FETs on the ASM-1 VCO.

[Joachim Verghese]

Arthur Harrison wrote:

> Has anyone considered using an integrated analog switch
> for the integrator reset?  (Or would that disqualify the oscillator
> from being a true "clone?")  These parts are available in a
> variety of form factors, and some newer devices have Rds(ON)
> specifications of 5 ohms, and very little charge injection.

I'm not sure how IC switches will react to the high discharge
current spike with such a low on-resistance. Also, I fear crosstalk
between switches may be a problem.

As for using MOSFETs, I've found that they work well in circuits
where the integrating cap is (dis)charged against a constant voltage
rail, and less well in designs with the cap in the feedback path
of an op-amp (I guess this was already mentioned).

One VCO design I've used a lot uses a BS250 P-channel MOSFET to
discharge the integrating cap to a local +5V rail. I guess almost
any P-channel MOSFET would so the job as long as its ABS(Vgs) is
around 3 or 4 volts, maximum.

I've deliberately slowed down the reset time to around 1 microsecond,
because I feel it doesn't need to be faster as long as you're working
in the audio range. Less chance of inter-VCO lock-up, etc...

I've posted a schematic at

http://www.netcontrol.fi/~jocke/circuits/osc1.gif


cheers,

-joachim