Digital Signal Processing Reference
In-Depth Information
Recorder
Sou r c e
FM s igna l
Multiplier
LP filter
VC O
Phase detector
Time domain
Frequency domain
110,0
107,5
105,0
102,5
100,0
97,5
95,0
92,5
90,0
4
3
2
1
0
-1
-2
-3
-4
40
30
20
10
0
-10
-20
-30
-40
10,0
7,5
5,0
2,5
0,0
-2,5
-5,0
-7,5
-10,0
25
20
15
10
5
0
-5
-10
-15
-20
10,0
7,5
5,0
2,5
0,0
2,75
2,50
2,25
2,00
1,75
1,50
1,25
1,00
0,75
0,50
0,25
0,00
20,0
17,5
15,0
12,5
10,0
7,5
5,0
2,5
0,0
10,0
7,5
5,0
2,5
0,0
20,0
17,5
15,0
12,5
10,0
7,5
5,0
2,5
0,0
Source signal
Source signal
FM signal
Difference signal Sum signal
VCO signal
Difference signal
at the output of LP-Filter
25
50
75
100
125
150
175
200
225
25
50
75
100
125
150
175
200
225
250
27
ms
Hz
Illustration 180:
Defective PLL without a controlled VCO
What happens if in the case of the PLL the feedback system is interrupted and the VCO is driven by a
constant “carrier frequency”? This analysis is very simple. We are dealing with a simple multiplication in
the time domain. In the frequency domain on the other hand the sum and difference signal is formed. If the
sum signal is filtered out by the lowpass signal this does not leave the original source signal; rather a
sideband of the “mid-frequency” f = 0 Hz appears in the frequency domain.
It is only the feedback system that brings back the source signal as it forces the VCO to synchronise itself
with the instantaneous frequency of the FM signal. As this FM instantaneous frequency changes in the
rhythm of the source signal this must also apply to the control voltage (the difference voltage at the output
of the lowpass filter).
The mid-frequency is set with the offset and the frequency swing of the VCO with the
amplifier. The module “delay” is prescribed in DASY
Lab
for feedback systems. The PLL
only clicks into place after a certain time. A change in the second decimal place in the
constant C in the two modules is sufficient to stop the PLL clicking into place. We see
how sensitively the feedback system reacts to the smallest changes in the VCO signal.
