T 7.2.1.3 Amplitude Modulation

TPS 7.2.1.3
Solutions
7 The ring modulator
Answers
7.1 The ring modulator provides an AM signal
with carrier suppression. The line of the
modulating signal is missing in the output
spectrum for f = f M .
7.2 In balanced modulators frequency-periodic
modulation spectra are produced.
Consequentily the wanted bands have to be
filtered out, i.e. the interfering sidebands
have to be suppressed for the sake of a narrow
transmission bandwidth.
7.3 All of the circuits which have been used for
modulation have one feature in common: a
non-linear characteristic. This is true in particular
for the mixer, multiplier, ring modulators
etc. A switch can be seen as a very
extreme case, its characteristic has abrupt
step changes.
Experiment results
Diagram 7.1-2: Dynamic characteristic of modulating signal
and modulated signal (CARRIER ON)
(1): modulating signal s M
(t)
(2): modulated signal
7.2 Spectrum at the output of the ring modulator
Table 7.2-1: Spectrum of the ring modulator
Signal parameter
Analyzer settings
A C : V V 1 : 2
f C : 20.0 kHz b : 100 Hz
f r : 50 kHz
A M : 2 V T : 40 s
f M : 2 kHz
Measurement
Theory
V 2
f
KHz
Name
S( n)
V
S AM (n)
V
S AM (n)
V
2 18 LSL2 5.20 1.30
2 22 LSL1 5.30 1.32
Diagram 7.1-1: Dynamic characteristic of modulating signal
and modulated signal (CARRIER OFF)
(1): modulating signal s M
(t)
(2): modulated signal
2 58 USL1 1.60 0.4
2 62 USL2 1.70 0.43
The modulation product has a certain similarity to
the PAM signal for the special case of a symmetrical
duty cycle t/T P = 50%.
Diagram 7.2-1: Spectrum of the ring modulator
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