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From: Chuck Simmons
Organization: You jest.
X-Mailer: Mozilla 4.61 [en] (X11; U; Linux 2.0.33 i586)
Subject: Re: R & C tempco repeatability
Date: Wed, 11 Sep 2002 13:41:52 GMT
NNTP-Posting-Date: Wed, 11 Sep 2002 06:41:52 PDT
fred bartoli wrote:
> Still for my 100kHz IF amplifier for which I still want really stable phase
> The requirements are such that low tempcos of R and C are not low enough to
> guarantee the specs for the active filters stages.
> Of course I can stabilize the temperature, or measure it and then apply a
> measured tempco to correct the value, but this is an added level of
> complexity I'd prefer not to have.
> For the active filters I can find at least a simple structure (Fliege) for
> which pairs of identical components provides opposite sensitivities for the
> Q and center freq, hence perfectly summing to zero *if* identical components
> have the same tempcos.
> Unfortunately, all the low tempco components I've found are specified +/-
> The way I understand that is that the manufacturer guarantee the tempco
> value for all the components range to be within the given band.
> But I guess that components value has some influence on the tempco versus
> value distribution, and the point that really interest me is how identical
> value components tempcos are related.
> Anyone having some experience with this component tempco repeatability ?
> How close can they come ?
> The components range is for an RC product of 1.59uS
I had one experience wrestling with this when I worked on a drift test
for V-F converters as a test engineer 20 odd years ago at Burr-Brown. I
had to put the RC components in the oven to avoid excessive "pulling"
from perhaps 100 parts in the oven at once. At that time I found
resistors that were repeatable enough (I had to call the manufacture to
get actual test data). Capacitors were a major problem. The ones I ended
up with were Corning glass capacitors in military specification at about
$40 each. These were specified very tightly so that frequency error due
to the capacitor could be predicted and removed from the drift test
data. I don't think Corning makes these anymore but I believe there is a
company that still makes glass capacitors. You might try to track down
glass capacitors and see if there is a grade of them that will work.
Zero tempco in capacitors with small spread in tempco is not something
you will find. The glass capacitors had a small nominal (+5ppm per
degree C is what I remember) with a very narrow range around it.
... The times have been,
That, when the brains were out,
the man would die. ... Macbeth
Chuck Simmons email@example.com
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