FETs on the ASM-1 VCO.

[Arthur Harrison]

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.


JV:

>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.

AH:

There is some concern if the value of the integrating capacitor is
relatively large.
A precise determination can be made by calculating the joules of
energy dissipated by the switch.  For a vast majority of synth applications,
there should be no danger to the switch whatsoever.  These switches,
after all, are integrated versions of the discrete JFETs or MOSFETs
routinely
used in all kinds of discharge applications.

If crosstalk is a concern, individual switches are available in eight-pin
packages.
The MAX319 is one such switch.

JV:

>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).

AH:

In such cases, you may have to contend with marginal Vgs conditions in
which the switch can not be fully enhanced.  The nice thing about many
integrated switches is that they contain the level shifters and other
goodies
to ensure proper enhancement of the output switch, regardless of Vds.
Also, the control can be treated as any logic input.

JV:

>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...

AH:

Agreed, limiting is a good idea in any case, just as bandwidth limiting is
a good idea in signal paths.

JV:

>I've posted a schematic at
>http://www.netcontrol.fi/~jocke/circuits/osc1.gif

AH:

I can see the reset limiter as R1, and that seems fine.  I was a little
concerned,
though, about possible instabilites in U1A resulting from C1's direct
connection
to its output.  Can C1 be moved across R6, instead, or does the transient
response of Q1's source node degrade too much?  Have you measured the
linearity of this oscillator and R1's relationship to linearity?

regards, Art