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The FM signal (see image 1) is applied to a 50 Ohms antenna, and I am seeking the required bandwith with the Bessel table (image 2). From the FM signal, I deduced that the information frequency is equal to 5 kHz and that the modulation index is equal to 2. So I also know that I have to look in the row of the table where mf = 2.0 and that j0, j1, j2, j3 and j4 exist in this case (significant sidebands).

The solution first says 4 x 5 kHz = 20 kHz, this I still understand (4 because components up to j4 exist and 5 kHz because that is the information frequency). Finally it says that BW = 2 x 20 kHz = 40 kHz. Why do we have to multiply the found bandwith by two? Is this always the case or does the found bandwith have to be multiplied by the modulation index?

Image 1: FM signal

FM signal

Image 2: Bessel table

enter image description here

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Multiply by 2 to take into account both upper and lower sidebands.

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  • \$\begingroup\$ Thank you! In the examples from our lectures, the modulation index always happened to be 2, that confused me a bit. \$\endgroup\$ – Dieter Nuytemans Dec 28 '18 at 17:51
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Define the dataeye you want, then determine the # of Bessel Coefficients (energy) required to implement that dataeye.

For starters, read up on GSM cellphone modulation: GMSK, or the general philosophy of MSK (minimium shift keying).

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