[sdiy] Temperature compensation results

[René Schmitz]

Hi Ian,

> 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).

> 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.

> I hope the above helps.  I've spent some time over the last month 
> analyzing drift processes, including detailed analyses of ideal and 
> non-ideal tempco resistors and of active compensation schemes.  Jim 
> Patchell was the one who pointed out to me the importance of 
> distinguishing between absolute and scale-factor drift.  I hope I can 
> find some time to write all this up, as it is easy to get confused on.

Sure it helps. Thank you!

> 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.

> 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.

Cheers,
  René

-- 
uzs159 at uni-bonn.de
http://www.uni-bonn.de/~uzs159