ANALOG COMMUNICATIONS

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ANALOG COMMUNICATIONS

• EE721

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MAIN TOPICS

• Introduction to Communication Systems
• Radio-Frequency Circuits
• Amplitude Modulation
• AM Receivers
• AM Transmitters
• Suppressed-Carrier AM Systems
• Test 1 4th week Test 2 7th week

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Elements of a Communication System

• Communication involves the transfer of
  information or intelligence from a source to a
  recipient via a channel or medium.
• Basic block diagram of a communication system

Channel
Source
Transmitter
Receiver
Recipient
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Brief Description

• Source analogue or digital
• Transmitter transducer, amplifier, modulator,
  oscillator, power amp., antenna
• Channel e.g. cable, optical fibre, free space
• Receiver antenna, amplifier, demodulator,
  oscillator, power amplifier, transducer
• Recipient e.g. person, speaker, computer

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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
• Types of modulation AM, FM, and PM

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Information and Bandwidth

• Bandwidth required by a modulated signal depends
  on the baseband frequency range (or data rate)
  and the modulation scheme.
• Hartleys Law I k t B
• where I amount of information
• k a constant of the system
• t time available
• B channel bandwidth

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Frequency Bands

• BAND Hz
• ELF 30 - 300
• AF 300 - 3 k
• VLF 3 k - 30 k
• LF 30 k - 300 k
• MF 300 k - 3 M
• HF 3 M - 30 M

• BAND Hz
• VHF 30M-300M
• UHF 300M - 3 G
• SHF 3 G - 30 G
• EHF 30 G - 300G

• Wavelength, l c/f

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Types of Signal Distortion

• Types of distortion in communications
• harmonic distortion
• intermodulation distortion
• nonlinear frequency response
• nonlinear phase response
• noise
• interference

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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

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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)CoSAn cos nw t SBn sin nw t (Fourier
  series)

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Effect of Filtering

• Theoretically, a non-sinusoidal signal would
  require an infinite bandwidth but practical
  considerations would band-limit the signal.
• Channels with too narrow a bandwidth would remove
  a significant number of frequency components,
  thus causing distortions in the time-domain.
• A square-wave has only odd harmonics

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External Noise

• Equipment / Man-made Noise is generated by any
  equipment that operates with electricity
• Atmospheric Noise is often caused by lightning
• Space Noise is strongest from the sun and, at a
  much lesser degree, from other stars

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Internal Noise

• Thermal Noise is produced by the random motion of
  electrons in a conductor due to heat. Noise
  power, PN kTB
• where T absolute temperature in oK
• k Boltzmanns constant, 1.38x10-23 J/K
• B noise power bandwidth in Hz
• Noise voltage,

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Internal Noise (contd)

• Shot Noise is due to random variations in current
  flow in active devices.
• Partition Noise occurs only in devices where a
  single current separates into two or more paths,
  e.g. bipolar transistor.
• Excess Noise is believed to be caused by
  variations in carrier density in components.
• Transit-Time Noise occurs only at high f.

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Noise Spectrum of Electronic Devices
Device Noise
Transit-Time or High-Frequency Effect Noise
Excess or Flicker Noise
Shot and Thermal Noises
f
1 kHz
fhc
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Signal-to-Noise Ratio

• An important measure in communications is the
  signal-to-noise ratio (SNR or S/N). It is often
  expressed in dB

In FM receivers, SINAD (SND)/(ND) is usually
used instead of SNR.
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Noise Figure

• Noise Figure is a figure of merit that indicates
  how much a component, or a stage degrades the SNR
  of a system
• NF (S/N)i / (S/N)o
• where (S/N)i input SNR (not in dB)
• and (S/N)o output SNR (not in dB)
• NF(dB)10 log NF (S/N)i (dB) - (S/N)o (dB)

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Equivalent Noise Temperature and Cascaded Stages

• The equivalent noise temperature is very useful
  in microwave and satellite receivers.
• Teq (NF - 1)To
• where To is a ref. temperature (often 290 oK)
• When two or more stages are cascaded

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High-Frequency Effects

• Stray reactances of components (including the
  traces on a circuit board) can result in
  parasitic oscillations / self resonance and other
  unexpected effects in RF circuits.
• Care must be given to the layout of components,
  wiring, ground plane, shielding and the use of
  bypassing or decoupling circuits.

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Radio-Frequency Amplifiers
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Narrow-band RF Amplifiers

• Many RF amplifiers use resonant circuits to limit
  their bandwidth. This is to filter off noise and
  interference and to increase the amplifiers
  gain.
• The resonant frequency (fo) , bandwidth (B), and
  quality factor (Q), of a parallel resonant
  circuit are

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Narrowband Amplifier (contd)

• In the CE amplifier, both the input and output
  sections are transformer-coupled to reduce the
  Miller effect. They are tapped for impedance
  matching purpose. RC and C2 decouple the RF from
  the dc supply.
• The CB amplifier is quite commonly used at RF
  because it provides high input impedance and also
  avoids the Miller effect.

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Wideband RF Amplifiers

• Wideband / broadband amplifiers are frequently
  used for amplifying baseband or intermediate
  frequency (IF) signals.
• The circuits are similar to those for narrowband
  amplifiers except no tuning circuits are
  employed.
• Another method of designing wideband amplifiers
  is by stagger-tuning.

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Stagger-Tuned IF Amplifiers
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Amplifier Classes

• An amplifier is classified as
• Class A if it conducts current throughout the
  full input cycle (i.e. 360o). It operates
  linearly but is very inefficient - about 25.
• Class B if it conducts for half the input cycle.
  It is quite efficient (about 60) but would
  create high distortions unless operated in a
  push-pull configuration.

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Class B Push-Pull RF Amplifier
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Class C Amplifier

• Class C amplifier operates for less than half of
  the input cycle. Its efficiency is about 75
  because the active device is biased beyond
  cutoff.
• It is commonly used in RF circuits where a
  resonant circuit must be placed at the output in
  order to keep the sine wave going during the
  non-conducting portion of the input cycle.

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Class C Amplifier (contd)
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Frequency Multipliers

• One of the applications of class C amplifiers is
  in frequency multiplication. The basic block
  diagram of a frequency multiplier

High Distortion Device Amplifier
Tuning Filter Circuit
Output
N x fi
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Principle of Frequency Multipliers

• A class C amplifier is used as the high
  distortion device. Its output is very rich in
  harmonics.
• A filter circuit at the output of the class C
  amplifier is tuned to the second or higher
  harmonic of the fundamental component.
• Tuning to the 2nd harmonic doubles fi tuning to
  the 3rd harmonic triples fi etc.

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Waveforms for Frequency Multipliers
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Neutralization

• At very high frequencies, the junction
  capacitance of a transistor could introduce
  sufficient feedback from output to input to cause
  unwanted oscillations to take place in an
  amplifier.
• Neutralization is used to cancel the oscillations
  by feeding back a portion of the output that has
  the opposite phase but same amplitude as the
  unwanted feedback.

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Hazeltine Neutralization
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Rice Neutralization
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Transformer-Coupled Neutralization
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Inductive Neutralization
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Oscillators
AV

• Barkhausen criteria for sustained oscillations
• The closed-loop gain, BAV 1.
• The loop phase shift 0o or some integer
  multiple of 360o at the operating frequency.

• AV open-loop gain
• B feedback factor/fraction

Output
B
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Hartley Oscillators
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Colpitts Oscillator
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Clapp Oscillator
The Clapp oscillator is a variation of the
Colpitts circuit. C4 is added in series with L in
the tank circuit. C2 and C3 are chosen large
enough to swamp out the transistors junction
capacitances for greater stability. C4 is often
chosen to be ltlt either C2 or C3, thus making C4
the frequency determining element, since CT C4.
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Voltage-Controlled Oscillator

• VCOs are widely used in electronic circuits for
  AFC, PLL, frequency tuning, etc.
• The basic principle is to vary the capacitance of
  a varactor diode in a resonant circuit by
  applying a reverse-biased voltage across the
  diode whose capacitance is approximately

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Crystals

• For high frequency stability in oscillators, a
  crystal (such as quartz) has to be used.
• Quartz is a piezoelectric material deforming it
  mechanically causes the crystal to generate a
  voltage, and applying a voltage to the crystal
  causes it to deform.
• Externally, the crystal behaves like an
  electrical resonant circuit.

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Packaging, symbol, and characteristic of crystals
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Crystal-Controlled Oscillators
Pierce
Colpitts
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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).

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Dual-Gate MOSFET Mixer
Good dynamic range and fewer unwanted o/p
frequencies.
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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
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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 1/2cos(X-Y) - cos(XY)
• Therefore, vo A/2cos(w1-w2)t-cos(w1w2)t.
• The last equation shows that the output of the
  balanced mixer consists of the sum and difference
  of the input frequencies.

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Balanced Ring Diode Mixer
Balanced mixers are also called balanced
modulators.
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Phase-Locked Loop

• The PLL is the basis of practically all modern
  frequency synthesizer design.
• The block diagram of a simple PLL

Vp
fr
fo
Phase Detector
Loop Amplifier
LPF
VCO
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Operation of PLL

• Initially, the PLL is unlocked, i.e.,the VCO is
  at the free-running frequency, fo.
• Since fo is probably not the same as the
  reference frequency, fr , the phase detector will
  generate an error/control voltage, Vp.
• Vp is filtered, amplified, and applied to the VCO
  to change its frequency so that fo fr. The PLL
  will then remain in phase lock.

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PLL Frequency Specifications
There is a limit on how far apart the
free-running VCO frequency and the reference
frequency can be for lock to be acquired or
maintained.
Lock Range
Capture Range
Free-Running Frequency
f
fo
fLC
fLL
fHC
fHL
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PLL Frequency Synthesizer
For output frequencies in the VHF range and
higher, a prescaler is required. The prescaler
is a fixed divider placed ahead of the
programmable divide by N counter.
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AM Waveform
AM signal es (Ec em) sin wct
ec Ec sin wct em Em sin wmt
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Modulation Index

• The amount of amplitude modulation in a signal is
  given by its modulation index

where, Emax Ec Em Emin Ec - Em (all pk
values)
When Em Ec , m 1 or 100 modulation.
Over-modulation, i.e. EmgtEc , should be
avoided because it will create distortions and
splatter.
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Effects of Modulation Index
m 1
m gt 1
In a practical AM system, it usually contains
many frequency components. When this is the case,
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AM in Frequency Domain

• The expression for the AM signal
• es (Ec em) sin wct
• can be expanded to
• es Ec sin wct ½ mEccos (wc-wm)t-cos
  (wcwm)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.

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AM Spectrum
Ec
mEc/2
mEc/2
fm
fm
f
fusf
fc
flsf
fusf fc fm flsf fc - fm Esf mEc/2
Bandwidth, B 2fm
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AM Power

• Total average (i.e. rms) power of the AM signal
  is PT Pc 2Psf , where
• Pc carrier power and Psf side-frequency
  power
• If the signal is across a load resistor, R, then
  Pc Ec2/(2R) and Psf m2Pc/4. So,

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AM Current

• The modulation index for an AM station can be
  measured by using an RF ammeter and the following
  equation

where I is the current with modulation and Io
is the current without modulation.
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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

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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

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Tuned-Radio-Frequency (TRF) Receiver

• The TRF receiver is the simplest receiver that
  meets all the basic requirements.

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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 ? BWfo/Q.
• Selectivity is not constant for a wide range of
  frequencies due to skin effect which causes the
  BW to vary with ?fo.

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Superheterodyne Receiver
Block diagram of basic superhet receiver
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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.

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Mixer and Local Oscillator

• The mixer and LO frequency convert the input
  frequency, fc, to a fixed fIF

High-side injection fLO fc fIF
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Autodyne Converter

• Sometimes called a self-excited mixer, the
  autodyne converter combines the mixer and LO into
  a single circuit

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IF Amplifier, Detector, AGC
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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.

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Diode Detector Waveforms
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Diagonal Clipping Distortion
Diagonal clipping distortion is more pronounced
at high modulation index or high modulation
frequency.
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Sensitivity and Selectivity

• Sensitivity is expressed as the minimum input
  signal required to produce a specified output
  level for a given (SN)/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.

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Shape Factor
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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.

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Image Frequency Rejection

• For a tuned circuit with a quality factor of Q,
  then the image frequency rejection is

In dB, IR (dB) 20 log IR
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IF Transformers

• The transformers used in the IF stages can be
  either single-tuned or double-tuned.

Double-tuned
Single-tuned
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Loose and Tight Couplings

• For single-tuned transformers, tighter coupling
  means more gain but broader bandwidth

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Under, Over, Critical Coupling

• Double-tuned transformers can be over, under,
  critically, or optimally coupled

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Coupling Factors

• Critical coupling factor kc is given by

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 kgtkc undercoupling, klt kc

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Piezoelectric Filters

• For narrow bandwidth (e.g. several kHz),
  excellent shape factor and stability, a crystal
  lattice is used as bandpass filter.
• Ceramic filters, because of their lower Q, are
  useful for wideband signals (e.g. FM broadcast).
• Surface-acoustic-wave (SAW) filters are ideal for
  high frequency usage requiring a carefully shaped
  response.

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Block Diagram of AM TX
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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.

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Transmitter Stages (contd)

• 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.

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Transmitter Stages (contd)

• 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.

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AM Modulator Circuits
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Impedance Matching Networks

• Impedance matching networks at the output of RF
  circuits are necessary for efficient transfer of
  power. At the same time, they serve as low-pass
  filters.

Pi network
T network
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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, overmodulation, or non-linearity
  is easier to observe with this method.

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Trapezoidal Pattern (contd)
mlt1
m1
mgt1
Improper phase
-VpgtVp
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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
  PtPc(1m2/2 ), then at m1, Pt 6 Psf .
• SSB also has a bandwidth reduction of half, which
  in turn reduces noise by half.

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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
USB
DSB-SC
BPF
or
AF Input
LSB
Carrier Oscillator
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Waveforms for Balanced Modulator
V2, fm
Vo
V1, fc
f
fc-fm
fcfm
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LIC Balanced Modulator 1496
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Filter for SSB

• Filters with high Q are needed for suppressing
  the unwanted sideband.

fa fc - f2 fb fc - f1 fd fc f1 fe fc
f2
where X attenuation of sideband, and ?f fd -
fb
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Typical SSB TX using Filter Method
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SSB Waveform
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Generating SSB - Phasing Method

• This method is based on the fact that the lsf and
  the usf are given by the equations
• cos (wc - wm)t ½(cos wct cos wmt sin wct
  sin wmt)
• cos (wc wm)t ½(cos wct cos wmt - sin wct
  sin wmt)
• The RHS of the 1st equation is just the sum of
  two products the product of the carrier and the
  modulating signal, and the product of the same
  two signals that have been phase shifted by 90o.
• The 2nd equation is similar except for the (-)
  sign.

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Diagram for Phasing Method
Balanced Modulator 1
Modulating signal Em cos wmt
Carrier oscillator
Ec cos wct
SSB output

90o phase shifter
90o phase shifter
Balanced Modulator 2
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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.

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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.

where Vp peak signal voltage and RL load
resistance
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Block Diagram of SSB RX
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SSB Receiver (contd)

• 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.

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SSB RX (contd)

• 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 BFO.