FETs on the ASM-1 VCO.

[Magnus Danielson]

>>>>> "IF" == Ian Fritz <ijfritz at earthlink.net> writes:

 IF> Magnus --
 IF> We discussed this a few months ago. What you write is fine as far as it
 IF> goes, but, as Terry Michaels pointed out, MOSFETS have a disadvantage in
 IF> capacitance, and also in leakage. In addition, if you are using an opamp
 IF> integrator, the rate at which the output can slew also matters.  It
 IF> would be interesting to see some actual data, though -- details on
 IF> spikes in the waveforms and on tuning accuracy and range. 

I am aware of the slewrate issue, but recall that I want to go from
1.5 us downto some figure below 1 us, the lower the better. Recall
that we are talking about a 5 V swing, while the LM311 makes a 15 V
swing in much less time (was it 25ns? it was fast anyway).

Actually, the falltime in itself is something like 300-600 ns on the
ASM-1, but a lot time is lost for some reason that I haven't dwelved
into yeat (my lazy bum).

 IF> BTW, my updated version of Terry's VCO switches pretty fast and only
 IF> needs a touch of HF compensation. Maybe it could be improved, but it's
 IF> hard to see why you would need anything better -- unless you
 IF> specifically need ultrasonic frequencies. 

Well, if you want to go deep ultrasound at maybe 200-500 MHz ;)

 IF> I've tried unsuccesfully several times to download your ASM-1
 IF> schematics, so I still don't know exactly what Gene's version of this
 IF> circuit looks like.

Strange, I just downloaded it all very nicely... using
Netscape/Mozilla "Save Link As" menuchoice of the left mouse button
work great. Lynx is failsave otherwise.

Gene's VCO is fairly straigth forward with nothing strange really.

Cheers,
Magnus