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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 Shawn,

    As far as my understanding goes, PAC takes all the small signal inputs (power supply, in my case) and propagates them through the system with one large signal input (LO, the input in my case). At the output spectrum, you see the harmonics of the LO with spurs created by the power supply around these harmonics. For supply input frequency of Fsup and input frequency Flo, you get at spectral peaks at:  kFlo, kFlo+Fsup, kFlo-Fsup.  

    The PAC varies Fsup at the given interval (much like conventional AC), so you get curves (AC responses) at every sideband.

    In addition to all this you might get a harmonic at Fsup (if a part of the small signal propagates directly to output, without modulation).

    All this runs under assumption that "small signal" is sort of linear, i.e. you won't see any non-linear effect related to Fsup, except those caused by modulation with Flo. For example, if you have several small signal sources (Fsup1, Fsup2, ...) there will be no intermodulations among them. Only modulations with Flo.

    The only difference between PAC and PXF (as far as I get it) is that PAC takes all PAC sources and sweeps them together. PXF zeroes all sources but one and sweeps that one only.

    Best regards,

    Yevgeny.

Reply

  • Dear Shawn,

    As far as my understanding goes, PAC takes all the small signal inputs (power supply, in my case) and propagates them through the system with one large signal input (LO, the input in my case). At the output spectrum, you see the harmonics of the LO with spurs created by the power supply around these harmonics. For supply input frequency of Fsup and input frequency Flo, you get at spectral peaks at:  kFlo, kFlo+Fsup, kFlo-Fsup.  

    The PAC varies Fsup at the given interval (much like conventional AC), so you get curves (AC responses) at every sideband.

    In addition to all this you might get a harmonic at Fsup (if a part of the small signal propagates directly to output, without modulation).

    All this runs under assumption that "small signal" is sort of linear, i.e. you won't see any non-linear effect related to Fsup, except those caused by modulation with Flo. For example, if you have several small signal sources (Fsup1, Fsup2, ...) there will be no intermodulations among them. Only modulations with Flo.

    The only difference between PAC and PXF (as far as I get it) is that PAC takes all PAC sources and sweeps them together. PXF zeroes all sources but one and sweeps that one only.

    Best regards,

    Yevgeny.

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