[sdiy] Temperature Compensated Exponential ConverterUsingSSM2164

[Magnus Danielson]

David G. Dixon 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?

I think you need to trim it since values varies and repeatability of a 
design needs to be achieved. Also, how do you repeatably achieve 
0,2678345615807 V or whatever we crank out as the optimal voltage?

> 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?

Carefull now! You are limited by the simplification of the models 
involved and that components change properties and is not perfectly 
repeatable, as well as they drift... when they age semiconductor doping 
slowly disperse... so there is no fixed setting and it never looks the 
same from device to device. There is no real accuracy and repeatability, 
but by adjusting it properly it looks like very good stability can be 
achieved and from that you can trim to some decent accuracy. Nothing 
stellar, but workable for the application at hand.

For instance, if the voltage is important, how it the repeatability of 
it achieved?

One thing which hits me is that the cancelation of V+ (+15V -> GND) that 
occurs in the traditional saw-oscillators (say ASM-1) can only be 
achieved with this design if V+ is nicely mirrored down to V-. An 
alternative is that the comparator voltage somehow is derived from V- 
rather than V+.

Cheers,
Magnus