[RE: VCDO]

[patchell]

Terry Michaels wrote:

> Message text written by Gene Zumchak
> >Guys,
>
>     I have another idea, and there is probably somebody that can implement
> it.  Use
> a simple voltage controlled high frequency oscillator and divide it down by
> the
> number of samples you want to have per waveform, say 64.  Use a
> phase-locked loop to
> lock the 64 times oscillator to the audio frequency you want to create.
> The audio
> frequency is taken from an audio exponential front-end VCO.  The result
> will have
> the same accuracy and stability of the your audio VCO.  The audio VCO is
> not
> intended to generate tones, just a square wave for phase locking and can be
> consequently as simple as possible.  Any comments?
>
> Gene Z (the other Gene)
> <
>
> An interesting idea, one problem you might have is the fact that the VCO in
> a phase locked loop can lock on a harmonic when you try to cover a
> frequency range greater than one octave.  It might be possible to pre-tune
> the high frequency oscillator with the control voltage driving the audio
> VCO, and just use the phase lock loop to do a narrow range correction to
> ensure tracking.  That gets around the false lock problem.
>
> Another possible concern is lock up time, you might end up with a permanent
> portamento.  If the loop damping is not critically damped, step changes in
> frequency might briefly slew past the desired pitch.
>
> This all sounds familiar, maybe it was already discussed some time back.
>
> Terry Michaels

    You can use a phase frequency detector that will get rid of the harmonic
lock problem.  The MC4044 and the CD4046 both have this type of detector in
it.  You can also use a single 74HC74 to make one as well.  I have also built
these into Xilinx Gate arrays also with no problem.

    -Jim


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