Electrical noise and mitigation - Part 3: Shielding and grounding (cont.), and filtering harmonics
We discussed earlier in this chapter about the earth loop being a primary mechanism of noise injection into sensitive signal circuits. One of the important noise mitigation measures is therefore the avoidance of ground loops altogether. We have also seen in the previous chapters that while keeping a separate ground for the sensitive equipment may resolve noise issues, it is an unsatisfactory solution from the safety point of view.
The correct approach is therefore to keep a common electronic ground but bond it firmly with the power system ground at the source point. Figure 8.30 shows an installation with a ground loop problem.
Figure 8.30 Ground loop problem
Here, the main computer system (bottom) and its user terminal are shown connected to the power circuit (including ground wiring) at two different points. A communication cable runs between the computer and its terminal. A ground loop is thus formed with the length of communication cable and the ground wire acting together in series.
Figure 8.31 shows one way in which this loop can be tackled, by bringing the two power and ground connections together to outlets at a single point.
Figure 8.31 Solution to the ground loop problem
This arrangement may not be feasible or practical to adopt. What is really possible is to introduce additional impedance in the ground loop so that the high-frequency noise prefers to take another low-impedance path and diverts itself away from the communication path. This is the principle behind the use of a longitudinal (or 'balun') transformer. Figure 8.32 demonstrates the action of this method.
Figure 8.32 Use of a 'balun' transformer for noise mitigation
8.12 Use of insulated ground (IG) receptacle
The IG receptacles are used in situations where we wish to avoid the mixing of sensitive equipment ground and the building power system ground at all points except the power source (say, the secondary of the shielded isolation transformer) thus avoiding ground loops from forming. Figure 8.33 shows such a receptacle.
Figure 8.33 An exploded view of IG receptacle
The receptacle frame has a separate ground connection, which is bonded to the general ground system through the metallic conduit to ensure safe conditions. But the grounding wire from the sensitive equipment is an insulating wire, which runs through the conduit directly to the ground point of the source. Figure 8.34 illustrates such a connection.
Figure 8.34 Grounding while using an IG receptacle