FM Audio Frequency Response

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amendiola

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Joined 2010
#1
Dear All:

I have a FM radio NarrowBand.
Max. frequency deviation : ±2.5 kHz [Narrow]

Channel bandwidth for max dev of 2.5 khz:
2*(2.5khz + 3.75khz) = 12.5 Khz

I put a value of 3.75 Khz to don't exceed the 12.5 Khz bandwidth limit of the FM narrow.

In this case is 3.75Khz the max audio frequency response that can transmit the FM Narrow Radio. ???

The radio is a Baofeng uv-5r.

Regards.
Alfredo Mendiola Loyola
Lima, Peru
 
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#3
amendiola said:
Max. frequency deviation : ±2.5 kHz [Narrow]

Channel bandwidth for max dev of 2.5 khz:
2*(2.5khz + 3.75khz) = 12.5 Khz

I put a value of 3.75 Khz to don't excede the 12.5 Khz bandwidth limit of the FM narrow.

In this case is 3.75Khz the max audio frequency response that can transmit the FM Narrow Radio. ???
Click to expand...


According to Carson's rule, if the channel bandwidth is 12.5 kHz and the frequency deviation 2.5 kHz, the maximum allowable modulating frequency is indeed 3.75 kHz. So if Carson's rule applies, 3.75 kHz is the best-case upper audio bandwidth limit before subtracting margins for the transition bands of the filters, carrier frequency offset et cetera.

I'm not sure, though, how accurate Carson's rule is when the frequency deviation is smaller than the modulating frequency. For frequency deviations well below the modulating frequency, the bandwidth is essentially twice the maximum modulating frequency because the higher-order Bessel terms are all negligible. So maybe you can allow a maximum audio frequency somewhere between 3.75 kHz and 6.25 kHz, minus margins for transition bands, carrier offset et cetera.
 
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