Classical modulation procedures - Signals, Processes, and Systems - page 259

Rec order

So ur c e FM s ig nal

Multiplier LP filter

Delay c * U r c + Ur VC O

Modules for calibration of VCO

7,00
6,25
5,50
4,75
4,00
0,50
0,25
0,00
-0,25
-0,50
1,00
0,50
0,00
-0,50
-1,00
1,00
0,50
0,00
-0,50
-1,00
6,50
6,25
6,00
5,75
5,50
0,50
0,25
0,00
-0,25
-0,50

Source signal

FM-signal (that is the input signal of the FM receiver)

Sum- and difference-signal at the output of the multiplier

VCO (Voltage controlled by the phase detector signal)

Signal at the output of
the phase detector
(difference frequency)

Here the PLL
locks in !

Demodulated source signal

6,5 7,0 7,5 8,0 8,5 9,0 9,5 10,0 10,5

s

Illustration 181: The PLL as a demodulator for FM signals

In the top block diagram you will find several more components than in the default block diagram of
Illustration 178. At the top left the FM signal which is to be demodulated is generated. The phase detector
- consisting of the multiplier and the lowpass filter - shows the phase difference between the FM and the
VCO signal. This output signal is processed for the adjustment of the VCO (setting the mid-frequency of
the VCO by offset (C + U R ) and the frequency swing (C < U R ). The feedback system operates with a certain

delay. You will see this by comparing the FM and the VCO signal. Feedback is only possible with
DASYLab by using the module “delay”.

If the FM signal were temporarily to show a higher frequency swing the PLL might also
get out of step. The transmitter frequency spacing and the frequency swing are therefore
fixed in the VHF range. The mid-frequency distance is 300 kHz, the maximum frequency
swing is 75 kHz, the highest LF frequency is roughly 15 kHz (“HiFi”). Our bandwidth
formula thus results in a transmitter bandwidth of roughly

B VHF = 2 ( 75 kHz + 2 < 15 kHz ) = 210 kHz