How phase modulation changes spectrum?
Phase modulation has the same sidebands as Amplitude modulation.

However the AM sidebands, modeled as rotating phasors, rotate in opposite directions and are synchronized in phase at the top and the bottom; if these sidebands are exactly balanced in energy, this synchronization avoids ANY phase perturbation to the Carrier, thus only Amplitude changes appear.

If you imbalance the energy, some Phase Modulation appears.

If you have imperfect rotational synchronization, some Phase Modulation appears.

More specifically, in AM the phasors are crossing at the top and at the bottom at the same times, and rotating in opposite directions.

In PM the sideband phasors are still rotating in opposite directions, but are synchronized at left and at right points.







simulate this circuit &ndash; Schematic created using CircuitLab

Notice any imbalancing of sideband energy causes AM-PM conversion, thus any finite bandwidth system has AM-PM.

To answer your question, the difference is in the phasing, and classic analog spectrum analyzers do not capture phase information. FFT based machines may have the information, but is "phase" displayed?