[sdiy] VCO Cap Switch

[Jim Patchell]

Jay Schwichtenberg wrote:

> Hello,
>
> I love this group, I was going through major withdrawal when it was down.
>
> I was looking into the reset circuits in VCOs and have some questions. I'll
> also say I'm more into stability of the oscillator than exact waveforms.
>
> 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?

    I am not familiar with the 2n7000.  Is this a mosfet or a jfet.  My
preference for reseting caps is jfets.  The J111 is the one I use.  You can get
them from future-active, although, I am a little confused now by their webpage.
Their web page indicates 500 unit lots, their catalog shows 1 unit lots....

>
>
> Has anyone used them in their VCOs?
>
> Next is the comparator. Most designs use LM311 and LM393 comparators, but
> some the faster LM319.

    I would not use the LM393...way to slow...

    The problem with the LM319 is that it is a dual...If you are going to use it
in a VCO, I would not use the other half for anything...until I did extensive
testing to make sure there were no soft sync problems.  But, yeah, it is a bit
faster than the LM311...which is what I am using right now, since I aquired a
whole bunch of them quite a while ago (I love it when local companies go out of
business...except when I am looking for a job).

>
>
> Any comments on this?
>
> 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?
>
> Thanks in advance.
> Jay