[sdiy] FET for VCO cap discharge

[JH.]

Here's something I want to share with you.

For the Frequency Shifter part of my Matrix FX I need a precise quadrature
VCO. Contrary to the usual triange core, or trapezoid core (Hi Don!),
I want to start with a saw waveform, and bend this to a sine and cosine
function with two waveshapers.

For this I need a precise saw waveform with less than 1us reset time,
with a frequency range from < 0.1 Hz to > 16kHz.

I started with this circuit (Hi Ian!):
http://home.earthlink.net/~ijfritz/sy_cir2.htm
But I wanted a 10nF capacitor instead of the 1nF, to stay away from leakage
(which might distort the waveform) at very low frequencies.

This is quite a demand on the discharge transistor, considering I wanted 
a JFET.

I found that the 2N4391 is not good enough for this. (It's great for
1nF, but for larger caps, I'd have to increase the discharge time,
which I didn't want.)

So I started paralleling devices. This works. But I also found a
quite common JFET that is better than the 2N4391 in this
application: the BF256C.
Two BF256's in parallel were enough to discharge 10nF in
Ian's circuit in ca. 500ns. I put in 3 in parallel just to be sure.

So I thought I'd pass this on - the good old unexpensive BF256C
may be a very good choice in that kind of oscillators.

One caveat, though: JFETs have huge tolerances, so I 
_might_  just have run into some relatively poor 2N4391's,
and into some relatively good BF256C's. I just had
3 of each to test. 

The circuit from Ian's site is obviously good enough to drive
3 gates in parallel. I've used CA3140 for the opamp and
LM311 for the comparator.
Ultra-precise tracking is not so important in my application,
and of cource I had to leave out the compensation
resistor R19.

JH.