[time-nuts] Noise source measurement

Sylvain RICHARD sr75pro at free.fr
Sat Oct 22 09:57:10 EDT 2011

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

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"

Have fun!


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