[sdiy] Through-zero FM oscillator

[Henry]

I reckon I can make a through zero linear FM oscillator, without going to
extremes....

Make a 20KHz sine oscillator which has quadrature outputs.  This is easy,
you can use a state variable design and use the BP and LP outputs.

Make another quadrature sine oscillator which is voltage controlled - just
use OTAs in the state variable design.  Arrange the currents so that a
control voltage of say 5V runs the voltage controlled oscillator at 20kHz,
10V runs it at 40kHz and 0V runs it at 0Hz.  Feed the quadrature outputs of
both oscillators into the four inputs of two four quadrant multipliers with
a sum at the outputs.

Now, your modulating signal varies the 5V CV from 0 to 10V, your output from
the multiplier is 0Hz at 5V CV and it is 20kHz at both 0V CV and 10V CV.
The through zero point is at 5V CV and if my intuition is correct, the phase
will all take care of itself, as the multiplier can do little else than get
it right.

OK, so I haven't built it yet, but surely this will do the job?  Very
similar to the "SSB style" frequency shifters, only easier to do cos you
don't need a wideband phase shift for the input signal.

Henry. Farnborough, UK.