Electromagnetic Interference - types, sources, causes, effects


Simply Electromagnetic Interference (EMI) is interference to an electronic device by electrical energy generated nearby. All the electronic equipments consist of electronic circuits made of electronic components and devices. Through these components the current of various frequencies flow, depending on signal frequency. According to electromagnetic theory, there is an EM field associated with flow of current. This field is radiated outwards and this can affect the operation of another device. So the phenomenon when EM field of one equipment or device disturb the working of another device is called Electromagnetic interference. The effect of EMI is becoming more and more important now a days due to the vast advancement in the field of electronics and mobile phone industry. EMI is also known as radio frequency interference (RFI). EMI can interrupt, block, degrade or restrict the effective performance of a circuit.

Types and sources of EMI  

There are mainly three general classes of EMI problems.

1. Conducted EMI,
2. Radiated EMI,
3. Electrostatic discharge

1. Conducted EMI 

It requires a wire or other conductor to transfer the EMI from one place to another. Ex: During surgical operations EMI may transfer through patient body to the ECG machine. Due to EMI, the electromagnetic field in the conductor is no longer consistently circulated and causes skin effect, hysteresis losses, eddy current losses etc. Electromagnetic troubles in the Electromagnetic field of a conductor will no longer be restricted to the exterior of the conductor and will emit away from it. Due to this EMI, the EM field around the conductor is not evenly distributed and will cause skin effect, eddy current losses, hysteresis losses, harmonic distortion etc.

2. Radiated EMI

Radiated EMI does not require any wire to carry interference and it is transmitted or radiated through space. In the case of radiated EMI, the circuit elements such as PCB traces can act as an antenna. The radiated EMI is caused due to induction without the physical contact of conductors. Radiated EMI mainly occur at high frequencies and the most horrible part of high frequency EM signal is that it makes each conductor an antenna. For example in the case of PCBs which includes capacitors and other semiconductor devices which are soldered to the bread board, the capacitors and soldering act like antennas producing and gripping EM fields.

Radiated EMI can be classified into two types. They are narrowband and broadband. Narrowband interference arises from radio and TV transmitters, pager transmitters, cell phones etc. Broadband interference results from subsidiary radio frequency emitters like electric power transmission lines, electric motors etc. An important characteristic feature of broadband EMI is the incapability to filter it out efficiently once it has come into the receiver. The standard line impedance stabilization networks (LISN) are used to prevent the conducted EMI. Active Noise Filters are also executed to lessen both usual and regular mode conducted EMI noise.

3. Electrostatic discharge

Electrostatic Discharge (ESD) is the rapid and temporary electric current that runs amid two objects at dissimilar electric potentials caused by straight contact or induced by an electrostatic field. Static electricity and electrostatic induction are the two main causes of ESD. ESD is the buildup of electrostatic potentials on tools, substance and the human body caused by any mechanical action.

EMI- causes and effects 

The extent to which EMI affects depends on the quality of design of a device. Also EMI varies with grounding methods, types of shielding etc. As told earlier, poor shielding of a device can be followed by severe effects of EMI. The internal circuits and layout can affect the internal EMI. Poor design can also enhance the effect of EMI.

The ongoing trend towards smaller, lighter, more- compact power supplies leads to higher power densities and switching frequencies which can lead to the serious issue of EMI related problems. A poorly shielded power supply, poor wiring lay out, improper grounding etc will cause EMI which can attenuate or superimpose on the original measured signal.

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