[sdiy] Temperature Compensated Exponential ConverterUsingSSM2164

[cheater cheater]

> 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?

D.

On Tue, Sep 1, 2009 at 8:58 AM, Magnus
Danielson<magnus at rubidium.dyndns.org> wrote:
> David G. Dixon wrote:
>>
>> I've done a little bit more careful analysis over a more realistic
>> temperature range of 19 to 31 deg C, and -3V to +3V control voltage, and
>> have now determined that, within these ranges, assuming perfectly matched
>> 2164 VCAs, it should be possible to achieve 1V/oct conformance to within
>> 0.02%.  This means, for example, that if A440 is exactly 440Hz, then A880
>> will be within +/- 0.18Hz of 880Hz!  In other words, with proper trimming
>> of
>> the tempco voltage V_TC and the feedback resistance R_FB, virtually
>> perfect
>> tempco can be achieved.  However, one does depend on the other, and the
>> result is VERY sensitive to achieving the correct value of V_TC.
>>
>> In case you are curious, here are the relationships (assuming 1V/oct,
>> 3300ppm/K, and 33.3mV/dB):
>>
>> R_FB = R_CV * log(2)*[1.0033^(1.0033/0.0033)]/1.5 = 0.546
>>
>> (i.e., R_FB = 54.6k if R_CV = 100k) and
>>
>> V_TC = log(1.0033)*(1.0033/0.0033)/1.5 = 0.290
>>
>> The values are somewhat sensitive to the gain coefficient (1/1.5), but
>> completely insensitive to the gain temperature coefficient (0.0033).  Here
>> are a few sensitivity results based on minimization of the sum of square
>> errors to demonstrate the point:
>>
>> GC      GTC      R ratio   V_TC
>> ----------------------------------------------
>> 1/1.5   0.0033   0.54542    0.28945  (baseline)
>>
>> 1/1.5   0.0030   0.54544    0.28946
>> 1/1.5   0.0036   0.54540    0.28943
>>
>> 1/1.45  0.0033   0.56423    0.29943
>> 1/1.55  0.0033   0.52782    0.28011
>>
>> A change of +/- 10% in the GTC only changes the values at the fifth
>> decimal
>> place!  Similar changes to the GC have more significant effects, but these
>> changes are far beyond expected tolerance levels.
>>
>> I think we have a winner!
>
> The one thing I worry about, is how feasable it is to actually do these
> adjustments, especially in a typical DIY environment (I can use any of the
> climate ovens at work, a luxury which few has) and also how stable they are,
> if they need to be very carefully tuned.
>
> So, how would a practical tuning procedure of such a design work? We can
> play with all the math we want, but it doesn't help the normal guy. A more
> complex synth could use one or more DACs and computer control to do
> auto-tuning, but for sanity reasons I want to see what a manual trimming
> would include. Maybe include heater at the SSM2164 and cut some PCB around
> it so it becomes more termically isolated so that it can be forced to
> exercise different temperatures? I think it would be hard to trim this
> without temperature cycling.
>
> Maybe include a trimmer to the reference current could be necessary here, to
> make the scale and tempco trimmings much more direct as the normal offset
> and scale trimmings now have become offset, scale and tempco trimmings, so
> the traditional offset/scale trimming loop becomes troublesome. Adding the
> reference trimmer allows the unscaled and scaled offsets to be trimmed
> independently of the scale trimmer. Using that simplification the scale and
> tempco balance can be handled easier and more direct.
>
> Now that there seems like it is possible to get good cancelation, we need to
> look at the practicality of achieving it.
>
> I think we need to have some heater/tempsensor hooked to it only for the
> purpose of trimming.
>
> Cheers,
> Magnus
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