Transcript Principles of Electronic Communication Systems

1
Principles of Electronic
Communication Systems
Third Edition
Louis E. Frenzel, Jr.
© 2008 The McGraw-Hill Companies
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Chapter 8
Radio Transmitters
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Topics Covered in Chapter 8
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8-1: Transmitter Fundamentals
8-2: Carrier Generators
8-3: Power Amplifiers
8-4: Impedance-Matching Networks
8-5: Typical Transmitter Circuits
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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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8-1: Transmitter Fundamentals
Every transmitter has four basic requirements:
1. It must generate a carrier signal of the correct
frequency at a desired point in the spectrum.
2. It must provide some form of modulation that causes
the information signal to modify the carrier signal.
3. It must provide sufficient power amplification to ensure
that the signal level is high enough to carry over the
desired distance.
4. 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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8-1: Transmitter Fundamentals
Transmitter Configurations
 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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8-1: Transmitter Fundamentals
Figure 8-1: A more powerful CW transmitter.
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8-1: Transmitter Fundamentals
Transmitter Types
 High-Level Amplitude Modulated (AM) Transmitter
1. Oscillator generates the carrier frequency.
2. Carrier signal fed to buffer amplifier.
3. Signal then fed to driver amplifier.
4. Signal then fed to final amplifier.
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8-1: Transmitter Fundamentals
 Low-Level Frequency Modulated (FM) Transmitter
1. Crystal oscillator generates the carrier signal.
2. Signal fed to buffer amplifier.
3. Applied to phase modulator.
4. Signal fed to frequency multiplier(s).
5. Signal fed to driver amplifier.
6. Signal fed to final amplifier.
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8-1: Transmitter Fundamentals
 Single-Sideband (SSB) Transmitter
1. Oscillator generates the carrier.
2. Carrier is fed to buffer amplifier.
3. Signal is applied to balanced modulator.
4. DSB signal fed to sideband filter to select upper or
lower sideband.
5. SSB signal sent to mixer circuit.
6. Final carrier frequency fed to linear driver and
power amplifiers.
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8-2: Carrier Generators
 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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8-2: Carrier Generators
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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8-2: Carrier Generators
Crystal Oscillators
 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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8-2: Carrier Generators
Figure 8-6: An emitter-follower crystal oscillator
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8-2: Carrier Generators
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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8-2: Carrier Generators
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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8-2: Carrier Generators
Figure 8-9: Using diodes to switch crystals.
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8-2: Carrier Generators
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.
 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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8-2: Carrier Generators
Figure 8-10: Basic PLL frequency synthesizer.
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8-2: Carrier Generators
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.
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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 highfrequency content smoothing the sine wave output.
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8-2: Carrier Generators
Figure 8-15: Basic concept of a DDS frequency source
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8-2: Carrier Generators
Direct Digital Synthesis
 DDS synthesizers offer some advantages over PLL
synthesizers:
 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
 Class C
 Switching
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8-3: Power Amplifiers
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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8-3: Power Amplifiers
 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.
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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 varyingamplitude signals.
 This type amplifier makes a good frequency multiplier
as harmonics are generated in the process.
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8-3: Power Amplifiers
 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.
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8-3: Power Amplifiers
Linear Amplifiers
 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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8-3: Power Amplifiers
Figure 8-21: A linear (class A) RF buffer amplifier
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8-3: Power Amplifiers
Linear Amplifiers
 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
Figure 8-23: A push-pull class B power amplifier
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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
Figure 8-27: Class C amplifier operation
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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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8-3: Power Amplifiers
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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8-3: Power Amplifiers
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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8-3: Power Amplifiers
Linear Broadband Power Amplifiers
 Newer wireless systems require broader bandwidth
than the previously mentioned amplifiers can
accommodate.
 Two common methods of broad-bandwidth amplification
are:
 Feedforward amplification
 Adaptive predistortion amplification
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8-3: Power Amplifiers
Linear Broadband Power Amplifiers
 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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8-3: Power Amplifiers
Figure 8-34: Feedforward linear power amplifier.
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8-3: Power Amplifiers
Linear Broadband Power Amplifiers
 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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8-3: Power Amplifiers
Figure 8-35: Concept of adaptive predistortion amplification.
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8-4: 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.
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8-4: Impedance-Matching
Networks
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 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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8-4: Impedance-Matching
Networks
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Figure 8-36: Impedance Matching in RF Circuits
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8-4: Impedance-Matching
Networks
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Networks
 There are three basic types of LC impedance-matching
networks. They are:
 L network
 T network
 π network
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8-4: Impedance-Matching
Networks
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 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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8-4: Impedance-Matching
Networks
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Figure 8-37a: L-type impedance-matching network in which ZL < Zi.
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8-4: Impedance-Matching
Networks
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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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8-4: Impedance-Matching
Networks
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Figure 8-40(a): π network.
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8-4: Impedance-Matching
Networks
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Figure 8-40(b): T network.
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8-4: Impedance-Matching
Networks
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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).
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8-4: Impedance-Matching
Networks
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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
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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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8-4: Impedance-Matching
Networks
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Figure 8-43: A toroid transformer.
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8-4: Impedance-Matching
Networks
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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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8-4: Impedance-Matching
Networks
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Figure 8-46: A transmission line transformer.
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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
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8-5: Typical Transmitter Circuits
Low-Power FM Transmitter
 A typical circuit might be made up of:
 A transmitter chip
 Power amplifier
 IC voltage regulator
 Voltage source.
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8-5: Typical Transmitter Circuits
Low-Power FM Transmitter
 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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8-5: Typical Transmitter Circuits
Figure 8-51: Freescale MC 2833 IC FM VHF transmitter chip.
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8-5: Typical Transmitter Circuits
Figure 8-50: Schematic of sections of the E-Comm transceiver.
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8-5: Typical Transmitter Circuits
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
 Remote-control lights and ceiling fans
 Garage door openers
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8-5: Typical Transmitter Circuits
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
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8-5: Typical Transmitter Circuits
Figure 8-52: The Freescale MC 33493D UHF ISM transmitter IC.
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