Rbe over temperature?

[Ian Fritz]

Hi Terry --

You're right -- it's the temperature derivative that has a Vbe factor, not
the voltage derivative.

Still, I think you need to be very sure that the tempco compensation is
correct before you look at higher-order effects like the temperature
dependence of the tracking correction. As you point out, this requires
precise low-frequency measurements.

  Ian


----- Original Message -----
From: "Terry Michaels" <104065.2340 at compuserve.com>
>
> Hi Ian:
>
> Sorry, I respectfully differ with you.  I said that Grant would see an
> incorrect scale factor setting, not necessarily incorrect pitch (or
> frequency).  Incorrect compensation for scale factor will cause a VCO to
> produce an incorrect frequency at all frequencies other than the point
> where the Vbe of the two expo transistors are equal.  It has exactly the
> same effect as the 1 volt/octave trimmer being set incorrectly.  All
> pitches will be off except at one point, the amount of error will increase
> as you go farther either side of that point.  That is a linear deviation
> from the correct 1 volt/octave VCO response.
>
> Grant said he had proper tracking at the lower frequency ranges, which to
> me indicates scale factor is properly adjusted and/or compensated there.
> Since tracking was off only on the high end of the range, his problem is
> due to a change in the Rbe/flyback time compensation.   That is a
> non-linear (or non straight line) deviation from the correct 1 volt/octave
> response.
>
> Of course this really depends on what Grant meant by "although the C2-C3
> range still looks OK. "  If tracking is perfect in that range, the problem
> has to be a change in the high frequency (Rbe) compensation.  OTOH,
> incorrect scale factor compensation will produce pitches that are very
> close to where they should be around the lower ranges of most VCO's which
> is usually where the Vbe's are equal.  Very close is not the same as
> perfect tracking, however.  The only way to be sure about this is to do
> very, very careful measurements of frequency vs. control voltage.
>
> Terry Michaels
>