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Chapter Five: Transmitters Introduction In spite of the wide variety of uses for transmitters, from toys to broadcasting transmitters, there are only a few topologies that are used in their design Transmitter Requirements A transmitter must generate a signal with the following criteria:

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Chapter five transmitters l.jpg
Chapter Five:Transmitters


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Introduction

  • In spite of the wide variety of uses for transmitters, from toys to broadcasting transmitters, there are only a few topologies that are used in their design


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Transmitter Requirements

  • A transmitter must generate a signal with the following criteria:

    • The correct modulation type

    • Must have sufficient power

    • Must generate at the correct carrier frequency

    • Should be reasonably efficient


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Frequency Accuracy and Stability

  • The accuracy and stability of the transmitter is fixed by the carrier oscillator

  • Exact requirements are determined by the application of the transmitter and by regulatory agencies


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Frequency Agility

  • Frequency agility is the ability to change operating frequency rapidly, without extensive retuning

  • Broadcast transmitters are rarely retuned

  • Other services, such as CB, require rapid and accurate retuning to other channels


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Spectral Purity

  • Spectral purity is a measure of the spurious signals generated by a transmitter

  • All transmitters generate frequencies other than the carrier and the sidebands required for the modulation scheme in use

  • All frequencies except the assigned transmitting frequency must be filtered out to avoid interference with other transmissions


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Power Output

  • There are a number of ways to measure transmitter power, depending upon the modulation scheme employed

  • Transmitters for full-carrier AM are rated in terms of carrier power

  • Suppressed-carrier AM transmitters are rated by peak-envelope power (PEP)

  • FM transmitters are rated by total power output


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Efficiency

  • There are two important reasonsfor efficient transmitter operation:

    • Most obvious is energy conservation

    • Power that enters the transmitter but does not exit via the transmitter output is converted into heat

    • Large amounts of heat require significant amounts of additional hardware to remove the heat, adding to the cost of the equipment


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Modulation Fidelity

  • An ideal communication system allows the original information signal to be recovered exactly, except for a time delay

  • Compression is often used to raise the overall modulation level of the signal

  • Compression distorts the overall dynamic range of the original signal, but results in an improved signal-to-noise ratio

  • Other types of distortion such as intermodulation and harmonic distortion must also be kept at a minimum


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Transmitter Topology

  • The figure at the right shows the block diagrams of some typical transmitters

  • There are many varieties of transmitters but most are based upon these structures


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AM Transmitters

  • AM transmitters are a “mature” technology, but are still in widespread use

  • Examples include:

    • Standard AM broadcast stations

    • CB radio

    • VHF aircraft radio


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AM Transmitter Stages

  • All of the stages of a transmitter (except the power amplifier and possibly the driver) operate at low power levels

  • This part of the transmitter, exclusive of the power-handling stages, is called the exciter

  • Other transmitter components include:

    • The oscillator stage

    • The buffer and multiplier stages

    • The driver stage

    • The power amplifier/modulator


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Output Impedance Matching

  • Most practical transmitters are designed to operate into a 50- Ohm resistive load to match the impedance of the coaxial cable that is used to carry the power to the transmitter

  • Transmitter output circuitry must be designed to transform the standard load resistance at the output terminal to whatever is required by the active device or devices



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An AM Citizens Band Transmitter

  • A CB radio is always found as part of transceiver as they are economical, compact, and convenient to install and repair


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Elements of a CB Transceiver

  • The oscillator is a frequency synthesizer

  • The audio circuitry consists of a microphone pre-amplifier, followed by an IC amplifier

  • The output circuit for the final amplifier is similar to a T network



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Modern AM Transmitter Design

  • AM transmitters have been built since the invention of the vacuum tube and their design has changed little

  • There are some new approaches that are now in use

  • High-power AM transmitters are large and expensive because of the power handled

  • Recent efforts to improve AM transmitters include the development of high-power solid-state power amplifiers and the use of pulse-duration modulation and switching amplifiers in the modulation process


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Modern AM Technologies

  • Solid-state RF power amplifiers

  • Pulse-duration modulators

  • Digital amplitude modulation


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Single-Sideband AM Transmitters

  • A typical SSB AM transmitter block diagram is illustrated below:


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Balanced Modulators for Double-Sideband Suppressed-Carrier Generation

  • Balanced modulators are used for DSSC generation

  • The output of a balanced modulator is shown here:


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Generating Single-Sideband Signals Generation

  • Bandpass filters may be used to filter out the unwanted sideband in an AM transmitter

  • The carrier is not filtered because of its large amplitude and proximity to the desired sideband

  • The carrier is typically nulled with a balanced modulator and then one of the sidebands is filtered


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SSB Generation Generation


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Mixing Generation

  • Mixing in a DSBSC AM transmitter is done by a carrier oscillator and a balanced modulator as shown below:


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Power Amplification Generation

  • Power amplification in a SSB transmitter must be linear

  • SSB typically uses much lower power levels than are found in commercial AM broadcast transmitters as SSB is usually used for point-to-point communications


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FM Transmitters Generation

  • FM Transmitters typically use the following components and configurations:

    • Direct-FM Modulators

    • Frequency Multipliers

    • Phase-Locked Loop FM Generators

    • Indirect-FM Modulators

    • Digital FM Modulators


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FM Stereo Transmitters Generation

  • FM stereo uses the baseband spectrum shown here:



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Transmitter Power Measurements Generation

  • Power measurements are typically quite ordinary but require attention to safety in doing so

  • High voltages and the possibility of RF burns are dangers to the technician


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