[time-nuts] Noise source measurement

Graham / KE9H timenut at austin.rr.com
Sat Oct 22 10:49:56 EDT 2011

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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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 
I think what you are measuring is more like -75dBm/Hz.

--- Graham / KE9H


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