On 10/22/2011 7:50 AM, Javier Herrero wrote:
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
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Javier:
When you say your Spectrum Analyzer bandwidth is set to 1 MHz, do you
mean the frequency span width of the screen?
I would think that your receiver bandwidth is the Video Bandwidth,
therefore 1 kHz.
Correction for noise bandwidth is an adjustment for power, proportional to
bandwidth, therefore 10log(BW1/BW2)
Correcting 1 KHz to 1 Hz is therefore a 30 dB adjustment, not a 60 dB
adjustment.
I think what you are measuring is more like -75dBm/Hz.
--- Graham / KE9H
==
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