[sdiy] PLL module

[Scott Gravenhorst]

"David G. Dixon" <dixon at interchange.ubc.ca> wrote:
>> Ah, well, an octave _divider_ is also useful as it drops a VCO output by
>> an octave.  I've
>> also used the divide by 3 circuit to do - uh, divide by 3 which produces a
>> perfect fifth
>> below the input frequency (actually I think it's an octave and a fifth
>> below).
>
>Yeah, I got that!  That's why it's more useful than a simple octave divider!
>Of course, I suppose one could extend this idea to the whole harmonic series
>by dividing by every integer up to 12.  Slap on a mixer for all the division
>outputs (perhaps suitably lowpass filtered to recover sine waves from the
>squares) and you've got a low-budget analog additive synthesis module!  (...
>or am I smoking crack (again)?)

I've often used it in an additive mode (though a square wave additive) since all 7
outputs are connected to a mixer.  In the case of my circuit, all of the outputs are
derived from a divide by 12 circuit.  Eg. several JK flops set up as toggle divide down
taping the divide by 12 chain to get 6F, 3F and 1.5F.  4F comes from the output of the
divide by 3 and 2F from 4F divided by 2.  The divider chain is really 3x2x2  To do the
more "harmonics" (in quotes because they are not sine) with one PLL, one would have to
create a divider with a larger value than 12.  30 would allow for 5F, but I believe that
the performance of the circuit will change such that the lag would be longer and even
more so if more dividers are added.  Of course, more PLLs could be used each with more
targeted dividers, but then coordinating the lags of each could either be artistically
useful or a PITA (or both).

>> My circuit can work with input signals other than a VCO to some degree or
>> other.  It
>> prefers a signal like a sawtooth VCO where the signal swing goes rail to
>> rail from the
>> perspective of the 4046 IC.  It also works best with one zero crossing per
>> cycle.
>
>Well, it's dead easy to convert any zero-crossing to a rising edge with a
>comparator, so I guess you could fire this thing with any wave.  However, as
>you say, it should be a clean wave and not, for instance, a resonant filter
>output with all those little siney wiggles -- that would surely confuse the
>hell out of it!

The 4046 has two phase comparators, the one I chose is largely immune to locking to a
harmonic whereas the other comparator will happily lock onto a strong harmonic.  Could be
interesting to have a switch to route to either one.  It will work with less clean waves
with the lock-to-harmonic caveat.

-- ScottG
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-- Scott Gravenhorst
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