PAC and PXF inconsistency?

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

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.

Yevgeny,

Your understanding of pxf is incorrect. Shawn's description is right, but maybe I can clarify by saying it again (maybe slightly differently).

When you do a pxf sweep, you are sweeping the output frequency, not the input frequency (so the opposite of pxf). The simulator then gives you the transfer function from an input (default, from any source) at each sideband to the output. So for example, if you have an output at F_out, you will get transfer functions from input frequencies of k*F_lo+F_out where k is the sideband number (so -5, -4, -3 ... 0 ..., 3, 4, 5 etc). Note that these are transfer functions, not signal levels caused by a particular size input signal, and also they are the transfer function caused by mixing with that multiple of the PSS fundamental.

For pac, you are specifying an input frequency, and then getting the output signal level (not transfer function) at each output sideband. So that's an input at F_in, and then a set of output signals at k*F_lo+F_in. For a given frequency sweep, the only place the results would match is the 0th sideband.

Now, in your case, you have an amplifier, so it's presumably not a LO frequency but some large signal interferer that you wish to look at intermodulation effects from a small signal on the supply. Say that large signal interferer was at 100MHz (just to keep it simple), and you were injecting a noise frequency of 10MHz. With 3 sidebands in the PAC analysis, you would see the amount of this signal at 10MHz, 90MHz,110MHz,190MHz,210MHz,290MHz, 310MHz.

If you did a pxf analysis, with the output frequency at 10MHz, it would tell you the transfer function from a noisy input on the supply (or anywhere else) at 10MHz, 90MHz,110MHz,190MHz,210MHz,290MHz, 310MHz. So in other words, you're finding out what the various means of getting a 10MHz output would be, as opposed to applying a 10MHz input.

Assuming that you are OK with converting the signal levels to transfer functions (commonly done by setting the PAC input signal to 1), if you were to run PAC with an input at 90MHz, and then look at the output at 10MHz (the -1 sideband), it should give the same as PXF would with a 10MHz output (the frequency on the form) and the transfer function probed at 90MHz (the -1 sideband). (the shape would be flipped though).

Hope that helps!

Regards,

Andrew

Wow!

Crystal clear. I stand corrected. Andrew and Shawn - thanks a million.

This explanation should probably have been in the manual somewhere (bet it is, and I just could not find it).

Best regards,

Yevgeny.