[sdiy] Soft Sync update

[harrybissell]

I did a soft sync like this.

The reset comparator sets the sawtooth amplitude via a voltage
divider on the inverting input.   I switched a higher value resistor
across the lower divider with an NPN transistor driven by the
syncronizing wave (through a small coupling cap to make a narrow pulse)

The sync pulse lowers the reset voltage an adjustable amount.  If the
resistor is a high value, soft sync is the result. Make the resistor a
short
and you get hard sync.  It works pretty well for pulling an oscillator
that is
close to an integral multiple of frequency in, but allows dissimilar
frequencies
to go out of lock. MUCH less dramatic than hard sync... but in my case
I'm trying
to eliminate beating between a VCO and an audio source.

Use a pot and you can dial in the amount of sync you like. By avoiding
the
input that is being fed the VCO sawtooth, you don't load the comparator
or mess
up the AC hysteresis (feedback cap)

H^) harry