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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The Essentials of Electronics and Electrical Equipment: Shielding

Effective EMI control prevents spurious signals from entering or leaving an enclosure. Shields and filters are the predominant techniques for controlling EMI.

Shields can involve many combinations of foils, conductive inks, paper, and adhesives. For example, there are now shields that consist of silver ink printed on 5-mil-thick polyester. After printing, a curing process removes nonconductive solvents. The result is a homogeneous shield that does not crack or delaminate when bent. The shield can be mechanically fastened and grounded with solder tabs. The cost is about the same as that for conventional laminated designs.

Carbon and stainless-steel fibers, combined with thermoplastics, provide an effective shield in many applications. Carbon fibers are usually classified as either PAN (polyacrylonitrile) or pitch. PAN fiber composites are selected for their high strength. Also, because PAN fiber has a higher aspect ratio (length to diameter ratio) than pitch, less is needed to provide a given conductivity.

Pitch-based fibers are not as strong as low-modulus PAN fibers. However, pitch fibers process easily into high-modulus products, making them attractive for stiffness-critical and thermally sensitive applications. The third carbon additive commonly used is carbon black. Carbon-black plastics are inexpensive, and are primarily for applications requiring high surface conductivity that allows dissipation of static charge.

Five factors affect plastic conductivity. The first is fill aspect ratio, which is proportional to conductivity. Second is loading level, also proportional to conductivity. The lowest fill loading needed to produce conductivity (generally defined as 10⁵Ω/sq) is called the critical concentration.
The third factor is resin type. The amount of fill needed for the critical concentration depends on the resin. For example, because nylon has a crystalline structure, its surface becomes conductive at lower fill concentrations than materials such as amorphous polycarbonate.


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4 ways in which noise can enter a signal cable and its control - PART 2

Continued from part 1… Read here.

Electromagnetic induction and RFI

In previous part of this technical article, I wrote about electrical noise occurs or is transmitted into a signal cable system in the following four ways:

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

3. Magnetic or inductive coupling

This depends on the rate of change of the noise current and the mutual inductance between the noise system and the signal wires.
Expressed slightly differently, the degree of noise induced by magnetic coupling will depend on the:

• Magnitude of the noise current
• Frequency of the noise current
• Area enclosed by the signal wires (through which the noise current magnetic flux cuts)
• Inverse of the distance from the disturbing noise source to the signal wires.

The effect of magnetic coupling is shown in Figure 1 below.

Figure 1 – Magnetic coupling

The easiest way of reducing the noise voltage caused by magnetic coupling is to twist the signal conductors. This results in lower noise due to the smaller area for each loop.

This means less magnetic flux to cut through the loop and consequently a lower induced noise voltage. In addition, the noise voltage that is induced in each loop tends to cancel out the noise voltages from the next sequential loop.

Hence an even number of loops will tend to have the noise voltages canceling each other out. It is assumed that the noise voltage is induced in equal magnitudes in each signal wire due to the twisting of the wires giving a similar separation distance from the noise voltage (see Figure 3).

Figure 3 – Twisting of wires to reduce magnetic coupling

The second approach is to use a magnetic shield around the signal wires (refer Figure 4).
The magnetic flux generated from the noise currents induces small eddy currents in the magnetic shield. These eddy currents then create an opposing magnetic flux Φ₁ to the original flux Φ₂. This means a lesser flux (Φ2 − Φ1) reaches our circuit!

Figure 4 – Use of magnetic shield to reduce magnetic coupling

Note: The magnetic shield does not require earthing. It works merely by being present. High-permeability steel makes best magnetic shields for special applications. However, galvanized steel conduit makes a quite effective shield.