[sdiy] Synthex Oscillator

[Gordonjcp]

On Sat, Jul 08, 2017 at 09:16:11AM +0100, Tom Wiltshire wrote:
> 

<lots snipped>

> So, what insights can you offer? Is my "discrete digital oscillator driven from divide-down technology" view of the circuit right at all? Is it really a discrete DCO design, like Mario seems to be claiming? What on earth is going on?!

What follows is a pure wild-ass guess, first-order conjecture of the
worst kind.

There's no way it's a digital current source because there's no way to
preload the counter.  The only way I could even vaguely see with that
layout was if there was some way to reset IC11/IC21 and then feed them a
predetermined number of clocks.

What I'm seeing here is in the "top half" of the diagram, a 12-bit
counter, which would give pretty decent frequency control.

In the bottom half is an eight-bit counter with a simple DAC.  C93 and
Q90 do indeed form a kind of output filter with Q90 discharging C93 when
the saw resets by driving its base low.  I'm guessing that R70/R72 keep
pins 5 and 2 of IC1H approximately low, with C69 allowing them to flick
high when the MSB goes high?  That would give you a quick reset pulse if
"SAW 1" was high.  I guess the DAC outputs are inverted by the XOR gates
or something, but 3H has to be doing something or one leg of each gate
is left floating.

I don't know what H2 does or MR2 but they seem related to the "DAC
rolled over" pulse too.  Something to do with syncing sawtooths?

Looks like the bottom 4013 is wired as a flipflop so depending on how
EOR1 is set, it'll either take the "other" input to the XOR gates from
/Q on the flipflop (that would be a triangle wave, I'd think - count up
then down - but wouldn't that make the triangle wave be an octave down?)
or from VCO2 for the "ring mod" effect.

The 12-bit frequency divider followed by the 8-bit DAC counter would
give you a range from about 4Hz to 15625Hz, which sounds about right.

-- 
Gordonjcp MM0YEQ