Consider Carson's rule for the occupied bandwidth of an FM modulated signal, which can be summarized as: 98% of the signal energy of an FM signal is contained in twice the maximum deviation plus twice the highest modulating frequency. This is an approximation.
In the case of commercial broadcast band, the maximum deviation is 75 KHz. With the original mono FM, the modulating signal was limited to 15 KHz, and thus the 98% power calculation results in about 180 KHz occupied bandwidth. Indeed, this was the original reason why commercial BCB stations were allocated 200 KHz with center frequencies spaced apart the same amount. Later, when stereo FM, then RDS, and eventually SCA features were added, the occupied bandwidth widened. SCA occupies 60-74 KHz, so occupied BW becomes 300 KHz or so. Modern stations are now allocated 400 KHz, and while the center frequencies are still in 200 KHz increments, no stations are assigned in a coverage area closer than 400 KHz. In practice, few stations carry SCA anymore, but most have RDS (55-59 KHz), so this works out to about 270 KHz. Finally, some stations carry a new product, HD2, which is added to the signal *after* the FM modulation, and occupies the outer 150 KHz of the 400 KHz allocation. (These are recognizable on a spectrum display by their square shape, a result of using OFDM). In this case the station does not use SCA, and filters the FM portion of the signal to 250 KHz, result in a slight distortion of the FM stereo and RDS products. Hope this clears some things up. -Johnathan
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