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# Injecting harmonic frequencies in Pspice

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#### micro7311

##### Junior Member level 2
Hi, I want to do an AC analysis in PSpice but I want to include other harmonic frequencies, say 2nd and 3rd on top of the fundamental, rather than a single AC voltage source, and observe the output. Is this possible? I know it can be done in transient analysis by adding 3 sinusoidal voltage sources in series and modifying their amplitude/frequency, but I need to do an AC sweep. Thank you.

I'd like to help but I don't understand the question.

When you put an AC 1 voltage signal it applies 1V input across the whole frequency range. So the harmonics are already in there, it just does not distinguish between them and the signals that are not harmonics of what you consider to be fundamental.

For example, say your fundamental is 1 kHz, and you have an amplifier as DUT, and assume this is an AC analysis from 1 Hz to 1 MHz. When you apply AC 1 V at the input, it would just put 1 V from 1 Hz to 1 MHz, for the entire swept spectrum. And then you would see at the output which bands are being attenuated and which bands are being amplified without distinguishing between harmonics of the frequency you consider as fundamental.

So what you are saying is basically running an AC sweep and looking at the output only at specific harmonic frequencies, which is just a regular AC analysis, there's nothing special about it. Also keep in mind that AC analysis is a linearized analysis, so 1 kHz AC signal will not create harmonics at the AC output.

So I think you're confusing AC analysis with actual transient signal that has AC (alternating, periodic) behavior. Because that's not AC analysis is.

One other thing you might mean would be PAC (periodic AC analysis) which accepts a periodic steady state and runs the AC analysis over that periodic operating point. In this case it makes sense to say that you want to run the AC analysis over the harmonics because in this case you actually need to consider harmonics of the operating point.

So if you can be more precise on your question, that'd be helpful.

Thank you for the response. Actually what I am looking to do is include other frequencies, not necessarily harmonics, into the original signal. I have a circuit which takes a frequency input and I suspect there are other parasitic frequencies imposed on the signal which is affecting the resonance frequency of the circuit. So what I would like to do is have two frequency sources, one at what I consider to be the main frequency, and another source at a multiple of the first at a reduced amplitude. Make sense?

Then you have to run a transient analysis. Because in pspice I don't think you can look at frequency transfer function using AC analysis. It would just give you the voltage gain from input to the output with the exact frequencies you want to put at the input.

What you can do however is running a parametric sweep with the transient setup which is exactly what you suggested. AC sweep does not work that way. It just takes an operating point, linearizes and gives you the gain, that's all.

micro7311

Points: 2