VCO hacks, tweaks

[Ian Fritz]

Hi all --

I've been working on upgrading the sawtooth VCO originally designed by
Terry Michaels (Electronotes v.62). My main goals have been to reduce
the temperature drift and to see if using modern op amps could improve
the speed and accuracy of the oscillator. Both tall orders, given the
excellent performance of the original.

To improve stability and temperature drift I first added on-board +/-
6.9V regulated supplies (from LM329 chips) and used them to supply the
critical voltages in the circuit for: (1) coarse frequency control, (2)
reference current in the expo converter and (3) ramp reset point.

For improved op-amp performance I chose the Burr Brown OPA132. This and
many other BB devices are now available from DigiKey. The OPA 132 was
chosen for its combination of low input current (5pA) fast slew rate
(20V/usec) high stability and moderate price (<$4). (The OPA 602 has
even better specs and only costs about a dollar more, but I haven't
tried it yet. )

For the comparitor I decided to try the LM 319, which is somewhat faster
than the original LM311. I'm not convinced that this has actually
improved performance very much, though. For the capacitor discharge
switch I chose the 2N4391 JFET (available from Mouser). It works almost
identically to the original KE4859, which is still available but a bit
harder to get a hold of.

The exponential converter uses the LM394CH matched pair. Temperature
compensation is done with a Q81 tempco resistor I had saved from a few
years ago.

Determing temperature drift can be tricky, as gradients and drafts can
mess up the measurement. I prototyped the circuit on a whiteboard, which
in turn was mounted on a wooden board. I put this assembly on a folded
over heating pad and covered it with a plastic lid. This seemed to give
fairly reproducible results. Temperature was monitored by an LM335
sensor heat sinked to the expo converter.

First I looked at the oscillator section alone, without the expo
converter. I was surprised to find quite a bit of temperature drift. By
probing around with heat from a soldering iron tip, I determined that
the discharge FET was the sensitive component. The problem turned out to
be that the discharge time was too short, causing a variation in the
discharge level due to the temperature dependence of the FET's channel
("on") resistance. So I increased the discharge-timing cap until this
source of drift went away. The residual drift was quite small, maybe
about 200ppm/K, but it's hard to get a reliable measurement at this
level.

Next I hooked up the expo converter and looked at the overall drift.
Converter drift actually involves a temperature-dependent scale factor.
In other words, when the differential base-emitter voltage (dVbe) is
zero, there is no temperature dependence of the output current, and
voltages above and below zero give positive and negative dependences,
respectively. Furthermore, these changes increase with increasing
magnitude of dVbe. So when someone quotes a value for VCO temperature
drift you have to look closely at how this relates to the operating
point of the converter. Conversely, converters should be designed so
that dVbe=0 corresponds to the center of the critical audio range. To me
this is 1-2kHz. This is all pretty simple and obvious, but I bring it up
because it does not always seem to be recognized.

I found that the temperature drift of the full VCO was in the opposite
direction from that of the uncompensated converter. This is sensible, as
the Q81 resistor has a tempco of 3500ppm/K vs. the 3300ppm/K (or so)
needed for compensation. This discrepancy was easily fixed by adding a
small metal film resistor in series with the Q81, producing a composite
with a smaller tempco. For this circuit a value of 174 ohms worked well.

The final result is an essentially negligible temperature dependence of
the VCO. At frequencies of 1kHz and 3kHz I observed zero frequency
change (i.e., <1Hz) over a 10K temperature range. At 200Hz the drift was
0.5Hz over a 15K range. So I can say that the drift is less than 0.1Hz/K
or alternately less than 200ppm/K (or 0.35cents/K) over the range of
200Hz-3kHz.

The circuit's tracking accuracy is perfect, as close as I was able to
measure it. I did this by setting the frequency at the octave values
between 110Hz and 28160Hz (by beating against a 440Hz xtal oscillator)
and measuring the corresponding input voltages. Within the 1mV accuracy
of my DVM, these were in exact 1.000V steps, indicating better than 0.1%
accuracy. I don't have the means to measure the switching reset time
accurately, but it looks like about 0.5usec, which is consistent with
the OPA132 slew rate and settling time.

The full circuit may be seen at my website:
http://home.earthlink.net/~ijfritz under wind synthesizer schematics.

  Ian