[sdiy] Temperature Compensated VCO attempt - help?

Magnus Danielson cfmd at swipnet.se
Sun Feb 2 02:32:31 CET 2003


From: Ian Fritz <ijfritz at earthlink.net>
Subject: Re: [sdiy] Temperature Compensated VCO attempt - help?
Date: Sat, 01 Feb 2003 18:02:08 -0700

Ian,

> Once again I have to remind you folks that it is totally meaningless to 
> give a drift figure unless you also give the corresponding deltaVbe.  At 
> deltaBve=0 there is no drift, and drift increases as deltaVbe deviates from 
> 0.  The uncompensated case is NOT 3300 ppm/C.  That is simply the 
> coefficient of 1/T, before being multiplied by deltaVbe.  It is not the 
> drift of the output current.

Correct observation, but in the used form is only for very rought comparision
of degree of improvement, as in how many decades better. Also, we are talking
about a multiplicative compensation of 1/T since we do multiply it by T. The
curve we do end up is certainly not also a linear curve, and it takes some
carefull measurements to estimate this curve and its characteristics.

So, in this case I think the comment where usefull only as a friendly reminder
but not very applicable to the situation at hand. The compensation scheme
discussed is targeted much more carefully than the classic tempco-resistor
approach and if correctly applied has much stronger theory for being relative
flat over a much wider range of frequencies.

I will have to look more carefully at Jim's work, as a complementary study, but
Scott's attempt was a darn near shoot IMHO.

Cheers,
Magnus



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