Measuring signals with spectrum analyzer

snipped-for-privacy@gmx.de a écrit :

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.

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Thanks,
Fred.

Hello Fred,

i hope, i got you right. But even if the IF (IF=3Dinput filter? antialiasing filter?) reduces the bandwith when a lower span is chosen, nevertheless the lower frequencies must be in the lower span with the same power for these lower frequencies are not filtered and so the power should not be less than with a wider span. If that would be the case, then the same phenomena would occure with much larger signals but it doesn't. I'm i right? Or do i get you completely wrong?

Thanks,

Jens

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.

Regards

Jens

snipped-for-privacy@gmx.de a écrit :

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.

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Thanks,
Fred.

snipped-for-privacy@gmx.de a écrit :

IF = intermediate frequency amplifier. An heterodyne SA is nothing but a precision receiver.

reduces the

I'm not sure I understand what you mean. Well, in fact I'm sure I don't understand at all.

Same.

For a given spectral noise density, the wider the filter bandwidth, the higher the power you'll observe at the filter output...

...and SA just display this power.

To get noise spectral *density* you have to relate the observed power to the bandwidth.

For a discrete signal spectrum the effect is different: widening the IF BW just (artificially) widens the displayed rays width.

Have a look at agilent app note (AN150 I think) "spectrum analysis basics".

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Thanks,
Fred.

All the figures look self consistent. Would seem you have something like a

20MHz bandwidth oscilloscope. The 10dB change you're seeing, emphatically implies that the spectrum analyser automatically increased it's measurement bandwidth by 10 times, when you switched from the small 3200HZ span to the much wider 25600Hz. You will have an option to override this 'auto bandwidth/sweep time' function and switch to a 'manual' filter setting. It a sensible world it should be described on page #1 of the operator manual :)

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Posted via a free Usenet account from http://www.teranews.com

Hello Fred, hello John,

thank you for your help. Fred: This Application Note helps a lot. John: Currently i have no time to care about the problem but i will pick your advice next week.

Again, thank you for your help.

Jens