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PAC and PXF inconsistency?

Yevgeny

Good day.


I am trying to simulate power supply noise propagation through an amplifier stage. The amplifier is driven by a large signal, which is the PSS beat. The circuit is slightly non-linear (THD around 30dB). I use harmonic balance for PSS engine. Now I add power supply interference (small signal); I try it in two different ways:

1) Run PAC analysis, with the only PAC source being power supply voltage source, look at the output spectrum (differential voltage).

2) Run PXF (differential voltage output) and look at the transfer function from the said power supply voltage source.

I deem the two analyses should have given same spectra, but they don't. In fact, the base harmonic (i.e. number "0") is exactly identical, all the others as not.

Can anyone please explain that to me?

UPD: I have tried to verify this with transient, i.e. ran a long TRAN simulation with both the input (LO) and the power supply (SUP) active; checked the output spectrum. Thus I validate a single frequency point of the SUP. It came out very close to PAC (and very far from PXF). So I better rephrase my question: where is my understanding of PXF went wrong? The image below shows these results. Peaks are come from the transient spectrum (1st harmonic). The brown and green curves are PAC results. The two remaining curves (flat ones) are PXF.

Many thanks,

Yevgeny.

Parents

  • Dear Yevgeny,

    > I deem the two analyses should have given same spectra, but they don't. In fact, the base harmonic

    > (i.e. number "0") is exactly identical, all the others as not.

    My understanding is that a PAC analysis maps a given input frequency (in this case the power supply modulation frequency) to the various output frequencies it will produce due to amplifier. You do note that the THD of the amplifier is better than 30 dB. Howver, I assume that is the THD from the amplifier input - not from an input on the supply rail. Hence, my guess is you observe significant sidebands at the amplifier output due to the supply noise modulation frequency.

    A PXF analysis maps an output harmonic back to the input frequencies that are required to generate that output harmonic. Hence, the two analyses, from my understanding, will not produce the same output spectra. PAC provides information on the output frequencies produced from a given input - and PXF provides insight on the input frequencies required to produce a given output frequency.

    Let me know your thoughts, or if I misinterpreted your question!

    Shawn

Reply

  • Dear Yevgeny,

    > I deem the two analyses should have given same spectra, but they don't. In fact, the base harmonic

    > (i.e. number "0") is exactly identical, all the others as not.

    My understanding is that a PAC analysis maps a given input frequency (in this case the power supply modulation frequency) to the various output frequencies it will produce due to amplifier. You do note that the THD of the amplifier is better than 30 dB. Howver, I assume that is the THD from the amplifier input - not from an input on the supply rail. Hence, my guess is you observe significant sidebands at the amplifier output due to the supply noise modulation frequency.

    A PXF analysis maps an output harmonic back to the input frequencies that are required to generate that output harmonic. Hence, the two analyses, from my understanding, will not produce the same output spectra. PAC provides information on the output frequencies produced from a given input - and PXF provides insight on the input frequencies required to produce a given output frequency.

    Let me know your thoughts, or if I misinterpreted your question!

    Shawn

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