4 ways in which noise can enter a signal cable and its control – Part 1

Few words about interference…

Noise, or interference, can be defined as undesirable electrical signals, which distort or interfere with an original (or desired) signal. Noise could be transient (temporary) or constant.
Unpredictable transient noise is caused, for example, by lightning.

Constant noise can be due to the predictable 50 or 60 Hz AC ‘hum’ from power circuits or harmonic multiples of power frequency close to the data communications cable. This unpredictability makes the design of a data communications system quite challenging.

Electrical noise occurs or is transmitted into a signal cable system in the following four ways:

1. Galvanic (direct electrical contact)
2. Electrostatic coupling
3. Electromagnetic induction (in part 2)
4. Radio frequency interference (RFI) (in part 2)

If two signal channels within a single data cable share the same signal reference conductor (common return path), the voltage drop caused by one channel’s signal in the reference conductor can appear as a noise in the other channel and will result in interference. This is called galvanic noise.
Electrostatic noise is one, which is transmitted through various capacitances present in the system such as between wires within a cable, between power and signal cables, between wires to ground or between two windings of a transformer. These capacitances present low-impedance paths when noise voltages of high frequency are present.
Thus noise can jump across apparently non- conducting paths and create a disturbance in signal/data circuits.

Electromagnetic interference (EMI) is caused when the flux lines of a strong magnetic field produced by a power conductor cut other nearby conductors and cause induced voltages to appear across them.

When signal cables are involved in the EMI process, this causes a noise in signal circuits. This is aggravated when harmonic currents are present in the system. Higher order harmonics have much higher frequencies than the normal AC wave and result in interference particularly in communication circuits.
Radio frequency interference involves coupling of noise through radio frequency interference. We will now describe these in some detail.


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