[sdiy] VCO Cap Switch

Takuya KATAYAMA cxh02175 at nifty.com
Tue Jun 4 20:14:43 CEST 2002


Hello Jay and the Lists!

My answer for the first question:
I've used some 2N7000s as analog switching device, its good enough in
audio frequency speed switching.
But, I think, I cannot recommend it as reset switch in V-F convertor /
VCO, because of its large off-state leakage current.
Generally speaking, MOS FETs have internal diodes across G-S or G-D for
static-discharge protection.
And these diodes are NOT good at reverse leakage current characteristics.
My personal experience says that use J-FETs for reset switch.

Answer for Second question:
Comparator-speed is effective to decide VCO's max. frequency and linearity.
But the max. freq. up to 20 kHz, there are nearly no difference between
311,339,319 even if 710!
It is effective to choice faster one in applications over 50 kHz
oscillation.
(If I make frequency up to 100 kHz VCO, use V-F converter IC such as LM331.)

==============================================================================
Takuya KATAYAMA, Muse Music Synthesizer Lab., Japan


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






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