[sdiy] Temperature compensation results

[Ian Fritz]

At 07:19 AM 6/9/2003, René Schmitz wrote:

>>That's the point!  The added offset I'm describing is *not* PTAT, whereas 
>>an added CV would be, because it would be multiplied by the tempco resistance.
>
>I somehow was assuming that you would introduce this at the base, so it
>would be affected by the tempco (if thats also at the base).

Oh, now I see how we got crossed up!  I should have said that I was 
assuming the tempco was in the feedback configuration. Sorry for the 
confusion.  If you have the tempco resistor at the base (and there are good 
reasons to do so) then the "trick" compensation offset could be applied to 
the base of the other transistor, instead of the usual ground.

>>The voltage you normally apply to the base (deliberately) is proportional 
>>to the input control voltage:  Vb = const  Vin.  If the constant is PTAT, 
>>then the converter is properly compensated.  If the constant is not PTAT, 
>>say just some fixed number, then the frequency response goes as exp{A Vin 
>>/ T} where A is independent of T.  This gives a temperature drift, but it 
>>is a "scale factor" drift, because of the Vin factor.  In other words, 
>>the drift rate (d/dT)Ln(f) is proportional to Vin.
>>What I described in my trick is different. It is an offset voltage 
>>independent of Vin and of T, i.e., Vb = const.  In this case, f goes as 
>>exp{B / T}.  This gives a drift that is an "absolute drift", by which I 
>>mean (d/dT)Ln(f) is constant (independent of Vin).  This is then made to 
>>cancel other sources of absolute drift, such as the integrating cap's tempco.
>
>Its somehow ironic that what we always try to fight is our friend here.

Yes, indeed.  A subtle point I didn't mention is that the compensation may 
not have the same T dependence as the drift source, so the cancellation is 
correct only in the linear approximation.  I did a calculation to determine 
the residual error in compensating a 100 ppm/K linear error with the 
exp(B/T) term, and it was very small, about 10 ppm/K average over a 10 K range.

>>I agree!  I wasn't ever sure I would be able to get there, but it seems 
>>OK.  And of all the people selling VCO's who say they don't drift, where 
>>is there one willing to give actual numbers???
>
>Well that would certainly be interesting.

Yes, it would be interesting, but it wouldn't be in their interest, 
economically speaking.  What musician would appreciate the drift 
specifications?  Also, if you are making promises you have to worry about 
the long term drift, which is more difficult to address.  In other words, 
how long will will my tweaked-up 50 ppm/K drift performance hold up?  After 
all, lab-grade instrumentation has to be regularly recalibrated.

>>Boy, good question.  At this level I would hate to rely on batch 
>>consistancy for all the parts.  But probably you could get a good start 
>>by initially using the same settings and then tweaking a bit.
>
>Its also more the question if the residual (pre-tweak) drift is because
>of nonobvious design issues, or if its a matter of tolerance and
>accuracy of the components. Only the experiment can tell.

Yeah, statistical studies, including accelerated ageing tests.  :-)

   Ian