T 7.2.1.3 Amplitude Modulation

TPS 7.2.1.3
Ring Modulator
s M
i
i
C
2
C
2
^↑Φ
^↓Φ
T / 2
T / 2
⎫
⎪
⎪
⎬ΣΦ
⎪
⎪
⎭
C
Φ C Φ
=+ −
2 2
C
! 0
0
f T
3 f T
5 f T f
Fig. 7-2: The principle of the differential transformer
Fig. 7-3: Dynamic characteristic and spectrum for the ring
modulator
lating signal alternates its flow through the output
transformer. The transformers have to be suitable
to process radio frequencies (RF). For that reason
ferrite is used as the core material. Earlier ring
modulators were manufactured exclusively with
differential transformers. Today they are being
overtaken more and more by ICs which operate
without transformers. Fig. 7-3 shows the dynamic
characteristic and the spectrum at the output of the
ring modulator. The AF signal is cancelled out in
the envelope of the modulated signal. Therefore,
the line at f = f M is missing in the spectrum
The following holds true for the spectrum of the
ring modulator:
4
1
1
sAM t [cos 2π fCt cos 2π3fCt cos 2π5fCt
π
3
5
( ) = ( ) − ( ) + ( )
−+ ...] A cos( 2π
f t)
M
M
2
2
= AM cos[ 2π
( fC − fM ) t]+ AM cos 2π
( fC + fM
) t
π
π
[ ]
2
2
− AM cos[ 2π
( 3fC − fM
) t]−
AM cos 2π ( 3 fC + fM
) t
3π
3π
+−
[ ]
(7-2)
If you consider the spectrum of the ring modulator,
it becomes clear that suitable filters have to be
used to separate the wanted sidebands from the
interfering sidebands.
Questions
7.1 What features does the ring modulator have?
7.2 What circuit techniques and measures are
additionally required, if you want to generate a
narrow band AM signal with a balanced
modulator?
7.3 Which feature is common to all modulating
components?
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