T 7.2.1.3 Amplitude Modulation

TPS 7.2.1.3
Solutions
5.3.2 Carrier recovery
Table 5.3.2-1:
Control characteristic of the VCO
U F
V
f VCO
kHz
0.5 0.0
corresponds precisely to the frequency difference
between the carrier and the auxiliary oscillation.
The frequency shift in the demodulated signal disappears
completely for f C = f Aux .
Synchronous demodulation with the aid of a
PLL-controlled VCO.
1.0 0.0
1.5 0.3
2.0 14.5
2.5 41.3
3.0 69.1
3.5 96.3
4.0 121.8
4.5 148.7
5.0 167.5
Diagram 5.3.2-2:
(1): Pilot tone at the CF transmitter
(2): Recovered pilot at the output of the PLL (receiver)
After lock-in of the PLL the original pilot signal
and the recovered signal have exactly the same
frequency. A fixed phase-shift occurs between
the two signals. This leads to a (constant)
amplitude error during synchronous demodulation.
Diagram 5.3.2-1: The control characteristic of the VCO in
the PLL circuit of the receiver
The synchronous demodulation is performed
with the aid of a free-wheeling VCO.
In the demodulated signal a constant frequency
shift occurs for f C ≈ f Aux . This is maintained during
variation of the signal frequency f M . It is only dependent
on the control voltage U F of the VCO,
which determines the frequency f Aux of the auxiliary
carrier. The frequency phase-shift between
the modulating signal and the demodulated signal
Diagram 5.3.2-3:
(1): 20 kHz original carrier at the transmitter
(2): The recovered auxiliary carrier at the receiver
A fixed phase-shift exists between the two carriers.
This phase-shift can assume 8 various values
due to the undefined starting conditions of the frequency
divider (f/8). These phase-shifts are associated
with amplitude errors in the demodulated
signal.
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