[sdiy] SSM2164 expo VCO finished

[David G. Dixon]

Further to our discussion here a few weeks ago about SSM2164-based expo
converters, I have now completed my first prototype of a saw-core VCO based
on this concept.  I am happy to report that the temperature compensation
works exactly as advertised.  I tuned the tempco voltage to exactly 289.5mV
(using a REF02 voltage reference trimmed to exactly 5V, and a 4.7k - 100R
trimmer - 261R resistor combination feeding the 2164 control voltage input,
with its 5k input impedance) and trimmed the inverting summer gain to
exactly 54.64% (using a 53.6k feedback resistor and a 2k trimmer).  Putting
warm things on or near the 2164 or the opamps has absolutely zero effect on
the output frequency!

I've designed a fairly standard saw core (using all TL07x -- no special
chips, comparators, etc).  With the right JFET the tracking is excellent.
PN4391 is preferred, although I'm using 2N5485 and getting good tracking up
to about 8kHz.  I'm using a simplified full-wave rectifier with a few small
caps for the triangle, a dead-standard TL07x-based comparator for the pulse,
and the Thomas Henry sine shaper based on two 2N3904 transistors.  The sine
waves are absolutely beautiful! -- much better than can be achieved with
LM13700.  Simulations suggest THD less than 0.7% is possible, and my ears
confirm this.  I was able to eliminate virtually all overtones, such that
when I run the sine wave through my 4P filter and adjust the cutoff
frequency, it's like turning a volume knob up and down -- no discernible
change in timbre.

I've also put in two switches: one for centre (triangle driven) or edge (saw
driven) pulse (a la Oakley) and one for sine (triangle-driven sine shaper)
or sigmoid (saw-driven sine shaper) -- remember that discussion?  The
sigmoid wave is very nice -- like a saw, only a little bit more "brassy" and
warm.  I'll be using this wave a lot.  It filters very nicely, too.

If anyone's interested, please email me privately for the schematic,
Excel-based layout, PCB transfer file, Multisim simulation file, front panel
file, etc.  The PCB is only 2" x 6".