Digital Signal Processing Reference
In-Depth Information
D
D
R
Quellsignal
AM-Signal
C
R
D
AM signal
Audio signal
C
.
1,5
1,0
0,5
0,0
-0,5
-1,0
-1,5
1,50
1,25
1,00
0,75
0,50
0,25
0,00
0,45
0,40
0,35
0,30
0,25
0,20
0,15
0,10
AM signal
Rectificated AM signal
Audio signal
0
25
50
75
100
125
150
us
Illustration 156:
Demodulation of two AM-Signals in the traditional manner
At the top you see the AM signal, the envelope of which represents the original signal. This signal is
rectified by the diode, i.e. the negative part of the AM signal is cut off. The R-C part acts like a lowpass
filter, i.e is so inert timewise that it does not notice “short-term” changes. It acts like a “floating
averager”.
If you look closely you will notice in the retrieved source signal slight steps at the bottom which derive
from this (imperfect) floating averaging.
For this reason the requirement m < 1 applies for this traditional form of AM.
In addition, as a real signal is certainly not sinusoidal the
modulation degree m
can-
not be meaningfully described as the relationship between two amplitudes of dif-
ferent sinusoidal voltages. Our above definition of the offset is simply more
meaningful.
If the original source signal is not clearly recognizable in the envelope, it cannot be
difficult to retrieve it by means of demodulation.
Since the beginnings of radio technology demodulation of the AM has been carried out
by means of an extremely simple circuit consisting of a diode D, a resistance R and a
capacitor C as shown in Illustration 156. The AM signal is first rectified by the diode, that
is, the negative region is cut off.
