TABLE OF CONTENTS BOYD WILLIAMSON and ROBIN MYLES
Hewlett-Packard Ltd.
Frequency division multiplexing (FDM) originated in the United States during World War I, with three voice circuits being carried on a pair of wires. The object of the exercise was, of course, to increase the capacity of a given amount of copper. From these early experiments, FDM has grown to today s huge networks, which carry thousands of circuits on a single coaxial tube or on a single radio carrier wave. The operating principle has remained unchanged.
The FDM process begins with a number of voice channels. Each of these is translated to a different frequency so that the channels are stacked side by side in the frequency domain. With only a few channels to be multiplexed, it is practicable to shift each individual channel directly to its new frequency slot. As the number of channels increases, this direct modulation method becomes more difficult to implement.
Each channel is translated by mixing it with a carrier from a stable local oscillator and filtering the resultant output signal to extract the desired sideband. The design of the channel filter determines how closely the channels can be stacked, so a compromise is made between filter complexity and line capacity. The cost of terminal equipment is balanced against that of line equipment. A high-capacity FDM trunk cable carries channels in a band from tens of kilohertz to tens of megahertz. Designing individual filters for each...
