[sdiy] Attempting the impossible?? Detuning waveforms?

[harrybissell]

Scott Gravenhorst wrote:

> Regarding detuning: what I have contemplated, is using PLL a little
> differently.  If the input of a PLL is driven by an XOR gate with a VCO
> square output driving one input and an LFO square output driving the other
> input, wouldn't the LFO cause extra phase reversals that would slightly
> increase the PLL output frequency?  And wouldn't the amount of increase
> depend upon the frequency of the LFO?  The loop filter would be important,
> I'm sure.  I'm not sure how useful this would be tho, prolly why I haven't
> tried it.

hmmm.  I don't think that would work.  Yes there would be extra phase reversals
but there would also be a lot of jitter. The LFO would not be syncronous with
the VCO
and the PLL would not be able to reconcile them.

I'd think of it as the XOR gate is a balanced modultor... the output should
contain
sum and difference frequencies. If you higpass filter that, you might get rid
of the
difference and have only the sum.  It would work OK for sine waves, but with
square
waves it would be really tough.

By the time you are done you'd have a frequency shifter...

I'd sooner get out the BBDs and use them !!!

yep, build another VCO.   ;^P

H^) harry