College of Engineering at Wadi College of Engineering at Wadi AlddawaserAlddawaser

AM TransmittersAM Transmitters

Workshop titled:

By:

IntroductionIntroductionIn 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 RequirementsTransmitter 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

Frequency Accuracy and Frequency Accuracy and StabilityStability

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

Frequency AgilityFrequency 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

Spectral PuritySpectral 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

Power OutputPower 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

EfficiencyEfficiencyThere are two important reasons for

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

Modulation FidelityModulation 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

Transmitter TopologyTransmitter 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

AM TransmittersAM TransmittersAM transmitters are a “mature”

technology, but are still in widespread use

Examples include:◦Standard AM broadcast stations◦CB radio◦VHF aircraft radio

AM Transmitter StagesAM Transmitter StagesAll 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

Output Impedance Output Impedance MatchingMatchingMost 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

An AM Citizens Band An AM Citizens Band TransmitterTransmitter

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

Elements of a CB Elements of a CB TransceiverTransceiver

The oscillator is a frequency synthesizerThe 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

Transmitter Section of a CBTransmitter Section of a CB

Modern AM Transmitter Modern AM Transmitter DesignDesign 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

Modern AM TechnologiesModern AM TechnologiesSolid-state RF power

amplifiersPulse-duration

modulatorsDigital amplitude

modulation

Single-Sideband AM Single-Sideband AM TransmittersTransmittersA typical SSB AM transmitter block diagram is

illustrated below:

Balanced Modulators for Double-Balanced Modulators for Double-Sideband Suppressed-Carrier Sideband Suppressed-Carrier GenerationGenerationBalanced modulators are used for DSSC generationThe output of a balanced modulator is shown here:

Generating Single-Sideband Generating Single-Sideband SignalsSignals

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

SSB GenerationSSB Generation

MixingMixing

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

Power AmplificationPower Amplification

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

FM TransmittersFM TransmittersFM Transmitters typically use the following

components and configurations:◦ Direct-FM Modulators◦ Frequency Multipliers◦ Phase-Locked Loop FM Generators◦ Indirect-FM Modulators◦ Digital FM Modulators

FM Stereo TransmittersFM Stereo Transmitters

FM stereo uses the baseband spectrum shown here:

Transmitter Power Transmitter Power MeasurementsMeasurements

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