[sdiy] Temperature Compensated Exponential ConverterUsingSSM2164

[cheater cheater]

I dunno about your synths, but mine get pretty warm.
I'd go all the way up to 50-60 degrees with my area of interest.
Bear in mind the whole VCO won't be the same temp - instead some parts
will be hotter while others will be colder. Which means stuff doesn't
extrapolate that easily.

D.

On Tue, Sep 1, 2009 at 7:08 PM, David G. Dixon<dixon at interchange.ubc.ca> wrote:
> 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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