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Pulsed RF signal Measurement using a spectrum analyser

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Newbie level 6
Nov 19, 2004
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rf signal measurement


I am trying to carry out some measurements for RF pulsed signals using a spectrum analyzer.

The Spectrum analyser model that I am using is a R & S FSP.

I am trying to get its mean power, mainlobe's peak power, etc. However, the problem that I am facing is that RBW seems to affect my measurements significantly. How should I go about choosing the right RBW and VBW? Are there some guidelines that I should follow?


pulsed signal spectrum analysis


Spectrum analyzer is like a power meter with a bandpass filter in front of it. If the bandwidth of the bandpass filter is too small, you do not let all of the RF power be detected. To complicate things, the center frequency of the bandpass filter is constantly changing, so if a pulse occurs when the bandpass filter is not tuned to that frequency, you again do not measure any power.

The first thing that you might want to do is set the analyzer so it is cenetered on the frequency desired and not sweeping. Then increase the RBW until you see no increase in signal power detected.

VBW just filters out changes (like pulsing power), so you want this set as wide as possible.

Average vs peak? Depends on the analyzer. Some only measure average, some can do both. Read the manual.

rf bandpass filter with spectrum analyzer

You have to set the Spectrum Analyzer for “Zero Span” to be able to measure Power vs Time.
Have to set the RBW of the internal filters wide enough to don’t affect the power measurement.

Read papers like these:
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spectrum analyzer in rf signal measurement


Thanks for the previous inputs.

I have tried the "zero span" mode and have managed to carry out some measurements.

However, I am having some problems with the RBW. I have tried using a signal generator to send a signal of a specified PRF and pulse width, and I have tried to set the RBW to the inverse of the pulse width as specified in the manual. But the power readings changes significantly even as I increase the RBW above the BW of the expected signal. Should I set my RBW to the max no matter the PRF and pulse width of the signal?

In the event that my signal BW is larger than my max level of RBW, how should I measure the Power level then?


pulse repetition frequency

If the pulsed signal BW reach the limit of your Spectrum Analyzer RBW, probably you have to use a dedicated RF Power Meter that have option Pulse Triggered Mode, like one of these:
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