FETs on the ASM-1 VCO.

[Magnus Danielson]

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