# How to Measure Phase Noise and Power Spectral Density - RF Cafe Forums

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arool
Post subject: How to measure Phase Noise and Power Spectral Density
Unread postPosted: Thu Dec 30, 2004 5:04 am

How to measure Phase Noise (dBc/Hz) and Power Spectral Density (dBm/Hz) using Spectrum Analyzer?

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Itay
Post subject:
Unread postPosted: Thu Dec 30, 2004 2:07 pm

Greetings Arool,

In most modern Spectrum Analyzers there is a built in feature that measures the Phase Noise and plots it in logarithmic graph: Phase Noise vs. Frequency.

However, if you are using older Spectrum Analyzer, you have to bring the carrier to the center by using MARKER>CF, then placing the reference marker on the carrier and moving the delta marker to the desired offset. The reading would be the Phase Noise (dBc/Hz). You have to remember to add the attenuation of the RBW filter= 10 log RBW to this reading. This filter further attenuates the Phase Noise. I am not sure if you can get the Spectral Density VALUE with Spectrum Analyzer. But you can measure the outpot power by Power Meter and make the calculation based on the Phase Noise result.

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arool
Post subject: PSD measurement
Unread postPosted: Fri Dec 31, 2004 4:54 am

PSD is used when measuring continuous broadband noise, and normalizes the power to an equivalent bandwidth of 1 Hz, irrespective of the actual bandwidth of the filter being used. For example, if a signal is measured at -93 dB in a 10 Hz bandwidth, then the spectral density would be -103 dB (in a 1 Hz bandwidth). But make sure that the sweep time in a given RBW = Span / RBW in sec. This makes it possible to compare noise measurements made with different bandwidth settings of the spectrum analyzer.

Posted  11/12/2012

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