8.8.3 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 8.22.
Figure 8.22 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 8.23).
Figure 8.23 Twisting of wires to reduce magnetic coupling
The second approach is to use a magnetic shield around the signal wires (refer Figure 8.24). 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 Ø1 to the original flux Ø2. This means a lesser flux (Ø2 - Ø1) reaches our circuit!
Figure 8.24 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.
8.8.4 Radio frequency radiation
The noise voltages induced by electrostatic and inductive coupling (discussed above) are manifestations of the near field effect, which is electromagnetic radiation close to the source of the noise. This sort of interference is often difficult to eliminate and requires close attention of grounding of the adjacent electrical circuit, and the earth connection is only effective for circuits in close proximity to the electromagnetic radiation. The effects of electromagnetic radiation can be neglected unless the field strength exceeds 1 V/m. This can be calculated by the formula:
Field strength = (0.173√power)/distance
where field strength is in volt/meter, power is in kilowatt and distance is in kilometer.
The two most commonly used mechanisms to minimize electromagnetic radiation are:
- Proper shielding (iron)
- Capacitors to shunt the noise voltages to earth.
Any incompletely shielded conductors will perform as a receiving aerial for the radio signal and hence care should be taken to ensure good shielding of any exposed wiring.
8.9 More about shielding
It is important that electrostatic shielding is only earthed at one point. More than one earth point will cause circulating currents. The shield should be insulated to prevent inadvertent contact with multiple points, which behave as earth points resulting in circulating currents. The shield should never be left floating because this would tend to allow capacitive coupling, rendering the shield useless.
Two useful techniques for isolating one circuit from the other are by the use of optoisolation as shown in Figure 8.25, and transformer coupling as shown in Figure 8.26.
Figure 8.25 Opto-isolation of two circuits
Figure 8.26 Transformer coupling
Although opto-isolation does isolate one circuit from another, it does not prevent noise or interference being transmitted from one circuit to another.
Transformer coupling can be preferable to optical isolation when there are very high speed transients in one circuit. There is some capacitive coupling between the LED and the base of the transistor which in the opto-coupler can allow these types of transients to penetrate one circuit from another. This is not the case with transformer coupling.
8.9.1 Good shielding performance ratios
The use of some form of low-resistance material covering the signal conductors is considered good shielding practice for reducing electrostatic coupling. When comparing shielding with no protection, this reduction can vary from copper braid (85% coverage), which returns a noise reduction ratio of 100:1 to aluminum Mylar tape, with drain wire, with a ratio of 6000:1.
Twisting the wires to reduce inductive coupling reduces the noise (in comparison to no twisting) by ratios varying from 14:1 (for four-inch lay) to 141:1 (for one-inch lay). In comparison, putting parallel (untwisted) wires into steel conduit only gives a noise reduction of 22:1.
On very sensitive circuits with high levels of magnetic and electrostatic coupling, the approach is to use coaxial cables. Double-shielded cable can give good results for very sensitive circuits.
Note: With double shielding, the outer shield could be earthed at multiple points to minimize radio frequency circulating loops. This distance should be set at intervals of less than 1/8th the wavelength of the radio frequency noise.
8.9.2 Cable ducting or raceways as magnetic shield
These are useful in providing a level of attenuation of electric and magnetic fields. These figures are valid for a frequency of 60 Hz for magnetic fields and 100 kHz for electric fields. Typical screening factors are:
- For 5 cm (2 in.) aluminum conduit with 0.154 in. thickness
- Magnetic fields 1.5:1
- Electric fields 8000:1
- Galvanized steel conduit (5 cm (2 in.), wall thickness 0.154 in.)
- Magnetic fields 40:1
- Electric fields 2000:1