Download ANALOG COMMUNICATIONS

Principles of Communication
EEC-502
Unit-I
• Introduction of Communication System
• Amplitude Modulation
Elements of a Communication System
Source
Transmitter
Receiver
Recipient
Description of Communication Elements
• Source: Analogue or digital like video, text or speech.
• Input Transducer: e.g. microphone that converts audio signal
into electrical signal.
• Transmitter: Transducer, amplifier, modulator, oscillator,
power amp., antenna. It modulates and transmits source data.
• Channel: Coaxial cable, optical fiber, free space etc.
• Receiver: Antenna, amplifier, demodulator, oscillator, power
amplifier, transducer. It demodulates the original signal from
modulated signal.
• Output Transducer: e.g. loudspeaker that converts electrical
signal into sound signal.
• Recipient: Person, speaker, computer etc.
Communication Channels
• Telephone channels
• Optical Fiber
• Mobile radio channel
Communication Channels
• Satellite channel
Modulation
• Modulation is the process of impressing information onto a
high-frequency carrier for transmission.
• Reasons for modulation:
–
–
–
–
to prevent mutual interference between stations
to reduce the size of the antenna required
Common processing
Narrowbanding
Frequency Bands
Baseband Signals
• The message signal generated from the information source is
known as baseband signal.
Analog baseband Signal
Digital baseband Signal
Band pass Signals
• Baseband signal is impressed upon a carrier, modulated signal is
produced. This signal has fixed band of frequencies around
carrier frequency so known as bandpass type.
Analog band pass Signal
Digital band pass Signal
Information and Bandwidth
 Bandwidth required by a modulated signal depends on the
baseband frequency range (or data rate) and the modulation
scheme.
 Hartley’s Law: I = k t B
where I = amount of information; k = system constant; t = time
available; B = channel bandwidth
 Shannon’s Formula: I = B log2 (1+ S/N) in bps
where S/N = signal-to-noise power ratio
Transmission Modes
 Simplex (SX) – One direction only, e.g. TV
 Half Duplex (HDX) – Both directions but not at the same
time, e.g. CB radio
 Full Duplex (FDX) – Transmit and receive
simultaneously between two stations, e.g. standard
telephone system
 Full/Full Duplex (F/FDX) - Transmit and receive
simultaneously but not necessarily just between two
stations, e.g. data communications circuits
Basic Analog Communications System
Baseband signal
(electrical signal)
Input
transducer
Transmitter
EM waves (modulated
signal)
Transmission
Channel
Modulator
EM waves (modulated
signal)
Carrier
Baseband signal
(electrical signal)
Output
transducer
Receiver
Demodulator
Time and Frequency Domains
• Time domain: An oscilloscope displays the amplitude
versus time.
• Frequency domain: A spectrum analyzer displays the
amplitude or power versus frequency.
• Frequency-domain display provides information on
bandwidth and harmonic components of a signal.
Non-sinusoidal Waveform
• Any well-behaved periodic waveform can be
represented as a series of sine and/or cosine waves plus
(sometimes) a dc offset:
e(t)=Co+SAn cos nw t + SBn sin nw t (Fourier series)
Amplitude Modulation
Waveforms of Amplitude Modulation
Analysis of Amplitude Modulated Carrier Wave
Let vc = Vc Sin wct
vm = Vm Sin wmt
The amplitude modulated wave is given by the equation
A = Vc + vm = Vc + Vm Sin wmt = Vc [1+ (Vm/Vc Sin wmt)]
= Vc (1 + mSin wmt)
m – Modulation Index. The ratio is Vm/Vc.
v = A Sin wct = Vc (1 + m Sin wmt) Sin wct
= Vc Sin wct + mVc (Sin wmt Sin wct)
v = Vc Sin wct + [mVc/2 Cos (wc-wm)t – mVc/2 Cos (wc + wm)t]
Frequency Spectrum of AM
Lower side frequency – (wc – wm)/2
Upper side frequency – (wc +wm)/2
Modulation Index
The ratio between the amplitude change of carrier wave to the
amplitude of the normal carrier wave is called modulation index.
Modulation Index
m
Em
Em ax Em in
or
Ec
Em ax Em in
where, Emax = Ec + Em; Emin = Ec - Em (all peak values)
When Em = Ec , m =1 or 100% modulation.
Over-modulation, i.e. Em>Ec , should be avoided
because it will create distortions and splatter.
Effects of Modulation Index
Effect of Over Modulation
AM in Frequency Domain
• The expression for the AM signal:
es = (Ec + em) sin wct
es = Ec sin wct + ½ mEc[cos (wc-wm)t-cos (wc+wm)t]
• The expanded expression shows that the AM
signal consists of the original carrier, a lower side
frequency, flsf = fc - fm, and an upper side
frequency, fusf = fc + fm.
AM Spectrum
Ec
mEc/2
mEc/2
fm
flsf
fm
fc
fusf
f
fusf = fc + fm ; flsf = fc - fm ; Esf = mEc/2
Bandwidth, B = 2fm
AM Power
Ptotal = Pcarrier + PLSB + PUSB
Considering additional resistance like antenna resistance R.
Pcarrier = [(Vc/√2)/R]2 = V2C/2R
Each side band has a value of m/2 Vc and r.m.s value of mVc/2√2.
Hence power in LSB and USB can be written as
PLSB = PUSB = (mVc/2√2)2/R = m2/4*V2C/2R = m2/4 Pcarrier
Ptotal = V2C/2R + [m2/4*V2C/2R] + [m2/4*V2C/2R]
= V2C/2R (1 + m2/2) = Pcarrier (1 + m2/2)
PT
Pc (1
m2
)
2
AM Current
• The modulation index for an AM station can be measured
by using an RF ammeter and the following equation:
I
Io
m2
1
2
where I is the current with modulation and
Io is the current without modulation.
Complex AM Waveforms
• For complex AM signals with many frequency components,
all the formulas encountered before remain the same, except
that m is replaced by mT. For example:
2
PT
mT
PC (1
); I
2
mT
Io 1
2
2
Limitations of Amplitude Modulation
•
•
•
•
Low Efficiency
Limited Operating Range
Noisy Reception
Poor Audio Quality
Block Diagram of AM TX
Transmitter Stages
• Crystal oscillator generates a very stable sinewave
carrier. Where variable frequency operation is
required, a frequency synthesizer is used.
• Buffer isolates the crystal oscillator from any load
changes in the modulator stage.
• Frequency multiplier is required only if HF or
higher frequencies is required.
Transmitter Stages (cont’d)
• RF voltage amplifier boosts the voltage level of the
carrier. It could double as a modulator if low-level
modulation is used.
• RF driver supplies input power to later RF stages.
• RF Power amplifier is where modulation is applied for
most high power AM TX. This is known as high-level
modulation.
Transmitter Stages (cont’d)
• High-level modulation is efficient since all previous RF
stages can be operated class C.
• Microphone is where the modulating signal is being
applied.
• AF amplifier boosts the weak input modulating signal.
• AF driver and power amplifier would not be required
for low-level modulation.
Linear AM Modulator Circuits
Square Law Diode Modulator
Trapezoidal Pattern
• Instead of using the envelope display to look at AM
signals, an alternative is to use the trapezoidal pattern
display. This is obtained by connecting the modulating
signal to the x input of the ‘scope and the modulated
AM signal to the y input.
• Any distortion, over modulation, or non-linearity is
easier to observe with this method.
Trapezoidal Pattern (cont’d)
m<1
m
m=1
Vm ax Vm in
Vm ax Vm in
m>1
Improper
-Vp>+Vp
phase
Amplitude Demodulators
• Demodulators, or detectors, are circuits that
accept modulated signals and recover the original
modulating information.
Diode Detector
Waveforms of Diode Detector
Double Side Band –Suppressed Carrier(DSB-SC)
In this information is contained in two sidebands only and carrier is
suppressed.
Double Side Band –Suppressed Carrier(DSB-SC)
Balanced Modulator
Balanced Modulator
• A balanced modulator is a circuit that generates a
DSB signal, suppressing the carrier and leaving only
the sum and difference frequencies at the output.
• The output of a balanced modulator can be further
processed by filters or phase-shifting circuitry to
eliminate one of the sidebands, resulting in a SSB
signal.
Waveforms for Balanced Modulator
V2, fm
Vo
V1, fc
fc-fm fc+fm
f
Mathematical Analysis of Balanced Modulator
• V1 = A1sin ct; V2 = A2sin mt
• Vo = V1V2 = A1A2sin ct sin mt
= ½A1A2{cos( c- m)t – cos( c+ m)t}
• The equation above shows that the output of the
balanced modulator consists of a lower side-frequency
( c - m) and an upper side-frequency ( c+ m)
Suppressed-Carrier AM Systems
• Full-carrier AM is simple but not efficient in terms of
transmitted power, bandwidth, and SNR.
• Using single-sideband suppressed-carrier (SSBSC or
SSB) signals, since Psf = m2Pc/4, and Pt=Pc(1+m2/2 ),
then at m=1, Pt= 6 Psf .
• SSB also has a bandwidth reduction of half, which in
turn reduces noise by half.
Generating SSB - Filtering Method
• The simplest method of generating an SSB signal is to
generate a double-sideband suppressed-carrier (DSB-SC)
signal first and then removing one of the sidebands.
Balanced
Modulator DSB-SC
USB
BPF
AF
Input
Carrier
Oscillator
or
LSB
Filter for SSB
• Filters with high Q are needed for suppressing the
unwanted sideband.
fa = f c - f2
fb = fc - f1
fd = fc + f1
fe = f c + f 2
Q
f c anti log( X dB / 20) where X = attenuation of
4 f
sideband, and f = fd - fb
Typical SSB TX using Filter Method
SSB Waveform
Generating SSB - Phasing Method
Phasing vs Filtering Method



•
Advantages of phasing method :
No high Q filters are required.
Therefore, lower fm can be used.
SSB at any carrier frequency can be generated
in a single step.
Disadvantage:
Difficult to achieve accurate 90o phase shift across
the whole audio range.
Peak Envelope Power
• SSB transmitters are usually rated by the peak envelope
power (PEP) rather than the carrier power. With voice
modulation, the PEP is about 3 to 4 times the average or
rms power.
PEP
Vp
2
2 RL
where Vp = peak signal voltage
and RL = load resistance
Non-coherent SSB BFO RX
Coherent SSB BFO Receiver
RF SSBRC
RF
input
signal
RF amplifier
and
preselector
IF SSBRC
IF amp. &
RF mixer
bandpass
filter
RF LO
Carrier recovery
and frequency
synthesizer
IF
mixer
BFO
Demod.
info
Explanation of SSB Receivers
• The input SSB signal is first mixed with the LO signal
(low-side injection is used here).
• The filter removes the sum frequency components and
the IF signal is amplified.
• Mixing the IF signal with a reinserted carrier from a
beat frequency oscillator (BFO) and low-pass filtering
recovers the audio information.
SSB Receivers (cont’d)
• The product detector is often just a balanced modulator
operated in reverse.
• Frequency accuracy and stability of the BFO is critical.
An error of a little more than 100 Hz could render the
received signal unintelligible.
• In coherent or synchronous detection, a pilot carrier is
transmitted with the SSB signal to synchronize the RF
local oscillator and BFO.
Vestigial Side Band
Vestigial Side Band
Quadrature Amplitude Modulator
AM Receivers
•




Basic requirements for receivers:
Ability to tune to a specific signal
Amplify the signal that is picked up
Extract the information by demodulation
Amplify the demodulated signal
Two important receiver specifications:
sensitivity and selectivity
Tuned-Radio-Frequency (TRF) Receiver
• The TRF receiver is the simplest receiver that
meets all the basic requirements.
Drawbacks of TRF Receivers
• Difficulty in tuning all the stages to exactly the same
frequency simultaneously.
• Very high Q for the tuning coils are required for good
selectivity BW=fo/Q.
• Selectivity is not constant for a wide range of frequencies
due to skin effect which causes the BW to vary with fo.
Superheterodyne Receiver
Block diagram of basic superhetrodyne receiver:
Antenna and Front End
• The antenna consists of an inductor in the form of a
large number of turns of wire around a ferrite rod. The
inductance forms part of the input tuning circuit.
• Low-cost receivers sometimes omit the RF amplifier.
• Main advantages of having RF amplifier: improves
sensitivity and image frequency rejection.
Mixers
• A mixer is a nonlinear circuit that combines two signals
in such a way as to produce the sum and difference of
the two input frequencies at the output.
• A square-law mixer is the simplest type of mixer and is
easily approximated by using a diode, or a transistor
(bipolar, JFET, or MOSFET).
Dual-Gate MOSFET Mixer
Good dynamic range and fewer unwanted o/p frequencies.
Balanced Mixers
• A balanced mixer is one in which the input frequencies
do not appear at the output.
Ideally, the only
frequencies that are produced are the sum and
difference of the input frequencies.
Circuit symbol:
f1
f1+ f2
f2
Equations for Balanced Mixer
Let the inputs be v1 = sin w1t and v2 = sin w2t.
A balanced mixer acts like a multiplier.
Thus its output, vo = Av1v2 = A sin w1t sin w2t.
Since sin X sin Y = 1/2[cos(X-Y) - cos(X+Y)]
Therefore, vo = A/2[cos(w1-w2)t-cos(w1+w2)t].
The last equation shows that the output of the balanced
mixer consists of the sum and difference of the input
frequencies.
Balanced Ring Diode Mixer
Balanced mixers are also called balanced modulators.
Waveforms for Frequency Multipliers
Mixer and Local Oscillator
• The mixer and LO frequency convert the input
frequency, fc, to a fixed fIF:
High-side injection: fLO = fc + fIF
Autodyne Converter
• Sometimes called a self-excited mixer, the autodyne converter
combines the mixer and LO into a single circuit:
IF Amplifier, Detector, & AGC
IF Amplifier and AGC
• Most receivers have two or more IF stages to provide
the bulk of their gain (i.e. sensitivity) and their
selectivity.
• Automatic gain control (AGC) is obtained from the
detector stage to adjusts the gain of the IF (and
sometimes the RF) stages inversely to the input signal
level. This enables the receiver to cope with large
variations in input signal.
Sensitivity and Selectivity
• Sensitivity is expressed as the minimum input signal
required to produce a specified output level for a given
(S+N)/N ratio.
• Selectivity is the ability of the receiver to reject
unwanted or interfering signals. It may be defined by
the shape factor of the IF filter or by the amount of
adjacent channel rejection.
Shape Factor
SF
B 60 dB
B 6 dB
Image Frequency
• One of the problems with the superhet receiver is that
an image frequency signal could interfere with the
reception of the desired signal. The image frequency is
given by: fimage = fsig + 2fIF
where
fsig = desired signal
• An image signal must be rejected by tuning circuits
prior to mixing.
Image-Frequency Rejection Ratio
• For a tuned circuit with a quality factor of Q, its imagefrequency rejection ratio is:
IFRR
x
2
1 Q x
2
f image
f sig
f sig
f image
where,
In dB, IFRR(dB) = 20 log IFRR
IF Transformers
• The transformers used in the IF stages can be either
single-tuned or double-tuned.
Single-tuned
Double-tuned
Loose and Tight Couplings
• For single-tuned transformers, tighter coupling means
more gain but broader bandwidth:
Under, Over, & Critical Coupling
• Double-tuned transformers can be over, under,
critically, or optimally coupled:
Coupling Factors
• Critical coupling factor kc is given by:
kc
1
Q pQs
where Qp, Qs = prim. & sec. Q, respectively.
IF transformers often use the optimum coupling
factor, kopt = 1.5kc , to obtain a steep skirt and
flat passband. The bandwidth for a double-tuned
IF amplifier with k = kopt is given by B = kfo.
Overcoupling means k>kc; undercoupling, k< kc