Re: Measuring signals with spectrum analyzer

JensGraf@xxxxxx a écrit :
On 24 Mai, 12:41, Fred Bartoli
<fred._canxxxel_this_bartoli@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote:
JensG...@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 BWHi
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.

I have some supplements:

Hi again,

Now i know what you mean with IF. I don't have a heterodyne Analyser.
I have an Analyser which is based on FFT (signal processing).
Hope that information helps.


In fact I was expecting that (from 25.6kHz top frequency) but this changes nothing.
The bandwidth here is related to the frequency bin width, i.e. span/number of bins. If you have it, check your manual.

--
Thanks,
Fred.
.

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