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walid

10kHz FM modulation

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Guest SM2GXN
The correct and normal term for this is deviation and in this case the deviation is 20kHz not 10Khz.


The example with center frequency 100mhz with a frequency shift from 99.99-100.01mhz give 10khz deviation and 20khz swing, swing is the total frequency range covered by the FM-transmitter.
Using 100mhz as center, 2khz modulation frequency and a deviation of 10khz will result in a modulation index of: Mod.index = deviation from carrier (khz)/Modulated audio frequency (khz) =10/2=mod.index 5.

deviation depens on the modulation index we chose


Not fully right, deviation is proportional to the amplitude of the modulating signal. Think of this, if a symetrical audio is fed to a resonant circuit where a varicap is used, then with a certain audio amplitude the result will be equal deviation on each side of the center frequency as long as the amlitude is inside the linear region of the varicap, higher amplitude will make the deviation bigger.
Higher index will increase amplitude of the sidebands and ofcourse increase deviation somewhat because of higher content in the harmonics.

Let me hear what you think guys  ;D

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hi 4-1000A
your answer is excellent
but still one point:
in your example you calculate a mod.index 5
i know now (after your answer) that i can control this by varying the amplitude of the modulating sig. what is the best mod.index
if i construct a FM Tx what is the optimum mod.index

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Guest SM2GXN

What are you going to transmitt?
Speech or music?
And what kind of receiver will be the lucky one to listen to your transmission?  ;D

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walid
Hi 4-1000A

I see from what you said that the mod. index is affected by:
1- the freq. of the modulating signal and
2- the amplitude of the modulating signal.
if this is the case, how can the designer control or achieve the desired mod.index he want.
AND is it correct if we say that controlling the modulating signal amplitude by using a variable resistor can control the mod.index
thanks.

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Hi Walid,
FM radio stations and TV audio don't use a variable resistor to keep their modulation index controlled. They use a compressor/limiter electronic circuit to control the level of the audio automatically. The max allowed deviation for mono broadcast FM is 75kHz.

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