FETs on the ASM-1 VCO.

[Ian Fritz]

Magnus --

We discussed this a few months ago. What you write is fine as far as it
goes, but, as Terry Michaels pointed out, MOSFETS have a disadvantage in
capacitance, and also in leakage. In addition, if you are using an opamp
integrator, the rate at which the output can slew also matters.  It
would be interesting to see some actual data, though -- details on
spikes in the waveforms and on tuning accuracy and range. 

BTW, my updated version of Terry's VCO switches pretty fast and only
needs a touch of HF compensation. Maybe it could be improved, but it's
hard to see why you would need anything better -- unless you
specifically need ultrasonic frequencies. 

I've tried unsuccesfully several times to download your ASM-1
schematics, so I still don't know exactly what Gene's version of this
circuit looks like.

  Ian
  http://home.earthlink.net/~ijfritz


Magnus Danielson wrote:
> 
> 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