[sdiy] CA3080 switch driving voltage

[Ian Fritz]

Hi JH --

That was my old work on my ultrasonic VCO (Electronotes #112).

I found a 60 mV drive to give the best tracking.  I'm still waiting for a 
good explanation of this effect.  Maybe now that I have a better 'scope I 
could characterize it better.

Please let me know if you figure it out.  It seems that the chip doesn't 
like the input transistors to be saturated, for some reason.

   Ian


At 01:24 PM 9/15/2003, jhaible wrote:
>I remember I've seen this somewhere (Electronots?), but
>I don't remember _where_.
>
>If you build a triangle VCO core,
>opamp integrator, 3080 providing the integration current,
>schmitt trigger to switch the 3080's current direction,
>
>what is the optimal voltage and impedance to drive
>the 3080 input, in order to get the best linearity
>of frequency vs. control current ?
>
>At first, you might think maximum overdrive with
>low impedance would be best (like 5V and 0 Ohm)
>to get immediate switching (no RC time constant
>with parasitic C's), but apparently such a configuration
>is "too much" and degrades the linearity of
>Iabc -> Iout operation. (Yes, speaking of switching!
>Maybe base current? Maybe too much common
>mode signal? Don't know ...)
>
>So, where is the optimum? I vaguely remember
>a resistance of a few hundred ohms, but I
>don't remember the voltage at all. Certainly
>more than 100mV, certainly less than 5V.
>
>JH. (Working on an improoved version
>         of the FS-1)