# how to use a spectrum analyzer?

This is a new topic to me so it would be great if someone could give me a few pointers. I have an amplifier that I would like to get the frequency response for and a spectrum analyser with Probes A, B, and an Input probe.

My understanding is that Probe A and the Input probe go at the input of my amplifier, and Probe B is at the output. I have read that you do not want to put too much voltage in the Input probe, as this will probably blow up the spectrum analyzer. My question is regarding Probe B. My output voltage is designed to be over 150V (minimum), so do I need to step down this value before measuring with Probe B? It seems to me that if I put a voltage divider to make this output voltage smaller, it would affect the magnitude reading of my frequency response. Is this true, and if so, what should I do about this? Or would it be fine to place Probe B at my output?

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Do you have a model number for the spectrum analyzer? –  Scott Seidman Sep 18 '12 at 21:16
Thanks for your replies Scott. Unfortunately, i do not have the model number for it. Just looking for some general information here. –  suzu Sep 19 '12 at 0:39
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## 2 Answers

A voltage divider, assuming the amp has low output impedance and the analyzer has high input impedance, should affect all frequencies equally, so you don't care all that much. In fact, since log(AB)=log(A)+log(B), voltage division just means you are subtracting a constant from all dB readings. What frequency range are you interested in, by the way? Sounds like you're interested in audio, but I just want to make sure we're not talking about very high frequencies, where non-ideal resistor-type things might have a real impact.

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I am interested in sub MHz frequencies, mostly in the range of kHz. So it seems like you would just have to multiply the gain of the voltage divider (Vo/Vin) to the frequency response? or could you say whatever divider you put there wouldn't matter, as there is infinite input impedance at the input to the spectrum analyzer anyway? –  suzu Sep 19 '12 at 0:37
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Normally a spectrum analyzer does not produce any output signal, and it does not provide any stimulus to your device under test (DUT).

If you want to test amplifier gain using a spectrum analyzer, you also need to have something like a synthesizer to provide stimulus to the DUT. Connect the synthesizer to your DUT's input; connect the DUT output to the spectrum analyzer. Then the spectrum analyzer tells you the amplitude output from the DUT. You can also use the spectrum analyzer to measure harmonic distortion (by looking at the output at harmonic frequencies), or to measure the DUT's output noise (by turning off the input signal).

If you have an instrument that provides a single-frequency stimulus and then automatically measures the output as the input frequency is swept, that is called a network analyzer, not a spectrum analyzer. While the network analyzer is usually more convenient for measuring gain (and return loss) of an amplifier, it's (except for very new models) not able to measure harmonic distortion or noise.

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So between the DUT and the spectrum analyzer, there is no need to step down the output voltage of the DUT? I'm just not sure the Probe B can handle this voltage if it is large. –  suzu Sep 19 '12 at 0:33
You'll need to look at the specs for your specific spectrum analyzer. There will be a maximum input power. If the output of your DUT is more than that, you need to reduce the power, maybe using an attenuator, before applying it to the spectrum analyzer input. –  The Photon Sep 19 '12 at 0:40
I am assuming your DUT is designed for input and output to/from a 50 Ohm RF system, because that is the type of thing that spectrum analyzers are normally used to test. –  The Photon Sep 19 '12 at 0:43
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