[sdiy] VCO Cap Switch

Tim Ressel madhun2001 at yahoo.com
Tue Jun 4 18:54:33 CEST 2002

--- Jay Schwichtenberg <jays at aracnet.com> wrote:
> Basically is the capacitance and leakage current for
> the 2N7000 an issue for
> what we're doing?

Leakage will cause frequency errors. Capacitance is
probably small enough not to directly frequency, but
may slow down the switch which will affect it.

> Has anyone used them in their VCOs?

Not me. I use an NTE466. Seems to work fine.

> Next is the comparator. Most designs use LM311 and
> LM393 comparators, but
> some the faster LM319.

I haven't noticed the LM311 in my design to be a
significant source of error. Its delay time is very
short compared to the period of the oscillator. I have
noticed the integrator opamp can be a big factor. When
the FET shorts the cap, the current output of the
opamp spikes. The opamp I used didn't like that. A
nice modern expensive opamp fixed it.

> 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?

<Dream Mode On>
0.01% of the smallest period the oscillator.
<Dream Mode Off>

Most designs recognize the finite nature of reset and
compensates in the expo convertor with a bleeder setup
across the B-E junction. Plenty of examples.


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