Transcript Double Side Band Supressed Carrier

Single Side Band Suppressed Carrier
Professor Z Ghassemlooy
Electronics and IT Division
School of Engineering
Sheffield Hallam University
U.K.
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Content
•
•
•
•
•
Theory
Transmitter Implementation
Detector Implementation
Power Analysis
Summary
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Single Side Band Suppressed Carrier
Carrier
From DSB-SC spectrum:
• Information m is carried twice
• Bandwidth is is high
LSB
USB
c - m c c + m
Question: Why transmit both side bands?
Ans:
Single frequency
Question: Can one suppress one of the side bandcarrier?
Ans.: Yes, just transmit one side band (i.e SSB-SC)
But what is the penalty?
System complexity at the receiver
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SSB-SC - Implementation
• Frequency discrimination
Message
m(t)
DSB-SC
Band pass
filter
c+ c
c(t ) 
Multiplier
Local oscillator
c(t) = cos ct
Band pass
filter
c- c
c(t ) 
Upper sideband
ME c
cos (c  m )t
2
Lower sideband
ME c
cos (c  m )t
2
c(t )  Em cosmt cosct
MEc
MEc

cos(c  m )t 
cos(c  m4)t
2
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SSB-SC - Waveforms
B = m
USB
B = 2m
Bandwidth B = m
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SSB-SC - Implementation cont.
• Phase discrimination (Hartley modulator)
v(t) =Em cos mt cos ct + Em sin mt sin
ct
= Em cos (m - c)t LSB
Em cos mt
Message
m(t)
X E cos  t cos  t
m
m
c
Carrier cos ct
90o
phase shift

90o
phase shift
sin ct
Em sin mt
X
+
SSB-SC
signal
-
Em sin mt sin ct
v(t) =Em cos mt cos ct - Em sin mt sin
ct
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= Em cos (m + c)t
USB
SSB-SC - Hartley Modulator
• Advantages:
– No need for bulky and expensive band pass filters
– Easy to switch from a LSB to an USB SSB output
• Disadvantage:
– Requires Hilbert transform of the message signal. Hilbert
transform changes the phase of each +ve frequency
component by exactly - 90o.
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SSB-SC - Detection
• Synchronous detection
SSB-SC
ME c
v(t ) 
cos mt
4
Low
pass
filter
Multiplier
Local oscillator
c(t) = cos ct
Message signal
ME c
y (t ) 
cos (c  m )t  cos c t
2
y (t ) 
Condition:
•Local oscillator has the same
frequency and phase as that of the
carrier signal at the transmitter.
ME c
ME c
cos (  mt ) 
cos (2 c   m )t
4
4
information
high frequency
Low pass filter
m
2c+m
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SSB-SC - Synch. Detection cont.
• Case 1 - Phase error
SSB-SC
v(t ) 
Low
pass
filter
Multiplier
Local oscillator
c(t) = cos (ct+)
y (t ) 
y (t ) 
Condition:
•Local oscillator has the same
frequency but different phase as
that of the carrier signal at the
transmitter.
MEc
cos (m  )t
4
Message signal
ME c
cos (c  m )t  cos (c t  )
2
ME c
ME c
cos ( mt   ) 
cos (2 c t   m   )
4
4
information
high frequency
Low pass filter
m
2c+m
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SSB-SC - Synch. Detection cont.
• Case 1 - Frequency error
SSB-SC
v(t ) 
Low
pass
filter
Multiplier
Local oscillator
c(t) = cos
(c+)t
Condition:
•Local oscillator has the same
phase but different frequency as
that of the carrier signal at the
transmitter.
y (t ) 
y (t ) 
ME c
cos (m  )t
4
Message signal
ME c
cos (c  m )t  cos (c  )t
2
ME c
ME c
cos ( m   )t 
cos (2 c   m   )t
4
4
information
high frequency
Low pass filter
m +
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2c+m +
SSB-SC - Power
• The total power (or average power):
PT  SSB  SC
1  MEc / 2 
 

R
2

( MEc ) 2

8R
2
• The maximum and peak envelop power
PP  SSB  SC
( MEc )

4R
2
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SSB-SC - Summary
• Advantages:
–
–
–
–
Lower power consumption
Better management of the frequency spectrum
Less prone to selective fading
Lower noise
• Disadvantage:
- Complex detection
• Applications:
- Two way radio communications
- Frequency division multiplexing
- Up conversion in numerous telecommunication systems
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