T 7.2.1.3 Amplitude Modulation

TPS 7.2.1.3
Solutions
ter suited for the assessment of moduation distortions
than direct display of the modulated signal in
YT modus. Between the modulating and the
modulated signal there arise, e.g. severe phase
delay distortions, if the modulation is performed
using the LP filter connected in series. These visible
distortions on the oscilloscope are frequencydependent.
They belong to the group of linear distortions
and are caused by the phase response of
the electronic components (especially the LP filter).
5.2 Spectrum of the DSB
5.2.1 DSB
Table 5.2.1-1: DSB spectrum
Signal parameter Analyzer settings
A C : 2.0 V V 1 : 2
f C : 20.0 kHz b : 100 Hz
f r : 50 kHz
A M : 2.0 V T : 40 s
f M : 2 kHz
Table 5.2.1-2: DSB spectrum
Signal parameter Analyzer settings
A C : 2.0 V V 1 : 2
f C : 20.0 kHz b : 100 Hz
f r : 50 kHz
A M : 1 V T : 40 s
f M : 3 kHz
Measurements
Theory
Measurements
Theory
V 2
f
KHz
Name
S( n)
V
S AM (n)
V
S AM (n)
V
V 2
f
KHz
Name
S( n)
V
S AM (n)
V
S AM (n)
V
2 18.00 LSL 4,5 1.1 1
2 20.01 carrier 8.5 2.1 2
2 22.01 USL 4.5 1.1 1.00
2 17.00 LSL 2.1 0.52 0.5
2 20.01 carrier 8.4 2.2 2.00
2 23.01 USL 2.2 0.55 0.50
Diagram 5.2.1-1: DSB spectrum
Diagram 5.2.1-2: DSB spectrum
With f M = 3 kHz the frequency of the modulating
signal s M (t) already lies in the cutoff range of the
LP filter. For that reason using a filter can lead to
the attenuation of the amplitude at the modulator
input and thus to a reduction in the modulation
index.
From the spectra it follows that:
– With increasing signal frequency f M the
USLs are shifted away from the carrier in
the direction of higher frequencies. This frequency
response of the USL is called the
normal position, high signal frequencies also
lie in the modulation spectrum at high
frequencies.
– With increasing signal frequency f M the LSLs
shift further away from the carrier into the
lower frequencies. The frequency response
of the LSLs is thus called the inverted position
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