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Reply-To: "fred bartoli"
From: "fred bartoli"
Subject: Re: R & C tempco repeatability
Date: Thu, 12 Sep 2002 11:46:03 +0200
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NNTP-Posting-Date: 12 Sep 2002 11:44:22 MEST
Bill sloman a écrit dans le message :
> "fred bartoli" wrote in message
> > Still for my 100kHz IF amplifier for which I still want really stable
> > deviation.
> > The requirements are such that low tempcos of R and C are not low enough
> > guarantee the specs for the active filters stages.
> > Of course I can stabilize the temperature, or measure it and then apply
> > 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)
> > which pairs of identical components provides opposite sensitivities for
> > Q and center freq, hence perfectly summing to zero *if* identical
> > have the same tempcos.
> > Unfortunately, all the low tempco components I've found are specified
> > something.
> > 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
> > value distribution, and the point that really interest me is how
> > 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
> Why don't you use a super-hetrodyne/lock-in system?
> Demodulate in-phase and in quadrature with a pair of synchronous
> crystal-controlled 100kHz clocks, which can be very stable indeed
> (particularly if you are dividing down from a good 10MHz
> source) and absolutely precisely in quadrature, then look at the product
> frquencies with the bandwidth and precision you actually want.
you're precisely describing what I'm planning to do : my app is a
DC->200/500MHz VNA and there will be a pair of IF/synchronous demodulator
boards. My concern is about the IF phase drift vs temperature. If not well
matched between the 2 channels (or corrected) this phase drift will induce
phase error in the result.
If I stick to the easily findable components, I have for each filter stage :
2 x 30ppm/°C for the caps
2 x 10ppm/°C for the resistors
for a total of 80ppm/°C, not including the opamps GBW tempco effects.
A Q of 10, at 100kHz give about 11°/KHz of phase drift, hence a total (for a
change by 10°C) :
80ppm/°C x 100kHz x 10°C x 11°/kHz = 0.88° of drift per IF board.
If I don't assume some degree of matching in tempcos, this is a possible
total error of 1.8° per 10°C.
My target is almost 100 smaller for this part of the analyser.
If I assume a tenfold improvement as suggest Spehro, I'm still off by 10
As I don't want to go the high price way (glass caps...) and also don't want
to control the temperature (to reach the target I'll need a 0.1°C control)
it seems the best way is a time multiplexed calibration. (maybe 1 point out
of 100 or 200)
I did rejected this at first because I was afraid of cal signal leakages :
the input refered noise floor is 0.15uV (6kHz BW), but sync demodulation
will easily lower this.
Now considering all the implications it seems I have no other choice and
after some thinking it may not be as difficult as I first thought.
This will also rise the point of the demodulators clocks leakages : can I
put the IF amp and the demodulator on the same board ? A (huge :-) ) 100nv
leakage on a 100R impedance from a 5V source at 100kHz is a 0.3fF parasitics
!!! All that not considering ground plane pollution.
And maybe I should want 10nV or 1nV to exploit all the possible dynamic
range of the detector.
Do you have some experience about this (demodulator and IF on the same
> Bill Sloman, Nijmegen
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