FETs on the ASM-1 VCO.

[Arthur Harrison]

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.
Maxim and Analog Devices come to mind as possible sources.
(No pun intended!)

-Art

-----Original Message-----
From: Magnus Danielson <magnus at analogue.org>
To: synth-diy at mailhost.bpa.nl <synth-diy at mailhost.bpa.nl>
Cc: Magnus Danielson <magnus at analogue.org>
Date: Monday, December 14, 1998 3:37 PM
Subject: FETs on the ASM-1 VCO.


>Hi there!
>
>Since the questions keeps poping up of which FETs that might be
>usefull for the ASM-1 VCO reset curcuit (Q3 in my schematic).
>
>Let's collect information on what would work and what that would not
>work. There ought to be some experience on this out there, and maybe
>some people have made experiments.
>
>While at it, let's pop an idea of mine and see how it survives the
criticism.
>I have been considering using a MOSFET, that would be interesting
>since there is MOSFETs offering Rds(on) resistances way below 1 ohm,
>which could reduce the reset fall-time and thereby can the reset-time
>be reduced. The reduction of the reset-time can then reduce the R17
>resistance for the France compensation and thus the waveform
>distorsion experienced at higher frequencies. BTW, the compensation
>dimensioning work like this:
>
> t      = R   * C (1)
> reset    17    3
>
>Given C3 and treset we get
>
>       t
>        reset
> R   = ------ (2)
> 17     C
>          3
>
>The floor raise that R17 causes is linear with the control current and
>thus the frequency. Thus, it is obvious that R17 should be minimized
>while still having the effect of compensating the reset time and thus
>the reset time itself should be minimized, thus giving the reason for
>my discussion of the Rds(on) in Q3. Cutting the reset time is ofcourse
>also involving the cursuitry around U4, thus R18 and C3.
>
>Cheers,
>Magnus