frequency modulation index

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liusupeng

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when modulation index is big, the signal to noise ratio is good. while modulation index is small, speed is higher. So in typical applications such as wireless communication, can any one provide me some example about the practical modulation index range used?
 

FM radio Fmod max = 20KHz, Fdev = 75KHz mod index = 3.75. Narrowband FM (amateurs/police/fire. . ) Fmod max = 3KHz, Fdev = 5KHz mod index = 1.6. Analogue satellite Tv transmission Fmod max = 5MHz, Fdev 27 MHz mod index = 5.4 . The thing to think about is triangulation noise. The demodulated noise varies as bandwidth^2. So it important to utilise the whole bandwidth. To help with the demodulated noise FM radio audio is "pre-emphasizied" i.e. given a treble boost with a time constant of 75 microseconds. This is removed in the receiver with a "de-emphasis" circuit with the same time constant which also removes some of the noise.
Frank
 
Hi chuckey,
Thanks for your reply! do you know any system with modulation index <1?
 

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