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Subject: Re: Wideband gain measurement
X-Newsreader: Microsoft Outlook Express 6.00.2800.1106
Date: Wed, 09 Oct 2002 09:29:05 GMT
NNTP-Posting-Date: Wed, 09 Oct 2002 11:29:05 MET DST
"John C. Price" schrieb im Newsbeitrag
> Thanks for the suggestions. The gain is not inverting, but I have toyed
> with the idea of inverting and then summing the outputs, just by using A-B
> on a good scope. The problem is that the amp has phase shift near the
> limits of the passband just where its most important to know the gain
> This phase shift generates a quadrature signal if you sum the outputs, so
> one would need a phase sensitive ac null detector. A lock-in that works
> from 1 Hz to 1 MHz would be perfect, but not available. Right now I am
> thinking of making the precision resistive attenuator that Robert Baer
> suggests, and then comparing the output with the original signal using a
> pretty fancy wideband true rms voltmeter meter. Some of them work from 10
> Hz to better than 1 MHz with 1% accuracy, such as the Ballantine 323, if I
> have the model number correct. I would still have to do something else
> the lowest frequencies - maybe I will settle for comparing traces on a
> digital scope.
you can greatly increase the accuracy with a precision adjustable attenuator
and your scope as a display.
now adjust the generator that the amp is in a linear region without
overload, high enough out of the noise and hits a horizontal line(90%) on
set the attenuator course to-30dB and adjust the fine scale until both
traces are of equal height. Write down the value, it corresponds to the gain
of your amp.
do this for all frequencies.
Be sure to use the same probes and identical settings on both scope
channels. This way you can eliminate the errors of the scope and the
generator and can approach the accuracy of the attenuator +/- 1 line screen
resolution(8 bit and 80%screenheight it's already 1%). With an analog scope
you will get a higher accuracy!
Since the voltmeter cannot measure simultaneously (unless you have two), the
accuracy might not be as high because of short time variations and noise.
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