Classical modulation procedures - Signals, Processes, and Systems - page 255

T i me domain Sp ectrum F req. domain

Time d omain Sp e c tr u m Fr eq . do ma in

Si n u s VC O LP filter Ab s - v a l u e LP filter

Sinus VCO LP fi lter Abs - valu e LP filte r

Time domain Frequency domain

66,0
65,5
65,0
64,5
64,0
63,5
63,0
62,5
62,0
1,00
0,75
0,50
0,25
0,00
-0,25
-0,50
-0,75
-1,00
1,00
0,75
0,50
0,25
0,00
-0,25
-0,50
-0,75
-1,00
0,8
0,7
0,6
0,5
0,4
0,3
0,2
0,1
0,0
0,525
0,500
0,475
0,450
0,425
0,400
0,375

2,00
1,75
1,50
1,25
1,00
0,75
0,50
0,25
0,00
1,0
0,9
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

0,30
0,25
0,20
0,15
0,10
0,05
0,00
0,07

0,05

0,03

0,01

0 50 100 150 200 250 300

125 175 225 275 325 375 425 475 525

Hz

ms

Illustration 177: Demodulation of an FM signal at the filter edge with a small frequency swing

If this is compared carefully with Illustration 148 the smaller frequency swing can be recognised in the FM
signal (top series), the FM signal (mirrored) at the filter edge and in the rectified signal. As a result the FM
signal is mirrored only in the more linear part of the filter edge. The retrieved source signal has a much
smaller amplitude (ca 0.05 V) but is only slightly distorted in a non-linear way. There is only a very small
2nd harmonic to be seen in the spectrum bottom right.

The PLL has been assembled from components with which we are familiar: multiplier,
lowpass filter and VCO (voltage-controlled oscillator)). It would, however, probably be
too difficult for you to describe the function of PLLs from Illustration 178 from scratch.
There are two reasons for this. First, the multiplier is used here for a “special purpose”,
second, there is feedback. It is generally difficult to predict the behaviour of locked-loop
systems because all locked loop systems are non-linear.