FM bandwidth calculation

[fran1942]

Hello, I am approximating the bandwidth of an FM signal using Carson's rule.
I am finding that it is consistently less than the method of visually summing the frequency amounts between each significant sideband as viewed on a spectrum analyser.
Is there a particular reason for this ? Is it because Carson's rule is simply an approximation method and not that accurate ?

Thanks kindly for any ideas.

 

[LvW]

The spectrum of an FM signal is determined by Bessel functions and, therefore, theoretically unlimited.
Due to practical reasons the usable bandwidth is limited. One such method was given by Carson.

Quote wikipedia: Any modulated signal will have an infinite number of sidebands and hence an infinite bandwidth but in practice all significant sideband energy (98% or more) is concentrated within the bandwidth defined by Carson's rule. It is a useful approximation, but setting the arbitrary definition of occupied bandwidth at 98% of the power still means that the power outside the band is only about 17 dB less than the carrier inside