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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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