© 2008 The McGraw-Hill Companies 1 Principles of Electronic Communication Systems Third Edition Louis E. Frenzel, Jr.

© 2008 The McGraw-Hill Companies 2 Chapter 8 Radio Transmitters

© 2008 The McGraw-Hill Companies 3 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

© 2008 The McGraw-Hill Companies 4 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.

© 2008 The McGraw-Hill Companies 5 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.

© 2008 The McGraw-Hill Companies 6 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.

© 2008 The McGraw-Hill Companies 7 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.

© 2008 The McGraw-Hill Companies 8 8-1: Transmitter Fundamentals Figure 8-1: A more powerful CW transmitter.

© 2008 The McGraw-Hill Companies 9 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.

© 2008 The McGraw-Hill Companies : 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.

© 2008 The McGraw-Hill Companies : 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.

© 2008 The McGraw-Hill Companies : 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.

© 2008 The McGraw-Hill Companies : 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.

© 2008 The McGraw-Hill Companies : 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.

© 2008 The McGraw-Hill Companies : Carrier Generators Figure 8-6: An emitter-follower crystal oscillator

© 2008 The McGraw-Hill Companies : 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.

© 2008 The McGraw-Hill Companies : 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.

© 2008 The McGraw-Hill Companies : Carrier Generators Figure 8-9: Using diodes to switch crystals.

© 2008 The McGraw-Hill Companies : 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.

© 2008 The McGraw-Hill Companies : 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.

© 2008 The McGraw-Hill Companies : Carrier Generators Figure 8-10: Basic PLL frequency synthesizer.

© 2008 The McGraw-Hill Companies : 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.

© 2008 The McGraw-Hill Companies : 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.

© 2008 The McGraw-Hill Companies : Carrier Generators Figure 8-15: Basic concept of a DDS frequency source

© 2008 The McGraw-Hill Companies : 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.

© 2008 The McGraw-Hill Companies : Power Amplifiers  The three basic types of power amplifiers used in transmitters are:  Linear  Class C  Switching

© 2008 The McGraw-Hill Companies : 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.

© 2008 The McGraw-Hill Companies : 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.

© 2008 The McGraw-Hill Companies : 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.

© 2008 The McGraw-Hill Companies : 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.

© 2008 The McGraw-Hill Companies : 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.

© 2008 The McGraw-Hill Companies : Power Amplifiers Figure 8-21: A linear (class A) RF buffer amplifier

© 2008 The McGraw-Hill Companies : 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.

© 2008 The McGraw-Hill Companies : Power Amplifiers Figure 8-23: A push-pull class B power amplifier

© 2008 The McGraw-Hill Companies : 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.

© 2008 The McGraw-Hill Companies : 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.

© 2008 The McGraw-Hill Companies : 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.

© 2008 The McGraw-Hill Companies : Power Amplifiers Figure 8-27: Class C amplifier operation

© 2008 The McGraw-Hill Companies : 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.

© 2008 The McGraw-Hill Companies : 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.

© 2008 The McGraw-Hill Companies : 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.

© 2008 The McGraw-Hill Companies : 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.

© 2008 The McGraw-Hill Companies : 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.

© 2008 The McGraw-Hill Companies : 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

© 2008 The McGraw-Hill Companies : 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.

© 2008 The McGraw-Hill Companies : Power Amplifiers Figure 8-34: Feedforward linear power amplifier.

© 2008 The McGraw-Hill Companies : 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.

© 2008 The McGraw-Hill Companies : Power Amplifiers Figure 8-35: Concept of adaptive predistortion amplification.

© 2008 The McGraw-Hill Companies : 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.

© 2008 The McGraw-Hill Companies : 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.

© 2008 The McGraw-Hill Companies : Impedance-Matching Networks Figure 8-36: Impedance Matching in RF Circuits

© 2008 The McGraw-Hill Companies : Impedance-Matching Networks Networks  There are three basic types of LC impedance-matching networks. They are:  L network  T network  π network

© 2008 The McGraw-Hill Companies : 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.

© 2008 The McGraw-Hill Companies : Impedance-Matching Networks Figure 8-37a: L-type impedance-matching network in which Z L < Z i.

© 2008 The McGraw-Hill Companies : 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.

© 2008 The McGraw-Hill Companies : Impedance-Matching Networks Figure 8-40(a): π network.

© 2008 The McGraw-Hill Companies : Impedance-Matching Networks Figure 8-40(b): T network.

© 2008 The McGraw-Hill Companies : 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).

© 2008 The McGraw-Hill Companies : 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.

© 2008 The McGraw-Hill Companies : 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.

© 2008 The McGraw-Hill Companies : Impedance-Matching Networks Figure 8-43: A toroid transformer.

© 2008 The McGraw-Hill Companies : 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.

© 2008 The McGraw-Hill Companies : Impedance-Matching Networks Figure 8-46: A transmission line transformer.

© 2008 The McGraw-Hill Companies : 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

© 2008 The McGraw-Hill Companies : 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.

© 2008 The McGraw-Hill Companies : 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.

© 2008 The McGraw-Hill Companies : Typical Transmitter Circuits Figure 8-51: Freescale MC 2833 IC FM VHF transmitter chip.

© 2008 The McGraw-Hill Companies : Typical Transmitter Circuits Figure 8-50: Schematic of sections of the E-Comm transceiver.

© 2008 The McGraw-Hill Companies : 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

© 2008 The McGraw-Hill Companies : 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

© 2008 The McGraw-Hill Companies : Typical Transmitter Circuits Figure 8-52: The Freescale MC 33493D UHF ISM transmitter IC.