Re: Measuring signals with spectrum analyzer

JensGraf@xxxxxx a écrit :
Hallo,

I tried to measure the noise of a DSP-output when no signal is applied
to the output. I measured the "rest"-noise with an oszilloscope. The
amplitude of the signal seems to be about 2mV. Then i wanted to
analyse the power spectrum of the noise. I connected the DSP-output to
a spectrum analyzer.
Now the problem is that i get different results when changing the
frequancy span of the spectrum analyzer. When i measure from 10Hz to
3.2KHz, the sprectrum of the measured noise lies at about -100dBVrms.
Increasing the span (10Hz to 25,6KHz) leads to a spectrum that lies at
about -90dBVrms. Compared to the previous measurement it is
approximately 10dB higher. Has anybody an explanation for this effect?
How can i ensure a precise measurement of the noise?


What you probably see (probably because we don't know which SA you have and which measurement you did exactly) is this:
when you increase the frequency span, you also increase the SA IF bandwidth (most SAs have frequency span, IF and video BW coupled in order to make the measurement in a 'reasonable' time).

What you want is a noise power density (nV/rtHz or uV/rtHz) and what the SA gives you is nV or uV. For each analyzed frequency it gives the total noise power that is within the IF BW around the analyzed frequency (hope that's clear enough). As you increase span, IF BW increases and so does the noise power.

To convert to PSD you'll have to check in your manual how the noise BW of your SA is related to IF BW (it may not be identical, but often is), and then divide the noise power by the sqare root of the noise BW.

Then the obtained figures should be identical.

Some analyzers also have the possibility to directly do noise PSD measurements at some spot frequencies during the sweep. Check your manual for this.


--
Thanks,
Fred.
.

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