Principles of ElectronicCommunication 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
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 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 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 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 Fundamentals Figure 8-1: A more powerful CW transmitter.
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 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 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 • 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 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 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 Generators Figure 8-6: An emitter-follower crystal oscillator
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 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 Generators Figure 8-9: Using diodes to switch crystals.
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 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 Generators Figure 8-10: Basic PLL frequency synthesizer.
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 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 Generators Figure 8-15: Basic concept of a DDS frequency source
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 • The three basic types of power amplifiers used in transmitters are: • Linear • Class C • Switching
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 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 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 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 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 Amplifiers Figure 8-21: A linear (class A) RF buffer amplifier
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 Amplifiers Figure 8-23: A push-pull class B power amplifier
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 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 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 Amplifiers Figure 8-27: Class C amplifier operation
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 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 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 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 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 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 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 Amplifiers Figure 8-34: Feedforward linear power amplifier.
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 Amplifiers Figure 8-35: Concept of adaptive predistortion amplification.
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 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.