Le 22/10/2011 14:50, Javier Herrero a écrit :
Hello all,
Perhaps a bit OT, but I'm measuring the output noise density of a
noise source at a puntual frequency. I've fed the noise output to a
8566B spectrum analyzer, BW set to 1MHz and video BW set to 1kHz so
the displayed trace is flat. I obtain a measurement of -45dBm, and I
understand that the noise density then is -105dBm/Hz.
From design variables, I was expecting a somewhat lower value, around
-110dBm/Hz, but between the NoiseCom noise source and the output there
are several things (attenuator, filter, amplifier, directional
coupler, variable attenuator, ...), so perhaps there are slight
differences between estimated insertion gains and losses accumulate up
to 5dB. Before dismount the system and look directly at the noise
source output and measure the losses/gains of each element, I would
like to know if I am doing this mesasurement right or am I commiting
some mistake?
Thanks! Best regards,
Javier, EA1CRB
Javier,
Short answer:
1) make sure that the noise level increases at least 10 dB when you
connect the noise source.
2) check that there is no hidden compression by manually controlling the
input attenuator and (not sure if this works on a 8566B) the IF amps /
reference level. Levels should remain constant. Don't fry your input
mixer: -10 dBm max for specified linearity. Total power in your case =
-105 dBm/Hz in many gigahertz plus (unknown) amplification.
3) use the noise marker (shift+M) to take care of the well-known ~2 dB
difference between proper power detection and what the analyzer does.
Long answer:
Agilent appnote 1303, aka 5966-4008E "Spectrum and Signal Analyzer
Measurements and Noise"
http://cp.literature.agilent.com/litweb/pdf/5966-4008E.pdf
Have fun!
--
Sylvain RICHARD
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