[sdiy] Temperature Compensated Exponential ConverterUsingSSM2164

[David G. Dixon]

This may be stupid, but...

What about a theoretical approach?  We already know that the trim values are
not sensitive to the gain temperature coefficient.  What if we just measure
the gain coefficient of the VCAs, calculate the necessary feedback resistor
value, install the nearest 1% resistor and/or trim to that value (by
measuring the resistance with a DVM), and just tune the VCO with the tempco
voltage trim by eliminating the beats at a two octave interval?

Alternatively, one could simply apply the theoretical resistance of 54.5k
and trim the tempco voltage.  According to my calculations, even if the
inverse gain coefficient is off by +/- 0.05 from the stated value of 1.5
(which is probably severe), the worst-case tempco conformance is still only
0.16% at 54.5k, at a CV of -3V, at 31 deg C.

It seems to me that the whole point of this design is that it compensates
for temperature very effectively.  It should be that much easier to achieve
excellent performance by trimming, not harder.  Why not take advantage of
this fact, using the math we know to be true?


> >>>I think we need to have some heater/tempsensor hooked to it only
> >>>for the
> >>>purpose of trimming.
> >>
> >>How will you know your temp sensor isn't lying?
> >
> >How precise do we need it to be? Maxim, for instance, makes some nice
> >digital temperature sensors with up to +/- 0.5 °C precision. I think
> >the bigger problem is to keep the sensor and the device under test at
> >the same temperature.
> 
> Yeah, I just used the old National whatever deg Kelvin sensor.  Being off
> by a couple of degrees doesn't much matter, especially since you only use
> the *change* in temperature.
> 
> I mount the board in an Al box, which spreads the heat out quite
> evenly.  As an extra, I surround the board with crumpled-up newspaper to
> eliminate drafts.  Oh, and I mounted the T sensor close to or incontact
> with the expo pair. Even if the sensor isn't well coupled it doesn't
> matter
> much, again because we are interested in temperature *differences*.
> 
> We do not need super precise or accurate results here.  But I agree that
> previous experience in doing this kind of work does help.
> 
> 
> Ian
> 
> 
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