gripes and more on cheap sampling

[Stopp,Gene]

Magnus has a good point here. One thing that I've seen with different
JFETs is they have different discharge characteristics. I haven't done
any methodical analysis, but between the PN4391/4392 and the NTE-466,
the sawtooth is discharged to a different level. The NTE-466 is the
best, with the discharge going all the way to ground or pretty darn
close to it. The other parts are worse, only making it down to about .5
to 1.5 volts, depending on the individual part. I would have suspected
on-resistance as the culprit, but the timing idea sounds like it may be
a problem as well.  I have not tried this cap trick, but next time I'm
messing with VCO's I'll give it a try. I have an ASM-1 with sockets for
the FETs, that I use just for tests like this. Fortunately, the NTE-466
is stocked at Fry's locally so I'm OK for now. Although the offset
sawtooth will oscillate, the level will be less than 5 volts and so the
level out of the level shifter/amplifier that follows will be less than
10 volts and not centered around ground. This can be worked around by
hand-picking resistors for this circuit to match the characteristics of
the individual FET, but it's a pain.

 - Gene

************************************************************************
*********
Hmm... this sounds like something that hit me...

If your VCO's get hung at +15V you will probably find some very high
noise there as well (you should not be able to hear it). If this is
the case I would suspect that the slow down cap of 18 pf sitting
across the LM 311 is too low. Hook on a few more pf (say 10 or 18 to
start with) and your VCO may just start oscillating on you. The 18 pf
there is very close to the limit for some JFETs and this causes the
JFET not to get enougth time to empty the cap and ith that hitting the
floor properly.

I must say that this problem was quite unsolvable without an scope and
quite some time of schematic analysis.

I have tried to make some notes about this specific problem on the web
page, but a more detailed description has not emerged since I have
been both lazy, bizzy and just forgot about it all...

Hope it helps...

Cheers,
Magnus