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.
Electrical noise occurs or is transmitted into a signal cable system in the following four ways:
- Galvanic (direct electrical contact)
- Electrostatic coupling
- Electromagnetic induction (in part 2)
- 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.
For situations where two or more electrical circuits share common conductors, there can be some coupling between different circuits with deleterious effects on the connected circuits. Essentially, this means that the signal current from one circuit proceeds back along the common conductor resulting in an error voltage along the return bus, which affects all the other signals.
The error voltage is due to the capacitance, inductance and resistance in the return wire. This situation is shown in Figure 1 below.
Obviously, the quickest way to reduce the effects of impedance coupling is to minimize the impedance of the return wire. The best solution is to use a balanced circuit with separate returns for each individual signal shown in Figure 2.
This form of coupling is proportional to the capacitance between the noise source and the signal wires. The magnitude of the interference depends on the rate of change of the noise voltage and the capacitance between the noise circuit and the signal circuit.
The size of the noise (or error) voltage in the signal wires is proportional to the:
- Inverse of the distance of noise voltage from each of the signal wires
- Length (and hence impedance) of the signal wires into which the noise is induced
- Amplitude (or strength) of the noise voltage
- Frequency of the noise voltage.
There are four methods for reducing the noise induced by electrostatic coupling. They are:
- Shielding of the signal wires
- Separating from the source of the noise
- Reducing the amplitude of the noise voltage (and possibly the frequency)
- Twisting of the signal wires.
Figure 4 indicates the situation that occurs when an electrostatic shield is installed around the signal wires.
The currents generated by the noise voltages prefer to flow down the lower-impedance path of the shield rather than the signal wires. If one of the signal wires and the shield are tied to the earth at one point, which ensures that the shield and the signal wires are at an identical potential, then reduced signal current flows between the signal wires and the shield.
The shield must be of a low-resistance material such as aluminum or copper. For a loosely braided copper shield (85% braid coverage) the screening factor is about 100 times or 20 dB that is, C3 and C4 are about 1/100 C1 or C2. For a low-resistance multi- layered screen, this screening factor can be 35 dB or 3000 times.
Twisting of the signal wires provides a slight improvement in the induced noise voltage by ensuring that C1 and C2 are closer together in value; thus ensuring that any noise voltages induced in the signal wires tend to cancel one another out.
Provision of a shield by a cable manufacturer ensures that the capacitance between the shield and the wires is equal in value (thus eliminating any noise voltages by cancellation).
Will be continued very soon… Stay in the tune ;)
Reference: Practical Grounding, Bonding, Shielding and Surge Protection – G. Vijayaraghavan, B.Eng (Hons) Consulting Engineer, Chennai, India