Chapter08
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Principles of Electronic Communication Systems8-1: Transmitter
Fundamentals
8-2: Carrier Generators
8-3: Power Amplifiers
8-4: Impedance-Matching Networks
8-1: Transmitter Fundamentals
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.
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8-1: Transmitter Fundamentals
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.
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Every transmitter has four basic requirements:
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 over the desired
distance.
It must provide circuits that match the impedance of the power
amplifier to that of the antenna for maximum transfer of
power.
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The simplest transmitter is a single-transistor oscillator
connected to an antenna.
This form of transmitter can generate continuous wave (CW)
transmissions.
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.
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Carrier signal fed to buffer amplifier.
Signal then fed to driver amplifier.
Signal then fed to final amplifier.
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Crystal oscillator generates the carrier signal.
Signal fed to buffer amplifier.
Applied to phase modulator.
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Carrier is fed to buffer amplifier.
Signal is applied to balanced modulator.
DSB signal fed to sideband filter to select upper or lower
sideband.
SSB signal sent to mixer circuit.
Final carrier frequency fed to linear driver and power
amplifiers.
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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.
The source of most carriers is a crystal oscillator.
PLL frequency synthesizers are used in applications requiring
multiple channels of operation.
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Crystal Oscillators
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.
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The Colpitts-type crystal oscillator is the most commonly used
crystal oscillator.
Feedback is derived from a capacitive voltage divider.
Transistor configuration is typically an emitter-follower.
The output is taken from the emitter.
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Crystal Oscillators
Pulling, or rubbering capacitors are used to make fine adjustments
to the crystal oscillator frequency.
Field-effect transistors (FETs) make good crystal oscillators. The
Pierce oscillator is a common configuration that uses a FET.
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.
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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.
Mechanical rotary switches and diode switches are often used in
this kind of application.
Diode switching is fast and reliable.
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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.
Frequency synthesizers are used in receivers as local oscillators
and perform the receiver tuning function.
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8-2: Carrier Generators
Phase-Locked Loop Synthesizer
The phase-locked loop (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.
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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.
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8-2: Carrier Generators
Direct Digital Synthesis
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 high-frequency
content smoothing the sine wave output.
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The frequency can be controlled in very fine increments.
The frequency of a DDS synthesizer can be changed much faster than
that of the PLL.
However, a DDS synthesizer is limited in its output
frequencies.
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8-3: Power Amplifiers
The three basic types of power amplifiers used in transmitters
are:
Linear
Linear Amplifiers
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 low-level AM or SSB
signals.
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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.
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8-3: Power Amplifiers
Class C amplifiers conduct for 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 varying-amplitude
signals.
This type amplifier makes a good frequency multiplier as harmonics
are generated in the process.
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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.
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Class A Buffers
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.
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Class B Push-Pull Amplifier
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° out of phase.
An output transformer couples the power to the antenna or
load.
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8-3: Power Amplifiers
Class C Amplifiers
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° of
the input.
Current flows through a class C amplifier in short pulses, and a
resonant tuned circuit is used for complete signal
amplification.
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8-3: Power Amplifiers
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.
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8-3: Power Amplifiers
Tuned Output Circuits
The pulse charges a capacitor, which then 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.
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8-3: Power Amplifiers
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.
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Neutralization
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.
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
Neutralization is a process in which a signal equal in amplitude
and 180° out of phase with the signal, is fed back.
The result is that the two signals cancel each other out.
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8-3: Power Amplifiers
Switching Power Amplifiers
A switching amplifier is a transistor that is used as a switch and
is either conducting or nonconducting.
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 to achieve a high level of efficiency.
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8-3: Power Amplifiers
Switching Power Amplifiers
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-frequency RF
amplifiers.
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Newer wireless systems require broader bandwidth than the
previously mentioned amplifiers can accommodate.
Two common methods of broad-bandwidth amplification are:
Feedforward amplification
Feedforward Amplification
With this technique, the distortion produced by the power amplifier
is isolated and subtracted from the amplified signal, producing a
nearly distortion-free output signal.
The system is inefficient because two power amplifiers are
required.
The tradeoff is wide bandwidth and very low distortion.
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Adaptive Predistortion Amplification
This method uses digital signal processing (DSP) to predistort the
signal in a way that when amplified, the amplifier distortion will
offset the predistortion characteristics.
The result is a a distortion-free output signal.
The method is complex, but is more efficient than the feedforward
method because only one power amplifier is needed.
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8-4: Impedance-Matching Networks
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.
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8-4: Impedance-Matching Networks
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.
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Networks
There are three basic types of LC impedance-matching networks. They
are:
L network
T network
π network
8-4: Impedance-Matching Networks
L networks consist of an inductor and a capacitor in various
L-shaped configurations.
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.
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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.
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.
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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).
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8-4: Impedance-Matching Networks
Transformers and Baluns
Although air-core transformers are used widely at RFs, they are
less efficient than iron-core transformers.
The most widely used type of core for RF transformers is the
toroid.
A toroid is a circular, doughnut-shaped core, usually made of a
special type of powdered iron.
Single-winding tapped coils called autotransformers are also used
for impedance matching between RF stages.
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8-4: Impedance-Matching Networks
Transformers and Baluns
Toroid transformers cause the magnetic field produced by the
primary to be completely contained within the core itself.
This has two important advantages:
A toroid does not radiate RF energy.
Most of the magnetic field produced by the primary cuts the turns
of the secondary winding.
Thus, the basic turns ratio, input-output voltage, and impedance
formulas for low-frequency transformers apply to high-frequency
toroid transformers.
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Transmission Line Transformers and Baluns
A transmission line or broadband transformer is a unique type of
transformer widely used in power amplifiers for coupling between
stages and impedance matching.
It is usually constructed by winding two parallel wires (or a
twisted pair) on a toroid.
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8-5: Typical Transmitter Circuits
Many transmitters used in recent equipment designs are a
combination of ICs and discrete component circuits. Two examples
are:
Low-Power FM Transmitter
Short-Range Wireless Transmitter
A transmitter chip
The heart of the circuit is the transmitter chip.
It contains a microphone amplifier with clipping diodes; an RF
oscillator, which is usually crystal-controlled with an external
crystal; and a buffer amplifier.
Frequency modulation is produced by a variable reactance circuit
connected to the oscillator.
It also contains two free transistors that can be connected with
external components as buffer amplifiers or as multipliers and
low-level power amplifiers.
This chip is useful up to about 60 to 70 MHz, and is widely used in
cordless telephones.
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Figure 8-51: Freescale MC 2833 IC FM VHF transmitter chip.
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© 2008 The McGraw-Hill Companies
Short-Range Wireless Transmitter
There are many short-range wireless applications that require a
transmitter to send data or control signals to a nearby
receiver.
Examples include:
Remote keyless entry (RKE) devices used to open car doors
Tire pressure sensors
Garage door openers
Short-Range Wireless Transmitter
Such transmitters are unlicensed, use very low power, and operate
in the FCC’s industrial-scientific-medical (ISM) bands.
A typical transmitter circuit might be composed of:
PLL used as a frequency multiplier
Output power amplifier