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Distortion in Telecommunication Circuits

 A signal is said to have suffered distortion if, after passing through a network, (passive* or active **) is not an exact replica of the original signal in respect of its amplitude wave shape.

* A passive network does not require power supply. It may consist of resistors, coils and condensers either singly or in different combinations e.g. attenuators and filters are passive networks.

** An active network is one that invariably requires power supply. It may consist of an electron tube, a transistor or an IC chip in combination with other components like resistors, coils, and condensers e.g. an amplifier or an oscillator.

Signal distortion is of two kinds.

1. Linear distortion
2. Non linear distortion

Linear distortion takes place in passive networks. Different types of linear distortion are;

1. Attenuation distortion
2. Phase distortion

Imperfect attenuation, frequency response and phase-frequency characteristic of a network cause attenuation and phase distortions respectively. Non-linear distortion takes place in active networks only and is caused by their excessive loading.

Different types of non-linear distortion are;

1. Amplitude distortion
2. Frequency distortion
3. Inter-modulation distortion

Attenuation distortion

The term attenuation distortion is employed to the case of a transmission system where there is variation of gain or loss with frequency. It is assessed with the system operated under steady- state condition by applying a series of sinusoidal wave- forms at different frequencies. Fig.1 shows the amplitude- frequency characteristics of an open wire line. Curve A depicts an ideal open wire line with no attenuation distortion. Curve B pertains to the practical one. Distortion in Telecommunication Circuits

As is clear from the figure, higher frequencies are attenuated more than the lower ones. An ideal band-pass filter should give same loss to all the frequencies of the pass-band but in actual practice it is not so. The above figure shows a typical attenuation distortion curve for a voice channel. Curve A shows the attenuation-frequency response of an ideal band pass filter and curve B shows the attenuation distortion in the case of a practical band pass filter. As such the network is designed in such a manner that attenuation distortion caused by it remains within permissible limits. Distortion in Telecommunication Circuits

However, in the case of open wire carrier lines, equalizers are used to bring amplitude distortion within limits.

 Phase distortion (Envelope delay distortion)

Phase distortion takes place when the time of propagation through a transmission system varies with frequency. Owing to the different relative phase relationships than existing, the output waveform may appear to be quite different from the input waveform, even though the same frequencies are available in the same relative amplitude. Distortion in Telecommunication Circuits

Non-linear distortion

“Non linear distortion” is the general name given to a certain type of distortion that occurs when the transmission properties of a system are dependent on the instantaneous magnitude of the applied signal. It is further sub-divided as under:

1. Amplitude distortion.
2. Harmonic distortion.
3. Inter-modulation distortion.

Amplitude distortion

It is defined as the variation of gain or loss of a system with the amplitude of the input. It is measured with the system operated under steady- state conditions with an input of sinusoidal waveform. Distortion in Telecommunication Circuits

Harmonic distortion

It is due to the production of harmonics in the output when a sinusoidal input of specified amplitude is applied. It is expressed as the ratio of the RMS voltage of all the harmonics in the output, to the total RMS voltage at the output.

Inter-modulation distortion

It is due to the production of combination frequencies in the output when two or more sinusoidal voltages of specified amplitude are applied at the input. For two parent frequencies p and q, the output may contain frequencies such as (p± q), (1p± q), (p± 2q) etc. in addition to the frequencies p and q.