[sdiy] VCO Cap Switch

harry harrybissell at prodigy.net
Wed Jun 5 05:40:05 CEST 2002

I've tried using the VN1206N  from Siliconix... and was not sucessful
with just ONE...

I had to use two in series... back to back as you would do in making
an AC switch that uses the body diode and one FET froward biased
in each direction.  Both gates are driven in the same polarity (parallel)

This worked OK... a single JFet outperformed it... but it did work.

Now... any guesses why I had to use two MOSFETs to reset a sawtooth
oscillator ???  (I'd like to know why...)

H^) harry

Ian Fritz wrote:

> At 10:25 AM 6/4/2002, Jay Schwichtenberg wrote:
> >First I was looking into using the 2N7000 as the FET to reset the cap.
> >They're easily available. In the archives there are a couple of things
> >mentioned that left me a bit confused.
> >
> >Basically is the capacitance and leakage current for the 2N7000 an issue for
> >what we're doing?
> >
> >Has anyone used them in their VCOs?
> I once heard from someone that the 2N7000 works very well. Many people are
> satisfied with MOSFETs for the discharge switch, so don't be afraid to try
> one. (They do have to be biased differently from a JFET, though, so be sure
> to watch that.) Capacitance and leakage are indeed issues, but these  trade
> off to a certain extent against the higher discharge currents afforded by
> JFET's are the first choice for most people. I like the KE4859 (PN4859,
> etc.) device originally suggested by Terry Michaels and the 2N4391, which
> is quite similar and a bit easier to find. These both will carry 50 mA of
> discharge current. After these, I like the 2N4392, which seems still easier
> to find, although it only carries 20 mA. Some people report good results
> with the J111, but its specs are not as good as the above choices.
> >Next is the comparator. Most designs use LM311 and LM393 comparators, but
> >some the faster LM319.
> >
> >Any comments on this?
> Stick with the 311, unless you need to use a dual (as some of my balanced
> tri VCO designs do). The 319 is difficult to work with, requiring reduced
> PS voltages, output pullup, etc.
> >Isn't it also more important to have a consistent reset time than a short
> >reset time if you aren't that critical of the waveforms?
> >
> >Also what should we keep the reset time under?
> A finite reset time causes warping of the tuning curve, but this can be
> compensated for with a resistor in series with the cap, at the sacrifice of
> some waveform amplitude (Franco compensation).  Moog-Hemsath compensation
> at the converter can be used to compensate for both reset time and Re error
> in practice, even though it doesn't have the correct functional form to
> compensate the reset error.
> Too short a reset time can lead to troubles such as increased spikes from
> parasitic capacitance, spikes on the PS lines leaking into other parts of
> the circuit, etc. That is probably one reason to use a JFET switch and live
> with the finite reset time. Around 0.5 us discharge time is typical.
>    Ian

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