junger d07 Operation Manual page 80

7.4.
Terms and
definitions
m = 0,5
Δf
= 1kHz
c
f = 2kHz
m
m = 1,0
Δf
= 1kHz
c
f = 1kHz
m
m = 5,0
Δf
= 1kHz
c
f = 0,5kHz
m
m = 10,0
Δf
= 1kHz
c
f = 0,1kHz
m
Frequency modulation
The sketch below shows the principal influence of the modulation index m= Δfc / fm to
the spectrum of an FM signal. On the left side the frequency deviation Δfc (amplitude
of the modulating signal) is constant but the modulating frequency fm changes. On the
right hand the modulating frequency fm is constant but the frequency deviation Δfc is
changing :
B = 2*f
m
2Δf
c
You can show that a bandwidth of approx B ~ 2 * (Δfc(max) + fm(max)) is needed for
FM modulation for high rejection of high frequency signals. With a maximum
frequency deviation of 75kHz and 15kHz cut-off-frequency of the modulating signal B
will be ~ 180kHz. Based on this fact the planning of transmitter positions and power
takes place for area-wide feed. To prevent disturbance in adjacent channels the
maintaining of the frequency deviation needs the highest attention.
Frequency deviation Δfc
Value of deviation of the mean frequency from the transmitting frequency of a FM
transmitter caused by the amplitude of the modulating signal.
Peak Deviation Δfc(max)
Maximum frequency deviation allowed for an FM transmitter. Defined by the ITU to
+ 75kHz
f
2Δf
c
Δfc + f
)
B = 2*(
m
f
f
B ~ 2 (Δf + f )
c m
f
m = 0,5
Δf
= 0,05kHz
c
f = 0,1kHz
m
B
2Δf
c
m = 1,0
Δf
= 0,1kHz
c
B
f = 0,1kHz
m
2Δf
m = 5,0
Δf
= 0,5kHz
c
f = 0,1kHz
m
B
2Δf
c
m = 10,0 B
Δf
= 1kHz
c
f = 0,1kHz
m
2Δf
c
f
f
c
f
f