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# simple quesiton about spectrum analyzer

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#### jayleung

##### Member level 5
I have a simple question about the spectrum analzyer.

Is it used to measure the power spectrum density(PSD), so the measure output unit should be dBm/Hz or W/Hz?

But it seems the real world spectrum analyzer output is dBm. So how can it convert back to the PSD? Is it related to the IF filter BW?

And the other question is about the power meter. Is it supposed to test the power over the whole spectrum? What are the differences between the output of the power meter and the spectrum analzyer?

A Spectrum Analyzer actually is a selective receiver, and measured power is related to the RBW (IF filter BW), when a power meter is a broadband power detector.

In a spectrum analyzer measurement if you use a RBW greater than the bandwidth of the measured signal, the power reading should be the same as a power meter reading.

### jayleung

Points: 2
If the channel contains more than one carrier then it will be easy with SA than the Power meter which just add these powers and give you single numerical value.

### jayleung

Points: 2
As an example, lets say your spectrum analyzer was set to have a resolution bandwidth of 1 KHz. That means it is letting in 1000 times as much power to the power detector than if it was set to a 1 Hz resolution bandwidth.

You make a measurement using the marker function, and at a particular frequency point in the spectrum the spectrum analyzer says that there is -50 dBm at that point. What is it saying to you? It is saying that at that frequency point, the AVERAGE power across a +/- 500 Hz bandwidth was -50 dBm. Since 1000 Hz is 1000 times a 1 Hz bandwidth, and 1000 is a 30 dB power ratio, then the actual power at that frequency spot in dBm/Hz will be -50 dBm/1000Hz = -80 dBm/Hz.

Note that if the slope of the spectrum is varying too much, you will get a distored view of the dBm/Hz, since it averages over the entire RBW window of frequencies.

(note, since the spectrum analyzer does not have a "brick wall" filter inside of its IF, when the RBW is set at some number, you get a little more than that thru the filter. If you read your manual carefully, you will find a calibration fudge factor (a dB or two) to compensate for this. When you use a spectrum analyzer's "phase noise measurement function", it applies that fudge factor automatically for you).

Points: 2