Transcript Principles of Electronic Communication Systems

Principles of Electronic
Communication Systems
Second Edition
Louis Frenzel
© 2002 The McGraw-Hill Companies
Principles of Electronic
Communication Systems
Second Edition
Chapter 7
Radio Transmitters
©2003 The McGraw-Hill Companies
Radio Transmitters
A radio transmitter takes the information to be
communicated and converts it into an electronic
signal compatible with the communication medium.
 This process involves carrier generation, modulation,
and power amplification.
 The signal is fed by wire, coaxial cable, or waveguide
to an antenna that launches it into free space.
 Typical transmitter circuits include: oscillators,
amplifiers, frequency multipliers, and impedance
matching networks.
Topics Covered in Chapter 7

Transmitter Fundamentals

Carrier Generators
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Power Amplifiers

Impedance-Matching Networks

Typical Transmitter Circuit
Transmitter
The transmitter is the electronic unit that accepts the
information signal to be transmitted and converts it
into an RF signal capable of being transmitted over
long distances.
Transmitter Fundamentals
Every transmitter has three basic requirements:
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It must generate a carrier signal of the correct
frequency at a desired point in the spectrum.
It must provide some form of modulation that causes
the information signal to modify the carrier signal.
It must provide sufficient power amplification to
ensure that the signal level is high enough to carry the
desired distance.
Transmitter Configurations
A continuous wave (CW) transmission can be generated
by a transistor oscillator.
 The oscillator generates a carrier and can be switched
off and on by a telegraph key to produce the dots and
dashes of the International Morse code.
 CW is rarely used today as the oscillator power is too
low and the Morse code is nearly extinct.
 The CW transmitter can be improved by adding a
power amplifier.
CW Transmitter With Amplifier
Transmitter Types
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High-Level Amplitude Modulated Transmitter
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Low-Level Frequency Modulated Transmitter
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Single-Sideband Transmitter
Carrier Frequency
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The starting point for all transmitters is carrier
generation.

Once generated, the carrier can be modulated,
processed in various ways, amplified and transmitted.
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The source of most carriers is a crystal oscillator.

PLL frequency synthesizers are used in applications
requiring multiple channels of operation.
Carrier Generators

Crystal Oscillator
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Frequency Synthesizer
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Phase-Locked Loop Synthesizer

Direct Digital Synthesizer
Crystal Oscillator
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The only oscillator capable of maintaining the
frequency precision and stability demanded by the
FCC is a crystal oscillator.
A crystal is a piece of quartz that can be made to
vibrate and act like an LC tuned circuit.
Overtone crystals and frequency multipliers are two
devices that can be used to achieve crystal precision
and stability at frequencies greater than 30 MHz.
Colpitts-Type Emitter-Follower
Crystal Oscillator

Feedback is derived from a capacitive voltage divider.

Transistor configuration is typically an emitterfollower.

The output is taken from the emitter.
Crystal Oscillator Emitter-Follower
Circuit
By Definition…
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Pulling, or rubbering capacitors are used to make fine
adjustments to the crystal oscillator frequency.
Field-effect transistors (FETs) make good crystal
oscillators. A FET is used in the Pierce oscillator.
An overtone crystal is cut so that it optimizes its
oscillation at an overtone of the basic crystal
frequency.
The term harmonic is often used as a synonym for
overtone.
Crystal Switching
If a transmitter must operate on more than one
frequency, but crystal precision and stability are
required, multiple crystals can be used and the desired
one switched on.
Crystal Switching Circuit
Frequency Synthesizers
Frequency synthesizers are variable-frequency
generators that provide the frequency stability of
crystal oscillators but the convenience of incremental
tuning over a broad frequency range.
 Frequency synthesizers provide an output that varies
in fixed frequency increments over a wide range.
 In a transmitter, a frequency synthesizer provides
basic carrier generation.
 Synthesizers are used in receivers as local oscillators
and perform the receiver tuning function.
Phased-Locked Loop Synthesizer
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The PLL consists of a phase detector, a low-pass
filter, and a VCO.
The input to the phase detector is a reference
oscillator.
The reference oscillator is normally crystal-controlled
to provide high-frequency stability.
The frequency of the reference oscillator sets the
increments in which the frequency may be changed.
PLL Frequency Synthesizer
Direct Digital Synthesis

A direct digital synthesis (DDS) synthesizer generates
a sine-wave output digitally.

The output frequency can be varied in increments
depending upon a binary value supplied to the unit by
a counter, a register, or an embedded microcontroller.
Direct Digital Synthesis (Continued)
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A read-only memory (ROM) is programmed with the
binary representation of a sine wave.
These are the values that would be generated by an
analog-to-digital (A/D) converter if an analog sine
wave were digitized and stored in the memory.
If these binary values are fed to a digital-to-analog
(D/A) converter, the output of the D/A converter will
be a stepped approximation of the sine wave.
A low-pass filter (LPF) is used to remove the highfrequency content smoothing the sine wave output.
Basic DDS Frequency Source
Power Amplifiers
The three basic types of power amplifiers used in
transmitters are:
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Linear

Class C
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Switching
Linear Amplifier
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Linear amplifiers provide an output signal that is an
identical, enlarged replica of the input.
Their output is directly proportional to their input and
they faithfully reproduce an input, but at a higher
level.
Most audio amplifiers are linear.
Linear RF amplifiers are used to increase the power
level of variable-amplitude RF signals such as lowlevel AM or SSB signals.
Amplifier Classes
Linear amplifiers are class A, AB or B. The class of an amplifier
indicates how it is biased.
 Class A amplifiers are biased so that they conduct
continuously. The output is an amplified linear reproduction of
the input.
 Class B amplifiers are biased at cutoff so that no collector
current flows with zero input. Only one-half of the sine wave
is amplified.
 Class AB linear amplifiers are biased near cutoff with some
continuous current flow. They are used primarily in push-pull
amplifiers and provide better linearity than Class B amplifiers,
but with less efficiency.
Amplifier Classes (Continued)
Class C amplifiers conduct for even less than one-half
of the sine wave input cycle, making them very
efficient.
 The resulting highly distorted current pulse is used to
ring a tuned circuit to create a continuous sine-wave
output.
 Class C amplifiers cannot be used to amplify varyingamplitude signals.
 This type amplifier makes a good frequency
multiplier as harmonics are generated in the process.
Amplifier Classes (Continued)
Switching amplifiers act like ON-OFF or digital
switches.
 They effectively generate a square-wave output.
 Harmonics generated are filtered out by using high-Q
tuned circuits.
 The ON-OFF switching action is highly efficient.
 Switching amplifiers are designated class D, E, F, and
S.
Class A Buffer Amplifier
A class A buffer amplifier is used between the carrier
oscillator and the final power amplifier to isolate the
oscillator from the power amplifier load, which can
change the oscillator frequency.
A Class A RF Buffer Amplifier
Class B Push-Pull Amplifier
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In a class B push-pull amplifier, the RF driving signal
is applied to two transistors through an input
transformer.
The transformer provides impedance-matching and
base drive signals to the two transistors that are 180
degrees out of phase.
An output transformer couples the power to the
antenna or load.
A Push-Pull Class B Power
Amplifier
Class C Amplifier
The key circuit in most AM and FM transmitters is the
class C amplifier.
 These amplifiers are used for power amplification in
the form of drivers, frequency multipliers, and final
amplifiers.
 Class C amplifiers are biased so they conduct for less
than 180 degrees of the input.
 Current flows through a class C amplifier in short
pulses and a resonant tuned circuit is used for
complete signal amplification.
Tuned Output Circuits
All class C amplifiers have some form of tuned circuit connected
in the collector.
 The primary purpose of a tuned circuit is to form the complete
AC sine-wave output.
 A parallel tuned circuit rings, or oscillates, at its resonant
frequency whenever it receives a DC pulse.
 The pulse charges a capacitor, which in turn, discharges into
an inductor.
 The exchange of energy between the inductor and the
capacitor is called the flywheel effect and produces a damped
sine wave at the resonant frequency.
Class C Amplifier with Tuned
Output Circuit
By Definition…

Any class C amplifier is capable of performing
frequency multiplication if the tuned circuit in the
collector resonates at some integer multiple of the
input frequency.

Self-oscillation exists when some of the output
voltage finds its way back to the input of the
amplifier with the correct amplitude and phase and
the amplifier oscillates.
More Definitions…
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When an amplifier circuit oscillates at a higher
frequency unrelated to the tuned frequency, the
oscillation is referred to as parasitic oscillation.
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Neutralization is a process where an output signal in
an amplifier is fed back to the input to prevent
oscillation.
Switching Power Amplifiers
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A class D amplifier uses a pair of transistors to produce a
square-wave current in a tuned circuit.
In a class E amplifier only a single transistor is used. This
amplifier uses a low-pass filter and tuned impedance-matching
circuit that achieves a high level of efficiency.
A class F amplifier is a variation of the E amplifier. It contains
an additional resonant network which results in a steeper
square waveform. This waveform produces faster transistor
switching and better efficiency.
Class S amplifiers are found primarily in audio applications
but have also been used in low- and medium RF amplifiers.
Vacuum Tubes
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Vacuum tubes are still widely used in electronics,
especially in communication.
The most common vacuum tube in use today is the
cathode-ray tube (CRT) which is the heart of TV sets,
monitors, and other devices with visual displays.
Vacuum tubes are also widely used for RF power
amplification.
A vacuum tube is a current-controlled device in
which a small input signal controls a much larger
current and produces amplification.
Vacuum Tube
Triode Tube Linear Amplifier
Impedance-Matching Networks
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Matching networks that connect one stage to another are very
important parts of any transmitter.
The circuits used to connect one stage to another are known as
impedance-matching networks.
Typical networks are LC circuits, transformers, or some
combination.
The main function of a matching network is to provide for an
optimum transfer of power through impedance matching
techniques.
Matching networks also provide filtering and selectivity.
Impedance Matching in RF Circuits
Networks
There are three basic types of LC impedance-matching
networks. They are:
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L network

T network

П network
L Networks
L networks consist of an inductor and a capacitor in
various L-shaped configurations.
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They are used as low- and high-pass networks.
Low-pass networks are preferred because harmonic
frequencies are filtered out.
The L-matching network is designed so that the load
impedance is matched to the source impedance.
L Type Low-Pass Network
T and π Networks
To get better control of the Q, or selectivity of a circuit,
matching networks using three reactive elements can
be used.
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A π network is designed by using reactive elements in
a configuration that resembles the Greek letter π.
A T network is designed by using reactive elements in
a configuration that resembles the letter T.
A π Network
A T Network
Transformers and Baluns
One of the best impedance-matching components is the
transformer.
 Iron-core transformers are widely used at lower
frequencies to match impedances.
 Any load impedance can be made to look like the
desired load impedance by selecting the correct value
of transformer turns ratio.
 A transformer used to connect a balanced source to an
unbalanced load or vice versa, is called a balun
(balanced-unbalanced).
Typical Transmitter Circuit
Many transmitters used in recent equipment designs are
a combination of ICs and discrete component circuits
such as:
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Single-chip FM transmitter IC
Digital shaping circuits
Power amplifiers
IC voltage regulators
Voltage source