[sdiy] 2164 Expo VCO tracking

[David G. Dixon]

OK, here's the deal:

The reason why the 2164 Expo VCO's tracking falls apart at high frequencies
is because the 2164's advertised 33mV/dB response falls apart above +20dB
(as advertised).

I doubled the reference current going into the 2164 (to double the frequency
at any given voltage) and immediately got excellent tracking up to 7kHz, and
only about -0.2% error at 14kHz.  In other words, I obtained virtually the
same tracking performance as lastnight, only at double the frequencies.

Other observations:

The Franco resistors are extraneous, but may help slightly.  Only a tiny bit
of HF trim is needed without them, and only at 7kHz (which needed about a
40Hz boost).  My reference (0V) frequency would have been about 920Hz, so
the +20dB frequency would have been about 9.2kHz.

The only "weird" thing about using the 2164 is that (at least with the
PN4391 JFET and an LM311 comparator for reset) the comparator output to the
gate must be filtered.  100pF or slightly smaller is ideal.  Otherwise, I
think that the reset spikes are injected into the ramp, and this is probably
what is causing the overshoot at the frequencies from 0 to +20dB.  With the
100pF cap, I can limit the error at those frequencies to about +0.1%, and
the edge of the ramp looks a lot cleaner.  Tracking below 0dB is excellent
regardless.

Also, I tried Achim's suggestion of running in Class AB mode, and this was a
total disaster.  I was correct when I said that I had been down that path
before.  Any resistor between the MODE pin and the positive rail distorts
the gain response, and the recommended 7.5k resistor sends it completely out
of whack (errors of about 100%).  This pin must remain unconnected.

Essentially, what all of this means is that one can get about 18 good
octaves out of this VCO.  However, tracking will only be precise about 3 to
4 octaves above the 0V (0dB) frequency.  Hence, that frequency should be set
to about 1/10 of the maximum frequency you care about tracking accurately.
If, for example, that frequency is set at 1kHz, then tracking should be
excellent up to 8kHz, and serviceable up to 16kHz.  However, for a +/-15V
system, the lowest frequency will be about 0.03Hz, which gives a period of
about 33s.  This is plenty low enough for most LFO purposes, but to get
lower would require a range switch.

Now I can finally put this naughty little puppy to bed and get on with my
life!  Thanks to everyone for your helpful suggestions and encouragement.