Chapter: Transmission Lines and Waveguides - Transmission Line Theory

Waveform Distortion

Waveform distortion is of two types: a)Frequency distortion b) Phase or Delay Distortion.

WAVEFORM DISTORTION

 

Waveform Distortion: Signal transmitted over lines are normally complex and consists of many frequency components. For ideal transmission, the waveform at the line-receiving end must be the same as the waveform of the original input signal. The condition requires that all frequencies have the same attenuation and the same delay caused by a finite phase velocity or velocity of propagation. When these conditions are not satisfied, distortion exists. The distortions occurring in the transmission line are called waveform distortion or line distortion.

 

Waveform distortion is of two types:

 

a)Frequency distortion

 

b) Phase or Delay Distortion.

 

a) Frequency distortion:

 

In general, the attenuation function is a function of frequency. Attenuation function specifies the attenuation or loss incurred in the line while the signal is propagating.

 

 

When a signal having many frequency components are transmitted along the line, all the frequencies will not have equal attenuation and hence the received end waveform will not be identical with the input waveform at the sending end because each frequency is having different attenuation. This type of distortion is called frequency distortion. That is, when the attenuation constant is not a function of frequency, frequency distortion does not exist on transmission lines.

 

In order to reduce frequency distortion occurring in the line,

 

a)The attenuation constant should be made independent of frequency.

 

b)By using equalizers at the line terminals which minimize the frequency distortion.

 

Equalizers are networks whose frequency and phase characteristics are adjusted to be inverse to those of the lines, which result in a uniform frequency response over the desired frequency band, and hence the attenuation is equal for all the frequencies.

 

b) Delay distortion:

 

When a signal having many frequency components are transmitted along the line, all the frequencies will not have same time of transmission,some frequencies being delayed more than others. So the received end waveform will not be identical with the input waveform at the sending end because some frequency components will be delayed more than those of other frequencies. This type of distortion is called phase or delay distortion.

 

It is that type of distortion in which the time required to transmit the various frequency components over the line and the consequent delay is not a constant.

 

This is, when velocity is independent of frequency, delay distortion does not exist on the lines. In general, the phase function is a function of frequency. Since v= ω / β, it will be independent of frequency only when β is equal to a constant multiplied by ω .

 

In order to reduce frequency distortion occurring in the line,

 

a)The phase constant _ should be made dependent of frequency.

 

b)The velocity of propagation is independent of frequency.

 

c) By using equalizers at the line terminals which minimize the frequency distortion.

 

Therefore, we conclude that a transmission line will have neither delay nor frequency distortion only if α is independent of frequency and β should be a function of frequency.

 

The value of the attenuation constant α has been determined that


In general α is a function of frequency. All the frequencies transmitted on a line will then not be attenuated equally. A complex applied voltage, such as voice voltage containing many frequencies, will not have all frequencies transmitted with equal attenuation, and the received for will be identical with the input waveform at the sending end. This variation ic=s known as frequency distortion.

 

Phase Distortion

 

All the frequencies applied to a transmission line will not have the same time of transmission, some frequencies delayed more than the others. For an applied voice voltage waves the received waves will not be identical with the input wave form at the receiving end, since some components will be delayed more than those of the other frequencies. This phenomenon is known as delay or phase distortion.

 

The of propagation has been stated that


It is apparent that ω and β do not both involve frequency in same manner and that the velocity of propagation will in general be some function of frequency.

 

Frequency distortion is reduced in the transmission of high quality radio broadcast programs over wire line by use of equalizers at line terminals These circuits are networks whose frequency and phase characteristics are adjusted to be inverse to those of the lines, resulting in an overall uniform frequency response over the desired frequency band.

 

Delay distortion is relatively minor importance to voice and music transmission because of the characteristics of ear. It can be very series in circuits intended for picture transmission, and applications of the co axial cable have been made to over come the difficulty. In such cables the internal inductance is low at high frequencies because of skinn effect, the resistance small because of the large conductors, and capacitance and leakance are small because of the use of air dielectric with a minimum spacers. The velocity of propagation is raised and made more nearly equal for all frequencies.

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Transmission Lines and Waveguides - Transmission Line Theory : Waveform Distortion |


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