[sdiy] 2164 expo VCO tracks!

[David G. Dixon]

Well, Damian, at the risk of offending our favorite Finn, who has apparently
HEARD enough about this module, I'll answer your questions:

> I think it's important that you get the right JFETs!

Absolutely.  Based on my experience with this VCO, it's THE critical thing.

> Also, maybe think about trying the approach with two or more HF trimmers?

Yes, I've thought about this.  It seems to me that, given a "decent" JFET
and a "reasonably fast" comparator, the key to excellent HF tracking is the
shape of the I-V response from the HF trim circuit.  Unfortunately, a diode
(or diode-connected transistor) simply does not give the correct response.
It undercorrects at the lower end, overcorrects in the middle, and
undercorrects again at the high end.  There is a simple circuit one can
build with a single opamp and a few diodes and resistors which will give a
steadily-increasing piecewise-linear gain response, with breakpoints and
slopes determined by the input resistors.  If the breakpoints were designed
to coincide with the required gain curve for "perfect" HF trim, then this
circuit would give excellent HF tracking, without trimmers.

> HF frequency is very important for FM and divide-down and the like.

The thing to understand here is that all of the HF tracking issues in my
circuit are around the integrator components, and have nothing to do with
the 2164 expo converter, which is essentially perfect.  Better integrator
components will give better tracking.  I simply did not have better
components on hand, but I always use IC sockets, so I can simply unplug the
opamps and replace them with better ones.  (The 5485 JFET will have to be
desoldered when I get my 4391's).

> Regarding the switching glitch:
> it's a feature, not a problem! If you can make its 'depth' or 'amount'
> controllable, then that's something very useful.

It's very tiny -- barely perceptible on my scope, and totally inaudible.
I'd use a waveshaper circuit to mess up the waveforms.

> Another important thing:
> how does it behave around other modules? Does it put switching noise
> on the voltage bus?

Absolutely not!  I've put 10uF electrolytics and 100nF ceramics on the
rails, just after they come onto the board (as I always do).  This
effectively isolates the module from the rest of the synth.  There are no
other decoupling caps on my board.  I could add a couple, but I personally
don't think it is necessary.