From: firstname.lastname@example.org (Tom Bruhns)
Subject: Re: Wideband gain measurement
Date: 9 Oct 2002 12:06:21 -0700
NNTP-Posting-Date: 9 Oct 2002 19:06:22 GMT
"John C. Price" wrote in message news:...
> 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 50
> and the input impedance is 8 pF in parallel with more than a G-Ohm. How do
> I do it? No problem making rough measurements with a scope but I doubt that
> 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?
As someone else suggested, a network analyzer; but also a precision
attenuator to check the analyzer's absolute accuracy. I don't believe
I'd have any trouble making the measurement with my Agilent 89410.
The rated absolute accuracy is +/-0.5dB, but you can easily check that
at zero dB "gain" by putting the same signal into both channels, and
then put your amplifier plus the attenuator into one path, and adjust
the attenuator for nominally 0dB net gain. The 89410 linearity is
plenty good enough for your measurement.
Out of curosity, I put the same (periodic chirp) signal into both
inputs of my 89410, and displaying frequency response between the two
inputs I see max deviation of about 0.012dB from zero--about 0.14%
error, from DC to 1MHz. Even that you could cal out if you wanted.
Then if you keep the input channels on the same ranges, it's just a
matter of relative accuracy to account for your not being able to
match your amplifier's gain exactly with your attenuator, and for the
89410, that's extremely good. (Properly done FFT-based analyzers
inherently have extremely good relative amplitude accuracy, as long as
you don't switch the input analog gain around.)
I suppose the other thing you should account for is the loading caused
by the 8pF... If you drive the input with a 50 ohm source, terminated
in 50 ohms at your amplifier's input, then at 1MHz there will be a
slight rolloff because of that capacitance. But it's small compared
with your desired accuracy, so shouldn't be an issue. In fact,
perhaps you need to understand that effect for the source impedance
that the amplifier will ultimately be used with anyway.