Not PWM, what you need is an op-amp as a gain stage, a rectifier (either diode or better still an op-amp based precision rectifier) to give you a DC...
As per FvM, don't confuse FM and AM bandwidth requirements. The deviation of standard FM broadcasts is far wider than the bandwidth of normal 455KHz...
I do not doubt any of the technical explanations about diode recovery time and how to snub the spike but I do have reservations about how relevant it...
There is a multitude of different, incompatible but similar functioning ICs for that purpose so I wouldn't risk experimenting with other types to see...
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