[time-nuts] Noise source measurement

Mike Feher mfeher at eozinc.com
Sat Oct 22 15:06:44 UTC 2011


Javier -

I believe you are indeed close. With the IF BW at 1 MHz the correction is
indeed 60 dB. The post detection filtering by the video filter only makes it
look nicer but does not come into play. I believe there are two correction
factors that you need to take into consideration, that, as I recall, comes
close to about 2.5 dB total. I may be off on that number, but, it is close
to that (I am sure you can Google it). You will read more because the filter
BW is off course not perfect, and the equivalent noise BW is greater than 1
MHz. I believe there is a correction for that. The other, is how the
detector inside the analyzer treats noise instead of a pure sine wave. Those
corrections will get you closer to your expected value, and, maybe the
others are due to your losses. 73 - Mike 

Mike B. Feher, EOZ Inc.
89 Arnold Blvd.
Howell, NJ, 07731
732-886-5960 (B)
908-902-3831 (C)

-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of Javier Herrero
Sent: Saturday, October 22, 2011 8:51 AM
To: Discussion of precise time and frequency measurement
Subject: [time-nuts] Noise source measurement

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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