Classical modulation procedures - Signals, Processes, and Systems - page 252

Time domain Frequency domain

10 75
50
2 0
-25
-50
-75
-100
150
100
50
0
-50
-100
250
200
150
10 5 0
-50
-100
-150
-200
-250

20,0
17,5
15,0
12,5
10,0
7,5
5,0
2,5
0,0
25,0
22,5
20,0
17,5
15,0
12,5
10,0
7,5
5,0
2,5
0,0
30
25
20
15
10
5
0

LP filtered noise

Bandwidth approximately 20 Hz

LP filtered noise

Bandwidth approximately 40 Hz

LP filtered noise

Bandwidth approximately 80 Hz

0 100 200 300 400 500 600 700 800 900 25 50 75 100 125 150

ms Hz

Time domain

Frequency domain

FM Signal with small frequency swing

4
2
0
- - -3
-4
3
1
- -2
- -4
2
0
- -2
- -4

0,9
0,8
0,7
0,6
0,5
0,4
0,3
0,2
0,1
0,0
0,8
0,7
0,6
0,5
0,4
0,3
0,2
0,1
0,0
0,7
0,5
0,3
0,1

FM Bandwidth approximately 250 Hz

FM Bandwidth approximately 500 Hz

FM Bandwidth approximately 800 Hz

87.5 92.5 97.5 102.5 107.5 112.5 117.5 122.5

500 750 1000 1250 1500

FM Signal with large frequency swing

ms

Hz

Illustration 175: FM spectrum of a realistic signal

Up to now we have largely used a sinusoidal source signal in order to obtain straightforward usable
results. A realistic signal has a stochastic component, that is, its future curve is not exactly predictable. As
a result the spectrum has a certain irregularity reminiscent of a noise signal. In the time domain (left) this
is not to be seen.

Here filtered noise signals of various bandwidths - see above - were frequency modulated. It can be seen
clearly that the bandwidth of the FM signal depends on the bandwidth of the source signal. Although the
same noise signal was lowpass filtered the real frequency swing depends on the bandwidth of the source
signal. You see the reason for this top left in the time domain: the maximum instantaneous values increase
with the bandwidth of source signal because the energy also increases with the bandwidth. The greater
these values, the greater the frequency swing.

Unlike the AM and single sideband modulation, the FM bandwidth is at least twenty times the bandwidth
of the source signal. The FM behaves extravagantly with the frequency band. If in spite of this the FM is
used, for example in the VHF range, this modulation process must offer considerable advantages
compared with the AM and single sideband modulation.