FETs on the ASM-1 VCO.

[Arthur Harrison]

-----Original Message-----
From: Fraser, Colin J <Colin.Fraser at scottishpower.plc.uk>
To: 'Arthur Harrison' <theremin1 at worldnet.att.net>; DIY
<synth-diy at mailhost.bpa.nl>
Date: Wednesday, December 16, 1998 3:23 AM
Subject: RE: FETs on the ASM-1 VCO.


AH:

>> 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!)

CF:

>I think I'll give this a try over my christmas holiday (if I can keep away
>from Tomb Raider 3 long enough...)
>I want to add a bit of extra logic to allow reset of the oscillator to a
>halfway point. This is to allow for normal oscillator sync plus an extra
>sync mode that resets a tri-wave output to the start or midpoint of its
>cycle depending on its current position.
>Maybe another sync mode with voltage control of reset point will be useful.
>This should be pretty easy with analogue switches, as long as I can find
one
>with very low on resistance.

>Colin f

AH:

In some theremin designs, I use an analog switch to reset the VCO to the
0v level.  This minimizes the "pop" when the oscillator is reinitiated.
By adding a little logic between the oscillator's output and the switch
control,
I can also synch the oscillator's turn-off to the first zero-crossing.
This minimizes the turn-off "pop" as well.
Your idea for selective synch seems especially nice for synth LFO apps.

-Art