What is transmission line noise?
Transmission Line Noise Defined.
Transmission lines are capable of generating noise and spurious voltages that are seen by the system as valid signals. Several such sources exist.
One source is coupling between noise currents flowing in the outer and inner conductors. Such currents are induced by nearby electromagnetic interference and other sources (e.g. connection to a noisy ground plane).
Although coaxial design reduces noise pick-up compared with parallel line, the potential for EMI exists. Selection of high-grade line, with a high degree of shielding, reduces the problem.
Another source of noise is thermal noise in the resistances and conductances of the line. This type of noise is proportional to resistance and temperature.
There is also noise created by mechanical movement of the cable. One species results from movement of the dielectric against the two conductors.
This form of noise is caused by electrostatic discharges in much the same manner as the spark created by rubbing a piece of plastic against woollen cloth.
A second species of mechanically generated noise is piezoelectricity in the dielectric. Although more\ common in cheap cables, one should be aware of it.
Mechanical deformation of the dielectric causes electrical potentials to be generated.
Both species of mechanically generated noise can be reduced or eliminated by proper mounting of the cable. Although rarely a problem at lower frequencies, such noise can be significant at microwave
frequencies when signals are low.
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