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Principles of Electronic Communication Systems. Third Edition Louis E. Frenzel, Jr. Chapter 8. Radio Transmitters. Topics 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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Principles of electronic communication systems

Principles of ElectronicCommunication Systems

Third Edition

Louis E. Frenzel, Jr.


Chapter 8

Chapter 8

Radio Transmitters


Topics covered in chapter 8
Topics 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


8 1 transmitter fundamentals
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.


8 1 transmitter fundamentals1
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.


8 1 transmitter fundamentals2
8-1: Transmitter Fundamentals

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.


8 1 transmitter fundamentals3
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.


8 1 transmitter fundamentals4
8-1: Transmitter Fundamentals

Figure 8-1: A more powerful CW transmitter.


8 1 transmitter fundamentals5
8-1: Transmitter Fundamentals

Transmitter Types

  • High-Level Amplitude Modulated (AM) Transmitter

    • Oscillator generates the carrier frequency.

    • Carrier signal fed to buffer amplifier.

    • Signal then fed to driver amplifier.

    • Signal then fed to final amplifier.


8 1 transmitter fundamentals6
8-1: Transmitter Fundamentals

  • Low-Level Frequency Modulated (FM) Transmitter

    • Crystal oscillator generates the carrier signal.

    • Signal fed to buffer amplifier.

    • Applied to phase modulator.

    • Signal fed to frequency multiplier(s).

    • Signal fed to driver amplifier.

    • Signal fed to final amplifier.


8 1 transmitter fundamentals7
8-1: Transmitter Fundamentals

  • Single-Sideband (SSB) Transmitter

    • Oscillator generates the carrier.

    • 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.


8 2 carrier generators
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.


8 2 carrier generators1
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.


8 2 carrier generators2
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.


8 2 carrier generators3
8-2: Carrier Generators

Figure 8-6: An emitter-follower crystal oscillator


8 2 carrier generators4
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.


8 2 carrier generators5
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.


8 2 carrier generators6
8-2: Carrier Generators

Figure 8-9: Using diodes to switch crystals.


8 2 carrier generators7
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.


8 2 carrier generators8
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.


8 2 carrier generators9
8-2: Carrier Generators

Figure 8-10: Basic PLL frequency synthesizer.


8 2 carrier generators10
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.


8 2 carrier generators11
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.


8 2 carrier generators12
8-2: Carrier Generators

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


8 2 carrier generators13
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.


8 3 power amplifiers
8-3: Power Amplifiers

  • The three basic types of power amplifiers used in transmitters are:

    • Linear

    • Class C

    • Switching


8 3 power amplifiers1
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.


8 3 power amplifiers2
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.


8 3 power amplifiers3
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.


8 3 power amplifiers4
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.


8 3 power amplifiers5
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.


8 3 power amplifiers6
8-3: Power Amplifiers

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


8 3 power amplifiers7
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.


8 3 power amplifiers8
8-3: Power Amplifiers

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


8 3 power amplifiers9
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.


8 3 power amplifiers10
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.


8 3 power amplifiers11
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.


8 3 power amplifiers12
8-3: Power Amplifiers

Figure 8-27: Class C amplifier operation


8 3 power amplifiers13
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.


8 3 power amplifiers14
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.


8 3 power amplifiers15
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.


8 3 power amplifiers16
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.


8 3 power amplifiers17
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.


8 3 power amplifiers18
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


8 3 power amplifiers19
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.


8 3 power amplifiers20
8-3: Power Amplifiers

Figure 8-34: Feedforward linear power amplifier.


8 3 power amplifiers21
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.


8 3 power amplifiers22
8-3: Power Amplifiers

Figure 8-35: Concept of adaptive predistortion amplification.


8 4 impedance matching networks
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.


8 4 impedance matching networks1
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.


8 4 impedance matching networks2
8-4: Impedance-Matching Networks

Figure 8-36: Impedance Matching in RF Circuits


8 4 impedance matching networks3
8-4: Impedance-Matching Networks

Networks

  • There are three basic types of LC impedance-matching networks. They are:

    • L network

    • T network

    • πnetwork


8 4 impedance matching networks4
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.


8 4 impedance matching networks5
8-4: Impedance-Matching Networks

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


8 4 impedance matching networks6
8-4: Impedance-Matching Networks

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.


8 4 impedance matching networks7
8-4: Impedance-Matching Networks

Figure 8-40(a): π network.


8 4 impedance matching networks8
8-4: Impedance-Matching Networks

Figure 8-40(b): T network.


8 4 impedance matching networks9
8-4: Impedance-Matching Networks

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).


8 4 impedance matching networks10
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.


8 4 impedance matching networks11
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.


8 4 impedance matching networks12
8-4: Impedance-Matching Networks

Figure 8-43: A toroid transformer.


8 4 impedance matching networks13
8-4: Impedance-Matching Networks

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.


8 4 impedance matching networks14
8-4: Impedance-Matching Networks

Figure 8-46: A transmission line transformer.


8 5 typical transmitter circuits
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


8 5 typical transmitter circuits1
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.


8 5 typical transmitter circuits2
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.


8 5 typical transmitter circuits3
8-5: Typical Transmitter Circuits

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


8 5 typical transmitter circuits4
8-5: Typical Transmitter Circuits

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


8 5 typical transmitter circuits5
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


8 5 typical transmitter circuits6
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


8 5 typical transmitter circuits7
8-5: Typical Transmitter Circuits

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


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