Hi,

75KHz is the nominal frequency deviation of a FM mono broadcast transmission, but now all of them are using stereo mode, adding two 38KHz sub-carriers. Mode is called 256-f3 because (75dev+38stereo+15max_audio_freq)x2 equals 256, nominal bandwidth.

Later they came with RDS, sub-carrier is at 57KHz, resulting a max bandwidth of  (57+75)x2 equal to 264, and filters should be as wide 280KHz. There are some 330KHz filters to be used in cascade with another one. I imagine that some are that wide to give a margin for some over-deviating transmissions.

Let us say that 280KHz@-6dB is a reasonable bandwidth.

73's
Jose