More on Fav Designs

[Joachim Verghese]

On Thu, 17 Apr 1997, Paul Schreiber wrote:

> First, no VCO has a "sound", sterile or otherwise. They may have THD, 
> but a pulse is a square is a saw. But what they DO have is drift and 
> have linearity errors.

Not all saw and pulse waves have fast edges. A saw/pulse wave
with 2us edges is brighter than one with 20us edges. Also,
VCOs with tri-to-saw conversion can sound different from
saw-based ones.

OTAH, I haven't done those listening tests for years, so I don't
know if I can hear the differences anymore. :(

> #1 overlooked component in a VCO is the integration capacitor.
> You just can't chunk any old cap in there.

I agree, and while we're on the subject of overlooked components
in VCOs, I'd like to point out that people tend to focus a little
too much on the exponential converter, it seems.

There are designs using just a 2N390x pair and a tempco resistor
which are more stable than some uA726-based designs. This is because
the stability of the remining parts of the VCO is being overlooked.

Actually, there are components which aren't considered to be part of
a VCO design at all, but which can affect stability, such as calibration
trimmers. A rheostat (variable resistor) -connected trimmer will drift
with temperature, while a potentiometer (variable potential divider)
will not, provided that the end-points are connected to low-impedance
sources (such as pwr supply lines or op-amp outputs).

[ARP used the latter approach for both tuning offset *and* scaling
trims, and came up with a simple stable VCO design in 1974 which they
never needed to change.]

> #3 discharge transistor. Best advice is use a DMOS FET, like an 2n7000. 
> They have a factor of 10 (to 1000!) lower ON resistance. This eliminates 
> errors at higher frequencies.

In some designs MOSFETs cause serious overshoots and glitches,
because of their capacitive nature (insulated gate). JFETs have
higher on-resistance, but are better for precision waveform
generation, IMO.

The on-resistance does affect high-frequency tracking, but so does
the bulk-emitter resistance of the exponentiating transistor, so
chances are that you'll have to compensate for HF errors anyway.

Great discussion, BTW!

Have a nice weekend, everyone.

-joachim