Subject: Re: Wideband gain measurement
From: "John C. Price"
Content-type: text/plain; charset="US-ASCII"
NNTP-Posting-Date: Tue, 08 Oct 2002 14:11:50 GMT
Organization: AT&T Broadband
Date: Tue, 08 Oct 2002 14:11:50 GMT
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 error.
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 for
the lowest frequencies - maybe I will settle for comparing traces on a
On 10/8/02 1:48 AM, in article 3DA28DE4.1125B4FC@earthlink.net, "Robert
> Ban wrote:
>>> I have a measurement problem that I have not encountered before. I want to
>>> measure the gain vs. freq. of an amplifier that works from 1 Hz to 1 MHz,
>>> and I want measurements accurate to 2%. The thing has a nominal gain of
>>> and the input impedance is 8 pF in parallel with more than a G-Ohm. How
>>> I do it? No problem making rough measurements with a scope but I doubt
>>> 2% is possible. Should I get an HP 3400? I have an FFT spectrum analyzer
>>> that seems to do the job below 25 kHz, but what about above that?
>>> Thanks for any suggestions,
>> Rent a Network Analyzer.
>> ciao Ban
>> electronic hardware designer
> Only a few have that kind of accuracy...