Amplitude Modulation 2.
29
Due tothe suppression of the carrier wave the power savingis increasing from
33.3 to 66.7%.
2.9 Generation of DSB-sC-AM
There are two ways of generating DSB-SC-AM such as
() Balanced modulator (ii) Ring modulator.
2.9.1 Balanced modulator
Bandpass filter
centred at t o.
Diode 1
C
V. V,
ZR
Cos of
Modulating
Mis signal
. V,
9i i Diode 2
Fig.2.16
the input of the two matched diode
The carrier voltage is applied in parallel to pushpull to the same two diodes as
voltage in
D, and D, where as the modulating
shown in Fig.2.16.
In this casea
V, =C() + m() =Ac Cos at +m Co_ w,mt
V, = C() + m () = C()- m ()
= Aç Cos Qçt- Am Cos wn!
The diode currents are given by
i = aV, + a,v
equations i &i', then we get
Substituting the values of V,
and V, from above
açt + A, Cos am) + a, (Aç Cos @çt tA, Cos )
i= 4(Aç Cos
4,A, Cos at + a,A¿ Cos' act +a,A Cos a!
=4A, Cos @t + a!
+2a,AçA Cos atCos
ant+ Cos
a,A; at +a,A, Cos a!
=4Aç Cos açt +a,A, Cos
i -2a,AAm Cos atCos@ct
2. 30 Communication Theory and Systems
V, k(-)
=2ka, A, Cos o! +4ka, A,nA Cos omCos ap!
=2ka, Am Cos ot +2ka, A,4(Cos(ç - m)! +Cos(c -On)t)
A bandpass filter is that type of filter which allows to passband of fre
quencies. Since the bandpass filter is centred around to, it willpass a nar
row band of freguencies centred at to with a small bandwidth of 2o th
preserve the sidebands.
Therefore, the output of BPF centered around to, is given by
Vo =2ka, AmAc(Cosl«c -m)t +Cos(aç +am))
Which is the expression for a DSB-SC signal.
Advantages
1) Suppression of carrier results in economy of power.
system in which one sideband
2) Itis commonly used in carrier current telephony required for transmission.
is filtered out to reduce the width of the channel
3) It increases the efficiency because the carrier is suppressed.
2.9.2 Ring modulator
telephony suppresses
The balanced ring modulator circuit widely used in carrier
there is no need fora band
Thus,
both modulating and carrier signals in its output.
only sideband pairs about carrier
pass filter at the output. The output contains modulator using four diodes are
frequency and several of its harmonics. A
ring
circuit for four diodes are connected in the
shown in Fig.2.17. In aring modulator
in the same manner. All the four diodes
form of a ring in which all four diodes point
controlled by asquare wave carrier signal c(t) of o, applied through a
in ring are
center-tapped transformer.
Modulated
DSB-SC
Modulating signal
signal
Carrier signal
modulator
diagram of a ring
Fig.2.17 Circuit
+) Amplitude Modulation 2. 31
W
Fig.2.18
m(t)
(i) (ii)
DSB-SC
(iii)
Fig.2.19 (i) Sinusoidal modulating wave
(ii) Square wave carrier
(ii) Modulated wave DSB-SC output of the ring modulator
balanced, the two
In case when diodes are ideal and transformer are perfectly
whereas the two inner
outer diodes are switched on if thecarrier signal is positive
shown in Fig.2.18
diodes are switched off and thus presenting very high impedance assignal m(t) by +1
() under this condition, the modulator multiplies the modulating
situation becomes reversed
Now, in case when carrier signal is negative, the
multiplies the modulat1ng
as shown in Fig.2.18 (ii). In this case, the modulator
signal by -1
wave carrier and
Hence, the ring modulator is a product modulator for a square sinuso1dal
modulating signals. Fig.2.19 shows the modulating signal (assuming a
Sgnal) square wave carrier signal and the modulated (DSB-SC) signal.
Consider the modulating signal V (0 and carrier signal c() such that
2. 32 Communication Theory and Systems
Vm() = A,ino:C(0) =ASina,!
The output voltage equals the productof two signals.
S() =A,A, Sina,t Sinon!
^mc(Coslo,
2
-Om)t - Cosfo, +o))
and other
The above equation shows that the output is free from the carrier
DSB-SC-AM is
higher order terms, and it contains only sideband terms this
generated.
modulator since it js
A ring modulator is also known as a double-balanced
balanced with respect to the baseband signal as well as the square wave carrier signal
From Fig.2.20it may beobserved that to avoid overlapping of sidebandswe must
have w, > W, m
-0c
A -30.
20
modulator
Fig.2.20 Spectrum of the DSB-SC output of the ring
Advantages
i) Its output is stable
activate the diodes.
ii) Require no external power source to
iii) Virtually no maintenance.
iv) Long life.
2.10 Generation of SSB-SC AM
SSB-SC-AM waves can be generated in two ways:
Frequency discrimination or filter
method
i)
ii) Phase discrimination method
divided into two types.
Phase discrimination method itself can be
a) Phase shift method
shift-weaver's method.
b) Modified phase
2.10.1 Filter method
method, firstly a DSB-SC signalis generated si
In a frequency discrimination modulator. After this
product modulator or a balanced
ply by using an ordinary
