Conductively Coupled Interference
The electrical fluctuations in a particular electrical device cause an interference an other electrical device connected in the same circuit. Such interference which coupled directly through the electrical conductors are called conductively coup interference. The common sources of such conductively coupled interference are,
1. The presence of common impedance ground path in a measuring circuit. This possible because many devices and instruments are connected to the ground through the same conductor. This problem is very dominant if the path has a substantial impedance to earth ground.
The problem can be minimized by using following procedure. All me low level analog signal grounds should be separate and the digital signal ground and power ground should be separate, till all of them are connected to a common point earth grounded conductor. The impedance of the common path, used by more than one instruments of a system should be as small as possible. The connections of all the ground leads should be made at a particular single point where the common impedance of the system ground path to earth ground has the smallest value.
2. The conductively coupled interferences due to the power transformers of the measuring instruments. When there occur starting and stopping of motors, relays which are supplied by power transformers, spikes and fluctuations in power line voltage get generated. Such noise spikes may affect the instruments from the same power line. The interferences are transmitted through the power lines. There exists a large capacitance between primary and secondary winding of the transformer. This capacitance can couple the current spikes, very strongly to internal power conductors of the instrument.
Providing a capacitive interference shielding between primary and secondary windings of power transformers is one way of reducing such interference. Such shielding is called Faraday shielding. It is made up of conducting metal foil which attenuates capacitive interference. And another way of reducing noise spikes is to identify the sources of such spikes and separate them from the local a.c. power distribution network.
3. Wrong connections of power supplies to load is also an important source of conductively coupled interference. If two loads are connected wrongly in parallel to the same supply then fluctuations in one load can affect other. Only remedy in such cases is it to identify incorrect connections and correct them.
4. The low level signals can get affected due to the unwanted conductively coupled interferences of frequencies other than the low level signals such effect of other frequency signals can be minimized using electronic filters the low level signal frequency is known. The noise reduction is also possible using properly designed filters.