Lecture Notes in “Analog Communications & Noise” Analog Modulation
Electrical Engineering | University of Anbar by: Dr. Mohammed AlMahamdy
3.2.3 SSB (Single Side Band)
DSB method results in doubling of 𝑚(𝑡) bandwidth, which is disadvantage when channel is
crowded or expensive. So, we can use only one sideband to be transmitted as it contains all the
information about 𝑚(𝑡).
DSB
f
-fc fc
SSB-U
f
-fc fc
SSB-L
f
-fc fc
MODULATION
(1) Filtering method:
SSB can be generated by filtering the DSB signal. In practice, this operation is not easy
because it is difficult to meet such filter requirements.
DSB
BPF
y(t)
DSB
SSB Filter
m(t) (BPF)
SSB
0 Fc
cos ωc t
(2) Phase shift method:
Since the ordinary AM signal is 𝑦(𝑡) = 𝑚(𝑡) cos(𝜔𝑐 𝑡), letting 𝑚(𝑡) = cos(𝜔𝑚 𝑡), so:
1 1
𝑦(𝑡) = cos(𝜔𝑚 𝑡) × cos(𝜔𝑐 𝑡) = 2 cos(𝜔𝑐 + 𝜔𝑚 )𝑡 + 2 cos(𝜔𝑐 − 𝜔𝑚 )𝑡 =DSB
Yields:
cos(𝜔𝑐 + 𝜔𝑚 )𝑡 = cos(𝜔𝑐 𝑡) cos(𝜔𝑚 𝑡) − sin(𝜔𝑐 𝑡) sin(𝜔𝑚 𝑡) ≡ Upper SSB
cos(𝜔𝑐 − 𝜔𝑚 )𝑡 = cos(𝜔𝑐 𝑡) cos(𝜔𝑚 𝑡) + sin(𝜔𝑐 𝑡) sin(𝜔𝑚 𝑡) ≡ Lower SSB
-34-
Analog Modulation Lecture Notes in “Analog Communications & Noise”
by: Dr. Mohammed AlMahamdy Electrical Engineering | University of Anbar
cos ωc t
+
-90o -90o SSB
sin ωc t
But cos 𝜃 = sin(𝜃 ± 90°), hence:
Upper SSB is 𝑦(𝑡) = 𝑚(𝑡) cos(𝜔𝑐 𝑡) − 𝑚
̂ (𝑡) sin(𝜔𝑐 𝑡)
Lower SSB is 𝑦(𝑡) = 𝑚(𝑡) cos(𝜔𝑐 𝑡) + 𝑚
̂ (𝑡) sin(𝜔𝑐 𝑡)
Where 𝑚
̂ (𝑡) is shifting the phase of 𝑚(𝑡) by 90°
The main problem of SSB systems is the practical realization of the 90° phase shifter.
DEMODULATION
y(t)
r(t)
LPF m(t)/2
PLL cos ωc t
̂ (𝑡) sin(𝜔𝑐 𝑡)
𝑦(𝑡) = 𝑚(𝑡) cos(𝜔𝑐 𝑡) ± 𝑚
𝑅(𝑡) = 𝑦(𝑡) cos(𝜔𝑐 𝑡)
= 𝑚(𝑡) cos2 (𝜔𝑐 𝑡) ± 𝑚
̂ (𝑡) sin(𝜔𝑐 𝑡) cos(𝜔𝑐 𝑡)
1 1 1
= 𝑚(𝑡) + 𝑚(𝑡) cos(2𝜔𝑐 𝑡) ± 𝑚̂ (𝑡) sin(2𝜔𝑐 𝑡)
2 2 2
The high frequency parts are removed by LPF, yields: 𝑚(𝑡)/2.
-35-
Lecture Notes in “Analog Communications & Noise” Analog Modulation
Electrical Engineering | University of Anbar by: Dr. Mohammed AlMahamdy
Illustration:
3.2.4 VSB (Vestigial Side Band)
Because of double BW usage in DSB method, and the selective-filtering and phase shifter
limitations in SSB method, VSB is a compromise between DSB and SSB.
Instead of rejecting one sideband completely as in SSB, a gradual cutoff of one sideband is
accepted: (VSB BW=one sideband +25% of the other sideband)
-36-
Analog Modulation Lecture Notes in “Analog Communications & Noise”
by: Dr. Mohammed AlMahamdy Electrical Engineering | University of Anbar
DSB
f
-fc fc
SSB-U
f
-fc fc
VSB-U
f
-fc fc
MODULATION AND DEMODULATION
y(t) y(t)
VSB
DSB r(t)
m(t) HI(f) HO(f) z(t)
PLL cos ω t
c
cos ωc t
𝑌𝑉𝑆𝐵 (𝑓) = {𝑀(𝑓 + 𝑓𝑐 ) + 𝑀(𝑓 − 𝑓𝑐 )} × 𝐻𝐼 (𝑓)
𝑟(𝑡) = 𝑦𝑉𝑆𝐵 (𝑡) cos 𝜔𝑐 𝑡 ⇔ 𝑅(𝑓) = 𝑌𝑉𝑆𝐵 (𝑓 + 𝑓𝑐 ) + 𝑌𝑉𝑆𝐵 (𝑓 − 𝑓𝑐 )
𝑍(𝑓) = 𝑀(𝑓){𝐻𝐼 (𝑓 + 𝑓𝑐 ) + 𝐻𝐼 (𝑓 − 𝑓𝑐 )} × 𝐻𝑂 (𝑓)
1
And to obtain 𝑧(𝑡) = 𝑚(𝑡), 𝐻𝑂 (𝑓) = 𝐻 (𝑓+𝑓 )+𝐻 (𝑓−𝑓 )
𝐼 𝑐 𝐼 𝑐
The VSB technique is used in the analog TV broadcasting system for video signals. The following
figure is a typical analog TV signal (Audio Video), why?
carrier
Audio
(FM)
Video (VSB)
Fc-1.25M Fc Fc+4.5M
-37-
Lecture Notes in “Analog Communications & Noise” Analog Modulation
Electrical Engineering | University of Anbar by: Dr. Mohammed AlMahamdy
3.2.5 AM Summary
(1) Full amplitude modulation (DSB-LC), in which the upper and lower sidebands are
transmitted in full, accompanied by the carrier wave. Accordingly, demodulation of an
AM signal is done rather simply in the receiver by using an envelope detector, for
example. It is for this reason we find that full AM is commonly used in commercial AM
radio broadcasting, which involves a single powerful transmitter and numerous
receivers that are relatively inexpensive to build.
(2) DSB-SC, in which only the upper and lower sidebands are transmitted. The suppression
of the carrier wave means that DSB-SC modulation requires much less power than full
AM to transmit the same message signal; this advantage of, however, attained at the
expense of increased receiver complexity. Suppressed carrier signals require
complicated circuits at receivers for synchronization. DSB-SC is therefore well suited for
point-to-point communication involving one transmitter and one receiver.
(3) SSB, in which only the upper sideband or lower sideband is transmitted. This is the
optimum form of CW modulation if is required the minimum transmitted power and the
minimum channel bandwidth for conveying a message signal from one point to another.
However, its use is limited to message signals with an energy gap centered on zero
frequency beside its complex systems.
(4) VSB, in which almost all of one sideband and a vestige of the other sideband are
transmitted. It requires a channel bandwidth that is between that required for SSB and
DSB-SC systems, and the saving in bandwidth can be significant if modulating signals
with large bandwidths are being handled, as in the case of television signals and high-
speed data.
-38-
