Transmission Media and Electronic Distortion

Communications are based on the sending of information by means of transmission media. This concept, the “transmission medium”, should be understood in a broad sense. The term “transmission medium” refers to the space (or electronic component) that separates any two points. Either a cable between two devices. Or a small track connecting two elements on an electronic printed circuit board. Or even the space separating two antennas thousands of kilometers apart.

In practice, at the physical level, the information sent on the transmission medium is reduced to electrical parameters (voltage, current, power, electric or magnetic field, etc.). Generically, all these parameters carrying information are called “signals”. This establishes the link that forms the theory of signal transmission media.

This section analyzes the distortion produced by the transmission media from the following perspective. First, the components that make up the signals are explained in a basic way. Next, the effects of the transmission media on these components are classified. Finally, each of these possible effects is discussed individually.

Signal Components

In general, signals can be decomposed into a sum of tones, each with its amplitude A, frequency f, and phase \varphi . As an example, the mathematical expression of a signal s(t) composed of two components, a(t) and b(t), reads as follows:

\begin{equation} s(t) = \underbrace{A_1\cos(2{\pi}f_1t +\varphi_1 ) }_{\text{a(t)}} \, + \,\underbrace{A_2\cos(2{\pi}f_2t +\varphi_2 ) }_{\text{b(t)}} \end{equation}

Signal composed of two tones prior to distortion in transmission media.
Example of signal s(t) consisting of two tones (A_1 = 1 , f_1 = 2 , \varphi_1 = 3{\pi}/4 ; A_2 = 1 , f_2 = 1 , \varphi_2 = {\pi}/4)

Disturbances caused by Transmission Media

The transmission media can produce different effects and disturbances on the signal s(t). These effects can be classified according to the parameter they directly affect: amplitude, phase, or frequencies of the individual tones that make up the signal.

On the other hand, a signal is said to have suffered distortion in the transmission medium when its “appearance” changes with respect to the original signal. This phenomenon occurs when the effects on amplitude and/or phase and/or frequency of the individual tones are not uniform. On the contrary, a transmitted and undistorted signal maintains the same “aspect” as the original signal. In the latter case, the amplitude and/or phase and/or frequency effects of the individual tones are uniform.

Combining both concepts, the following table makes a classification of the effects of the transmission media:

ParameterDistortionless EffectsDistortion Effects
AMPLITUDEAttenuation / GainAmplitude Distortion
FREQUENCIESLinear TransmissionNon-Linear Transmission
Classification of possible effects of a transmission medium on the individual components of a signal.

Note that this classification is not rigid. A transmission medium, or a component, can produce any combination of the effects presented.

In addition, it should be kept in mind that rarely is a medium totally innocuous with respect to any parameter. In general, when speaking of distortion-free cases, reference is made to situations where the distortion is so small that it can be neglected.

Finally, it should be noted that noise, in practice, is always present in one form or another in any component or transmission. Noise always generates distortion, although, if it is low enough, it can also be neglected.

Summary of Transmission Media Effects

Each of the cases in the table will be explained in more detail below. In summary, the table can be summarized as follows:

  • Amplitude: If the transmission produces the same gain/attenuation of the amplitude at all frequencies, the resulting signal can maintain its original appearance. Conversely, the signal will be distorted if there are at least two frequency components that are subjected to different gain/attenuation.
  • Phase: If the medium is non-dispersive, all tones suffer the same time delay. However, in a dispersive medium some frequencies are delayed with respect to others, distorting the signal.
  • Frequencies: If the medium is linear, the frequencies obtained at the output are already present at the input. On the contrary, a non-linear medium generates additional frequencies in the output signal, producing distortion.

Each of the disturbances shown in the table is explained individually below with simple examples. For a more formal analysis of the linear distortion, please refer to this link.

Amplitude Distortion =>

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