FM and Bessel functions

[fran1942]

Hello I am doing some experiments with FM modulation.
I have an RF generator connected to a function generator and a spectrum analyser.
I have been adjusting the modulation index and measuring the amplitudes of the side frequencies.

My question is: why is there a discrepancy between my measured side frequency amplitudes and the results I get from the Bessel function table ?
I know there is a fundamental reason for this but I am unsure what it is.

Thank you for any help.

 

[ahsan_i_h]

May be your function generator or rf generator are not making pure sine wave. Sine wave with harmonics.

 

[LvW]

Hello I am doing some experiments with FM modulation.
I have an RF generator connected to a function generator and a spectrum analyser.
I have been adjusting the modulation index and measuring the amplitudes of the side frequencies.

My question is: why is there a discrepancy between my measured side frequency amplitudes and the results I get from the Bessel function table ?
I know there is a fundamental reason for this but I am unsure what it is.

Thank you for any help.

Are you sure that the parameter you have called "modulation index" is identical to the parameter found on the horizontal axis of the Bessel functions?