Some phasemodulation thoughts

[Rene Schmitz]

Hi all!

I've been thinking about Maciej's proposal makeing a phase modulator from a
wavetable oscillator. 

Now my idea would be to use a analog sinewave generator (like the AD639).
(For all who don't know that chip: It can generate a voltage proportional
to sin(x) for +/-500degrees!)
However I would try to implement it using discretes, I've read the
discussion of that chip on Martin Czechs website, where the idea of the
internal circuits are shown. (http://www.geocities.com/SoHo/Museum/4459/)
I simulated a circuit which can do +/-360 degrees. (But I'm not sure about
temperature compensation. As of yet the "phase gain" depends still on
temperature.) 

The task would be to add the output of a sawtooth VCO together with the
modulation signal. 

To allow for deeper phasemodulation one could "wrap" the phase signal with
a circuit similar to the serge waveshaper. The folding should occur at
+/-(3/2)pi. Let pi=1 (volts :-) and set up a chain of full wave rectifiers
to fold the modulation voltage at +/-1.5V +/-3V +/-4.5V ...
The amplitude of the saw would have to be 1V (2Vpp).

However, the sinewave shaper is not necessary at all, for makeing a
triangle one can just use the waveshaper chain, and add a second signal to
a sawtooth input. The foldover points would have to be at multiples of the
sawtooth amplitude then. But doing a triangle like this and feeding it to a
sineshaper afterwards will not yield the same results!

Note the discussion of the Serge waveshaper at Martins site! He compared
the waveforms from Yamaha PM, with what you get when variing the input
amplitude on the waveshaper.

This circuit will make high demands on the sawtooth input, to avoid
glitches. But they could be compensated like in Jorgen Bergfors' sineshaper.

Bye
 René





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