02-06-2012, 03:40 PM
ELECTRO MAGNETIC INTERFERENCE & ELECTRO MAGNETIC COMPATIBILITY
ELECTROMAGNETIC INTERERENCE.doc (Size: 692 KB / Downloads: 38)
ABSTRACT:
Performance of analog circuits is often affected adversely by high frequency signals from nearby electrical activity. And the equipment containing the analog circuitry may also adversely affect external systems. ELECTROMAGNETIC COMPATABILITY is the ability of a device or unit of equipment or system to function satisfactorily in its electromagnetic environment without introducing intolerable electromagnetic disturbances to anything in that environment. The externally produced electrical activity may generate noise and is referred to either as electromagnetic interference (EMI) or radio frequency interference (RFI).
Designing of EMC can be done by: Cable coupling, Direct coupling, Grounding, Filtering and Shielding. The main EMC problems for electronic products are the emissions of internally-generated high frequencies which may interfere with on-board or nearby radio reception and susceptibility to transient or radio frequency interference from the external environment which may degrade the quality of analogue signals or corrupt digital processes.
The main advantages of EMC are reducing magnetically-coupled noise, reducing capacitance-coupled noise, reducing common-impedance noise, reducing externally induced interference, reducing system susceptibility to EMI. So, complete EMC assurance would indicate that the equipment under design should neither produce spurious signals, nor should it be vulnerable to out-of-band external signals.
INTRODUCTION:
Analog circuit performance is often affected adversely by high frequency signals from nearby electrical activity. And, equipment containing the analog circuitry may also adversely affect systems external to it. Complete EMC assurance would indicate that the equipment under design should neither produce spurious signals, nor should it be vulnerable to out-of-band external signals (i.e., those outside its intended frequency range). The externally produced electrical activity may generate noise and is referred to either as electromagnetic interference (EMI) or radio frequency interference (RFI). One of the more challenging tasks of the analog designer is the control of equipment against undesired operation due to EMI. EMI and or RFI are almost always detrimental. Once given entrance into the equipment, it can and will degrade its operation, quite often considerably. Misbehavior of this sort is also known as EMI or RFI susceptibility, indicating a tendency towards anomalous equipment behavior when exposed to EMI/RFI. There is of course a complementary EMC issue, namely with regard to spurious emissions.
Electromagnetic Interference:
The fundamental concept of electrical and electromagnetic interference involves a source and an affected device or system. The transfer of energy between systems can occur through radiation, conduction or induction. The actual transfer of energy is facilitated respectively through a transmission path, conductive path or through magnetic coupling. The interference that affects wireless communication links is typically the result of radiated or conductive energy transfer. The condition of a conductive affect occurs when the signal is picked-up by a conductor attached to the affected system.
The presence of electrical contributes to the surrounding levels of electrical and electro-magnetic noise and interference. The increasing demand and dependence on the use of electromagnetic transmission systems for information technology applications has enabled revolutionary communications capabilities to include cordless communication devices, wireless networking and satellite communication systems. As the number of these devices increases, there is an increase in electromagnetic radiation within the segments of the electromagnetic spectrum in which these systems operate. These emissions have the potential to interfere with the normal operation and function of electronic communication links and systems. The condition of electromagnetic energy disrupting the normal operation or function of electronic devices is known as electronic interference (EI).
By the very name of interference given to it, EMI is an unwanted signal at the signal receiver and in general methods are sought to reduce the level of the interference.
Types of Electromagnetic interference:
EMI - Electromagnetic Interference can arise in many ways and from a number of sources. The different types of EMI can be categorised in a number of ways.
One way of categorising the type of EMI is by the way it was created:
• Man-made EMI: This type of EMI generally arises from other electronics circuits, although some EMI can arise from switching of large currents, etc.
• Naturally occurring EMI: This type of EMI can arise from many sources - cosmic noise as well as lightening and other atmospheric types of noise all contribute.
Another method of categorising the type of EMI is by its duration:
• Continuous interference: This type of EMI generally arises from a source such as a circuit that is emitting a continuous signal. However background noise which is continuous, may be created in a number of ways, either manmade or naturally occurring.
• Impulse noise: Again, this type of EMI may be man-made or naturally occurring. Lightening, ESD and switching systems all contribute to impulse noise which is a form of EMI.
It is also possible to categorise the different types of EMI by their bandwidth.
• Narrowband: Typically this form of EMI is likely to be a single carrier source - possibly generated by an oscillator of some form. Another form of narrowband EMI is the spurious signals caused by intermodulation and other forms of distortion in a transmitter such as a mobile phone of Wi-Fi router. These spurious signals will appear at different points in the spectrum and may cause interference to another user of the radio spectrum. As such these spurious signals must be kept within tight limits.
• Broadband: There are many forms of broadband noise which can be experienced. It can arise from a great variety of sources. Man-made broadband interference can arise from sources such as arc welders where a spark is continuously generated. Naturally occurring broadband noise can be experienced from the Sun - it can cause sun-outs for satellite television systems when the Sun appears behind the satellite and noise can mask the wanted satellite signal. Fortunately these episodes only last for a few minutes.
Interference Mechanism:
There are many ways in which the electromagnetic interference can be coupled from the source to the receiver. Understanding which coupling method brings the interference to the receiver is key to being able to address the problem.
• Radiated: This type of EMI coupling is probably the most obvious. It is the type of EMI coupling that is normally experienced when the source and victim are separated by a large distance - typically more than a wavelength. The source radiates a signal which may be wanted or unwanted and the victim receives it in a way that disrupts its performance.
• Conducted: Conducted emissions occur as the name implies when there is a conduction route along which the signals can travel. This may be along power cables or other interconnection cabling. The conduction may be in one of two modes:
Common mode: This type of EMI coupling occurs when the noise appears in the same phase on the two conductors, e.g. out and return for signals, or +ve and -ve for power cables.
Differential mode: This occurs when the noise is out of phase on the two conductors.
The filtering techniques required will vary according to the type of EMI coupling experienced. For common mode lines are filtered together. For differential mode they may be filtered together.
• Inductive coupling: What is normally termed inductive coupling can be one of two forms, namely capacitive coupling and magnetic induction.
• Capacitive coupling: This occurs when a changing voltage from the source capacitively transfers a charge to the victim circuitry.
• Magnetic coupling: This type of EMI coupling exists when a varying magnetic field exists between the source and victim - typically two conductors may run close together (less than λ apart). This induces a current in the victim circuitry, thereby transferring the signal from source to victim.
By determining the form of coupling that exists and the way in which it is reaching the victim, it may prove to be that the most effective method of reducing the EMI is by putting measures in place to reduce the coupling and reduce the level of interference to an acceptable level.
Electromagnetic interference, EMI is present in all areas of electronics. By understanding the source, the coupling methods and the susceptibility of the victim, the level of interference can be reduced to a level where the EMI causes no undue degradation in performance.