chapter08 radio transmitters

© 2008 The McGraw-Hill Companies

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Principles of ElectronicPrinciples of ElectronicCommunication SystemsCommunication Systems

Third Edition

Louis E. Frenzel, Jr.


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Chapter 8Chapter 8

Radio Transmitters


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Topics Covered in Chapter 8Topics Covered in Chapter 8

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 8-1: Transmitter FundamentalsFundamentals

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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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: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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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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Figure 8-1: A more powerful CW transmitter.


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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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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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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 Generators8-2: 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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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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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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Figure 8-6: An emitter-follower crystal oscillator


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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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Figure 8-9: Using diodes to switch crystals.


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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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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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Figure 8-10: Basic PLL frequency synthesizer.


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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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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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Figure 8-15: Basic concept of a DDS frequency source


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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 Amplifiers8-3: Power Amplifiers

The three basic types of power amplifiers used in transmitters are: Linear Class C Switching


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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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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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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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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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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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Figure 8-21: A linear (class A) RF buffer amplifier


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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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Figure 8-23: A push-pull class B power amplifier


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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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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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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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Figure 8-27: Class C amplifier operation


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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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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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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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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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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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Figure 8-34: Feedforward linear power amplifier.


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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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Figure 8-35: Concept of adaptive predistortion amplification.


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8-4: Impedance-Matching 8-4: Impedance-Matching NetworksNetworks

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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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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Figure 8-36: Impedance Matching in RF Circuits


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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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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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Figure 8-37a: L-type impedance-matching network in which ZL < Zi.


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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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Figure 8-40(a): π network.


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Figure 8-40(b): T network.


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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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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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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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Figure 8-43: A toroid transformer.


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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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Figure 8-46: A transmission line transformer.


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8-5: Typical Transmitter 8-5: Typical Transmitter CircuitsCircuits

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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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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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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Figure 8-51: Freescale MC 2833 IC FM VHF transmitter chip.


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Figure 8-50: Schematic of sections of the E-Comm transceiver.


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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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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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Figure 8-52: The Freescale MC 33493D UHF ISM transmitter IC.

chapter08 radio transmitters

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