Transcript CHAPTER 2 – Part 2

CHAPTER 2 –
Part 3
AM Modulation – Types of AM
Methods
(SSB and VSB)
Why SSB?
• Shortage of AM and DSB signals is
basic information is transmitted
TWICE (2X).
Once in each sideband
Both sideband have identical information
No solid reason to transmit both!!
• One sideband may be suppressed, the
remaining sideband is SSB signal.
• Bandwidth SSB signal, BWSSB = fm(max)
Benefits of SSB Signals
• All power transmitted (one-half of power
in DSB method) represents intelligence.
• Occupy half spectrum space than AM and
DSB signals and less interference.
• Transmit stronger information and more
reliable over longer distances.
• Susceptible to less noise due to restricted
signal BW and receiver BW (major adv for
long-distance comm.)
• Experience less fading due to ionosphere.
How Fading Affect Signal?
• Fading means signal alternately
increase and decrease in strength as
pick-up by the receiver.
• The ionosphere bends all signals
compositions back to earth at slightly
different angles resulting canceling
out or adding up of the signals.
• Fading does not occur in SSB since
only one sideband transmitted.
Characteristic of SSB
• When no information is present, no RF
signal is transmitted (unusual condition) –
e.g voice pause
• A carrier modulated by a single tone audio
signal generates a constant frequency
pure sin wave at the rate of either the
upper or lower frequency.
• Modulating voice (varying amplitude and
freq) will create a complex RF SSB which
varies in freq and amplitude over narrow
spectrum bounded by voice BW.
SSB Power Content
• In practice an SSB transmitter generates
both sidebands and a switch is used to
select either USB or LSB transmission.
• Since SSB transmitter sends no carrier the
carrier power, PC = 0.
• The transmitter output is expressed in
peak envelope power (PEP) – maximum
power produced on voice amplitude peaks.
PEP Computation
• The PEP output power is computed by,
V = Voltage across tx antenna
R = antenna chac. impedance
2
2
Vp
Vrms
PEP (out) 

R
2R
Eq (2.17)
• The PEP input power is computed by,
PEP (in)  Vs I max
Eq (2.18)
Vs = dc supply voltage of final
amplifier stage
Imax = maximum final amplifier peak
current
Contd…
• PEP occurs only occasionally.
• Voice amplitude peaks are produced only
when very loud sound generated or words
emphasized.
• During normal speech levels, the average
power is typically only one-fourth or onethird of the PEP value with typical human
speech.
• Typical SSB transmitters designed to
handle the average power level on
continuous basis not the PEP.
Example 1
• Calculate the input and output PEP
assuming that a voice signal
produces a 120-Vpp signal across a
50- transmitting antenna and the
dc supply of the final amplifier stage
is 300 V with a peak current of 0.6
A. Hence evaluate the power
handling capability of a typical SSB
transmitter for this system.
Solution 1
• PEP (in) = VsImax = (300V)(0.6A)
•
= 180 W
• PEP (out) = Vp2/2R = (60)2/[2x50]
•
= 36 W
• Pavg = 1/4[PEP(out)] to 1/3[PEP(in)]
•
= 1/4[36W] to 1/3[36 W]
•
= 9 W to 12 W
Example 2
• A voice signal with a frequency range
of 300 to 3000 Hz modulates a 14.3MHz RF carrier. Determine the
frequency limit for the output signal
produced by SSB AM modulator and
its BW.
• Draw the time-domain wave and
spectrum using a constant 2 kHz sine
wave tone.
Solution 2
• The SSB signal may use either USB or
LSB.
• If chosen USB: Frequency band extends
from [14.3M + 300] to [14.3M + 3000]
i.e. 14,300,300 Hz to 14,303,000 Hz.
• If chosen LSB: Frequency band extends
from [14.3M – 3000] to [14.3M – 300] i.e.
14,297,000 Hz to 14,299,700 Hz.
• BW = fUSB(highest) –fUSB(lowest) or
fLSB(highest) – fLSB(lowest) = 2.7 kHz
Solution 2
Solution 2
Variations of SSB
Operations
• Single sideband suppressed-carrier
(SSBSC or simply SSB) – used for HF
mobile communications.
• Vestigial sideband (VSB) – used
worldwide for television (picture)
transmissions.
• Single sideband full-carrier (SSBFC) –
used in a low-distortion, compatible AM
broadcasting system whose transmitted
signal can be received by standard AM and
SSB receivers.
Contd…
• Single sideband reduced-carrier (SSBRC) – a
pilot carrier sys widely used in transmarine pointto-point radiotelephony.
• Single sideband controlled-carrier (SSBCC) –
another pilot carrier sys used in which power is
independent of modulation and output Pavg is
constant.
• Independent sideband (ISB) also called
twin-sideband suppressed-carrier (TSBSC) –
transmit two independent sidebands, each
containing different intelligence, frequently used
for HF point-to-point radiotelephony and primarily
used in military communications.
• We will discuss on the first THREE only.
Comparing Spectrum
of AM Methods
Figure 23 (a) & (b)
Contd…
Figure 23 (c) and (d)
Two Basic Methods for
SSB Generation
• Filter method.
• Phase cancellation method.
• Both methods use some form of balanced
modulator to suppress the carrier but
differ in the suppression of the unwanted
sideband.
• Commonly used SSB generators consist of
balanced modulator followed by an
extremely selective crystal, ceramic or
mechanical filter or phasing circuit.
Balanced Modulators
• The balanced modulator (BM) is a symmetrical
circuit that delivers an output signal containing
the sum and difference frequencies of the two
inputs.
• In theory the injected carrier is balanced out so
that it disappear in the output.
• Practical design characteristics determine the
extent to which the carrier can be balanced out.
– Existing BM suppress the carrier from 30 dB to 40 dB
below the PEP.
– The SSB filter further suppresses the carrier by an
additional 20 dB.
– Total carrier suppression of from 50 dB to 60 dB can be
reasonable be expected from SSB transmitter systems.
Refer Figure 24
Simplified Block Diagram of
a Filter-type SSB generator
Figure 24
Filter Method
• The widely used technique is filter method
which uses a BP filter to pass one
sideband while reject the other.
• The filters used in SSB transmitters must
have very fast rise and fall slopes and flat
BW characteristics.
• Crystal filters are commonly used in SSB
systems because they have very high Q
(selectivity) values.
• This method is not suitable for high
frequency signals (VHF and above).
Simplified Block Diagram of a
Filter-type SSB Transmitter
Figure 25
SSB Transmitter Action
Figure 26
Sideband Filters
• Voice transmission for adequate speech
intelligibility – audio frequency from 100
Hz to 3000 Hz
• After carrier suppression, one sideband
rejected without affecting the other.
• Closeness between sidebands require high
Q-filters for sharply defined skirts and
narrow pass band and flat band-pass
characteristics to pass the desired
sideband and reject the other.
Phase-cancellation Method
• The phase-cancellation method uses two matched
balanced modulators, both of which suppress the
carrier and have a double-sideband output.
• The two sideband signals are combined by a
summing circuit producing only the desired
sideband.
• Advantage of this method is SSB signal at the
operating frequency can be generated without
the frequency conversion.
• Disadvantage is the phase shifts and amplitudes
of the signals to the summing circuit must be
maintained.
Contd…
• This method can be used at higher
frequency band (VHF and above).
• Reconfigurable for any sideband
output i.e. upper or lower-generated
SSB signal.
• In case of upper sideband
modulation, the modulated output is
phase-shifted by /2.
Block Diagram of Phaseshift SSB Transmitter
Figure 27
Upper Sideband
SSB Modulator
Figure 28
Lower Sideband
SSB Modulator
Figure 29
Mathematical Analysis
of SSB Signals
• Recall from previous part that the
output of balanced modulator (DSB
signal) is
Vam( DSB) (t )  Vc m(t ) cos c t
Eq
(2.19)
 V V cos  t cos  t
c
m
m
c
Vc Vm
cos (c  m )t  cos (c  m )t 

2
• The expression is DSB signal contains
two sideband frequencies.
Contd…
• If one sideband is removed either by
filtering or canceling method, taking
USB to remain hence
Vc Vm
cos (c  m )t  Eq (2.20)
VUSB (t ) 
2
• Since all the transmitted power goes
into USB wave, yield  VcVm  2
Eq (2.21)


2


V
V
2
  c m
PT  
2R
8R
Contd…
• Demodulation of a single sideband signal
is achieved by multiplying it with a locally
generated synchronous carrier at the
receiver.
• Detectors using this principle are product
detectors which use balanced modulator
circuit.
• The carrier must be as closely
synchronized in frequency (≤100Hz) and
phase with the original carrier in the
transmitter to avoid distortion in the
modulated output.
Contd…
• To demonstrate the demodulation process,
multiply the received signal with Vccosct
yields,
V V

Vreceived (t )   c m cos (c  m )t Vc cos c t
 2
Eq (2.22)

2
Vc Vm
cos mt  cos (2c  m )t

2
• Low-pass filtering easily remove the 2nd term
which is the upper side freq at the second
harmonic of the local carrier freq, leaving only
the 1st term which is required information.
Demodulation using
carrier reinsertion
Figure 30
SSB Receiver (demodulator)
• SSB receivers are superheterodyne but
differ from standard AM receivers in that
additional circuitry must be used to
restore the suppressed carrier.
• Since SSB transmission is used
extensively in HF communications
systems, SSB receivers must be capable
of receiving signal in HF band (3 MHz to
30 MHz).
Block Diagram of
SSB Receiver
Figure 31
SSB Applications
• SSB Signals are
popularly used in
telephone systems
as well as in twoway radio.
• Two-way SSB
communications is
used in the
military, CB radio
and radio
amateurs.
Disadvantage of SSB
• SSB transmitters are more
complicated hence more expensive
than standard AM transmitters – due
to added cost and system complexity
involved in providing better
frequency stability in the transmitter
and receiver.
VESTIGAL
SIDEBAND
(VSG)
Vestigal Sidebands (VSB)
Overcomes the complexity problem
of coherent demodulation.
Obtained by passing one pass band
mostly and a vestige, just a trace, of
the other sideband of an AM/DSB-SC
modulation.
Easy to generate.
Bandwidth is ~ 25% greater than
SSB signals.
Contd…
Also called asymmetric sideband
system.
Compromise between DSB & SSB.
Low frequency including DC
components can also be transmitted
and received.
Average transmitted power equals
SSB + small power of carrier.
Contd…
 AM wave is applied to a vestigial sideband
filter, producing a modulation scheme –
VSB + C
 Mainly used for television video
transmission.
 VSB allows recovery of the base band
signal with a receiving system (envelope
detector) intended for AM signal.
 Avoid the necessity of coherent
demodulation.
 Introduce some distortion.
VSB Spectrum
Figure 32
Eq (2.23)
Demodulation of VSB Signal
Using envelope detector.
Mathematically written as
SVSB(t) = Ac cos ct + (mAc / 2) cos(c+m)t
= Ac (1 + (m/2)cos m t) cos c t –
(mAc /2) sin c t sin m t
Eq (2.24)
COMPARISON
Table 1
Summary of Various
AM Methods
DSB-SC
o Less transmitted power – all the
transmitted power is useful.
o Wide bandwidth.
o Requires a coherent carrier at the
receiver – increased complexity.
o Suited for point to point communication
involving one Tx and one Rx which would
justify the use of increased Rx
complexity.
Contd…
Conventional
o Sidebands are transmitted in full with
the carrier.
o Simple to demodulate ( envelope
detector).
o Wide bandwidth.
o Used in commercial AM radio
broadcasting - one TX, many
receivers
Contd…
SSB
o Good bandwidth utilisation (information
signal BW = modulated signal BW)
o Good Power efficiency.
o Demodulation is harder compares to
conventional – exact filter design &
coherent demodulation.
o Widely use for long distance transmission.
o Preferred in systems requiring a minimal
BW – multi channel carrier telephony.
Contd…
VSB
o Compromise solution between DSB &
SSB.
o Has lower power, less bandwidth
than DSB.
o Higher power, slightly greater
bandwidth than SSB.
o Standard for transmission of TV (low
frequency component).