Electronic filters have many applications in the telecommunications and data communications industry. One such application, which involves a multiple channel communications system employing a technique known as time-division multiplexing (TDM). 

In this system several channels are transmitted through a medium such as an optical fi ber, as shown here, or through a coaxial cable or waveguide. Multiplexing means combining several signals into one, and this is accomplished in TDM by allocating time slots for each channel so that each channel is transmitted at a particular time. 

If the signals are synchronized correctly there will be no interference between them. At the transmitter end a multiplexer is used to combine the signals, while at the receiver end a demultiplexer is used to separate the original channels.

However, when the channel signals arrive at the receivers they have deteriorated in shape and amplitude. In order to clean them up they are reconstructed by an integrator that sums up the incoming signal very much as in mathematical integration. Once this has been done a filter is used to pass the wanted channel frequencies while attenuating the unwanted signals such as noise.

The combined functions of the integrator and filter cause the transmitted channels to be reproduced. In this case, where three channels are involved, each filter will be designed to pass the particular channel frequency and its related information, hence a band of frequencies is passed by each filter.

This is an example of where filters are used to pass bands of frequencies such as the voice band (300–3400 Hz). However, filters can also be used to pass frequencies below a certain frequency while attenuating all frequencies above it. Similarly, it is possible to construct a filter which passes all frequencies above a certain frequency while attenuating all frequencies below it.

Other applications are the following: noise filtering; guard band separation of channels; bandpass selection; boosting and cutting certain bands in the frequency spectrum; and harmonic reduction. Some of these will be investigated later.

Sine waves of different amplitudes and frequencies are shown below.

It should be appreciated that the majority of filters have to be capable of handling a mixture of such sine waves, as shown in (e) ; the effect of reducing the amplitudes of the signals in figures(d)–(e) is shown in Figure (f) . Figure (g) shows what happens when the signal in Figure (b) is reduced and that in
Figure (a) is eliminated. 

It is therefore possible to use filters to alter amplitudes and frequencies, depending on the requirements of the system. Finally, the fi lters discussed in this chapter are used in sine or continuous wave circuits.

However, certain circuits such as integrators and differentiators utilize passive high-pass and low-pass networks to process square waves and produce wave shaping. When fed through a filter the square wave is modified: the high-frequency edges are rounded when passing through a low-pass fi lter, while the flat top and bottom are distorted when passing through a high-pass filter.

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