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DEVELOPMENTS IN THE DESIGN AND CONSTRUCTION OF

AMPLITUDE MODULATION RADIO TRANSMITTERS

1. Introduction

Since the beginning of human existence, mankind have always felt a need for

long-distance communications. In order to address this need, every possible

means has been exploited ranging from the very crude to the very

sophisticated. The discovery of electrical communication as an effective

means of telecommunications towards the end of the nineteen century

spurred the development of techniques, components and equipment that can

be traced to the Information Revolution the world is now experiencing.

Most of the initial efforts in the design and construction of radio

communication systems were concentrated on amplitude-modulation (AM)

systems (1, 2). This was because AM theory was well developed and

understood earlier in time than frequency modulation (FM). This paper

attempts to outline the major developments in AM transmitter design and

construction from the early days of radio communication to the present.

The discussion on AM radio transmitter presented in this paper are focussed

on low-power AM radio transmitters. This is because medium- and high-

power AM radio transmitters, which are usually broadcast transmitters,

employ proprietary designs and special components whose details are not

provided in the open literature for obvious reasons.

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2. Theory of Amplitude Modulation

AM modulation is achieved by using an information signal (e.g. an audio

signal) to linearly vary the amplitude of a radio-frequency (rf) sinusoidal

waveform called the carrier. The mathematical expression for the un-

modulated carrier is given by

The type of AM modulation described above is called double sideband (DSB)

full carrier AM modulation or conventional AM modulation. Other common

types of AM modulation util ize partial or total carrier and/or single sideband

suppression which results in more signal power to the modulation signal and

increased transmission efficiency. These types require a more sophisticated

receiver than the simple diode-detection AM receiver.

More efficient AM modulation schemes that are compatible with the simple

AM receiver include Dynamic Amplitude Modulat ion (DAM), Dynamic Carrier

Control (DCC) and Amplitude Modulation Companding (AMC). These

modulation techniques have been developed for large-power AM broadcast

transmitters.

3. AM Radio Transmitter Circuitry

The block diagram of Fig. 1 il lustrates the circuitry of a basic AM radio

transmitter. The detail of the circuitry is as varied as the individual circuit

designers. However, regardless of the differences in their design, every AM

radio transmitter performs the same basic function(s).

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The audio amplifier raises the level of the output of the information

transducer (e.g. mic) such that it is able to drive the AM modulator. The

design of audio amplifiers forms part of the electrical and electronics

engineering curriculum and its detail will not be presented in this paper.

The rf oscil lator generates the carrier waveform. The main requirement of

the circuit is the generation of a stable frequency that satisfies the stringent

frequency stability stipulated by the communications regulatory agency.

Designers have a choice between LC tuned or crystal tuned oscil lators.

Except where the condition does not permit, a crystal-tuned oscil lator is used

as the rf carrier oscil lator. A frequency synthesizer is preferred to a crystal

oscil lator in communication systems that require more than one stable

frequency. A frequency synthesizer has the same frequency-stability

tolerance as a crystal oscil lator or better. More details on the rf carrier

oscil lator are presented in Section Three.

The AM modulator uses the amplified audio signal to amplitude modulate the

rf carrier signal supplied by the carrier oscil lator thereby producing an AM

wave at its output. A lot of simple circuits exist in the literature for achieving

amplitude modulation. Some popular designs are discus sed in Section Four.

The rf power amplifier raises the power of the AM wave to a level sufficient

to allow the radiated radio wave to cover the desired distance. The majority

of the works that are done in AM radio transmission are focussed in this

area. This is because the power developed by this stage determines the

effectiveness of the transmitter. Furthermore, the power efficiency of the

stage has a significant impact on the cost of operation of the transmitter

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an important performance criterion for AM radio transmitters. Details of

practical rf power amplifier designs for AM radio transmitters are presented

in Section Five.

The antenna coupling network impedance-matches the rf power amplifier to

the antenna so that the transmitter is able to develop the desired power.

This network is usually in the form of a passive tuned circuit.

The antenna for AM transmission is a vertical wire of appropriate length that

is well grounded and installed in a place where the ground has good

conductivity. Since this length is impractical for experimental work, there is

usually a need to add a loading coil to the antenna so that it can resonate at

the operating radio frequency thereby radiating a large proportion of the

power developed by the rf power amplifier.