Ping: Gurus. MAT-04 EXPO for (2) VCO

[Jim Patchell]

    I am working on the same problems at the moment, although, I have no
results at this time to share (because I haven't been able to touch my
soldering iron for a couple of weeks, DARN!), I will share my thoughts...

    I don't really see where thermal effects will be too much problem.
The voltage across the exponential transistors is only 0.6 volts and if
the current gets up to 1 ma, this is only 0.6 mWatts.

    The soft sync issure is a little more dicey.  There is going to be
some finite capacitive coupling between the two halves.  Using pins
1,2,3-14,13,12 combinations looks like would minimize this since there in
a conductive trace from one substrate pin to the other right across the
chip that should help to minimize coupling.  Another option would be one
suggested by J.H. to me when I asked a similar question and that is to add
a cascode connected transistor to help isolate the exponential pairs from
the oscilator.  This does cause extra complexity, because you have to drop
the voltage on the exponential pair another diode drop down.  It may be
that the cascode transistor is not needed since the exponential converter
is feeding into the inverting input of an opamp.  If the opamp can keep
this point at a virtual ground, even when the capacitor is discharging,
you might not have any problem.  But, this is something that somebody has
to try.

    The mini modular might not be a good platform to try this on, since
the PC board was not layed out for it.  You will have to have some
relatively long connections to some rather sensitive points in the
circuit.

    Just my opinions....

    -Jim

Harry Bissell wrote:

> I'm building the Chris MacDonald "Mini-Modular"
>
> I'm considering using the MAT-04 for the expo... in fact I'm even
> thinking
> on ONE MAT-04 shared by two oscillators.
>
> The MAT-04 has a separate substrate so that should not be a problem...
> but
> what about soft-sync, or even thermal effects between transistor pairs
> ???
>
> Comments please...
>
> H^)   harry  (i've never really done this sort of thing before... have
> you ????)