[sdiy] VCO Cap Switch

[Ian Fritz]

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 
MOSFETs.

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