Although the bandwidth of a phase modulated signal is theoretically
infinite, the vast majority of the energy is in a finite bandwidth. So
if you filter signal components beyond that bandwidth, you can still
recover the modulation information adequately. What you have to do,
then, is to design the tank circuit so its response isn't too narrow to
pass the modulation information. If it is too narrow, it will decrease
and distort the modulating phase shift.

If you were to build a tank circuit that acted as a perfect "flywheel",
i.e., had zero bandwidth, you wouldn't even be able to pass a code or
voice waveform through it -- everything would come out as a single
frequency, single amplitude sine wave, or nothing at all. (It would also
take an infinite time for it to respond to a signal.) For amplifying
some very broadband types of signals, tank and other tuned circuits are
avoided altogether.

Roy Lewallen, W7EL

gary wrote:
Can someone explain to me how a rf carrier that is phase shift
modulated, for lets say digital transmission, can be amplified in an
amp with a tank circuit output. Why doesnt the tank "flywheel effect"
prohibit or inhibit the abrupt phase changes in the signal. thanks
73

Gary
W4AF