I am trying to understand as much as possible the aircraft replies of ADS-B signals. It uses a PPM (pulse position modulation) scheme with a carrier frequency of 1090 plus minus 1Mhz. You can find all the important specifications about ADS-B reply signals on this Link, by clicking on annex 10 volume IV page 35-36 if you are interested.
I think I understand almost everything except things related to the "spectrum":
What is the reply spectrum? how does it affect the signals frequency in practice?
I plotted the amplitude alongside the phase and noticed that for a lot of signals of my dataset: there is a clear pattern where if amplitude increase, then phase decrease and vice-versa. Even though for ADS-B signals, the behaviour of the phase is unspecified. Could this be related to the spectrum and potential small frequency changes when the transmitter changes amplitude?