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aaron_do

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Noise Measurement on Spectrum Analyzer
Apr 26^th, 2007, 12:06am

Hi all,

I want to do an input referred noise (IRN) measurement for a 2 MHz low pass filter. I have a spectrum analyzer and a noise source (minimum noise level output is -30 dBm). for now i just want a rough measurement, and i'll get better equipment later. How would I measure the IRN?

The noise source outputs -30 dBm, so I assume that is the total integrated power. At the spectrum analyzer, I read about -60 dBm at the passband frequency with a resolution bandwidth of 100Hz. So my thinking is that i have to integrate the noise over the entire bandwidth. So i was thinking as an approximation i could use the -3 dB bandwidth divided by the resolution bandwidth (of the spectrum analyzer) and multiply that by -60 dBm. Next divide by the gain of the filter, and subtract the source noise (-30 dBm). So that would be the answer.

Is that correct? Also what is a good way to integrate the noise over the desired bandwidth?

thanks,
Aaron

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there is no energy in matter other than that received from the environment - Nikola Tesla

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mg777

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Re: Noise Measurement on Spectrum Analyzer
Reply #1 - May 5^th, 2007, 8:35am

First, do you see the filter shape on the spectrum analyzer without any input? If you don't see a shape then it means the analyzer's grass dominates. You'll have to broadband amplify the LPF output before going to the analyzer.

Next connect the noise source thru a variable attenuator (if you're using a NoiseCom source they usually have an attenuator built in). Twiddle the att till you get a 3 dB increase in the low-freq noise. The two PSD's will be approximately equal at this point. With broadband noise don't mess with integration. Simply use the analyzer's PSD function - modern analyzers have very good DSP-based PSD estimators.

M.G.Rajan
www.eecalc.com

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aaron_do Senior Fellow Offline Posts: 1398	Re: Noise Measurement on Spectrum Analyzer Reply #2 - May 5^th, 2007, 6:52pm thanks for the reply, i'm still a little confused though...which two PSDs are you talking about: the PSD from the noise source to the DUT to the spectrum analyzer and? thanks, Aaron
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mg777 Senior Member Offline Posts: 131	Re: Noise Measurement on Spectrum Analyzer Reply #3 - May 5^th, 2007, 7:14pm The PSD of your low-pass filter noise and the PSD of the noise source with attenuator. There's also the issue of thermal noise if your LPF has a resistive input termination. M.G.Rajan
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