want the get fractions of Hz difference _constantly_, without locking.
But for an audio VCO with audio rate thru-0-FM, you would pass the
critical region very quickly. So would there be a locking problem
at all ?

JH.