Analysis of Electromagnetic Compatibility - Assignment Example

Objective: The computer room in an established bank headquarters consists of 12 workstations in association with printers, data farms, server equipment and “invaluable data”. The fundamental need was uninterrupted power supply and safe preservation of data. The heating, ventilating and air-conditioning system takes care of temperature and humidity requirement for proper functioning of the IT system. ITC equipment supplied and installed by various manufacturers of international standards and repute, lie close to each other and are connected to the same single phase power supply through screened cables of superior quality. Uninterrupted power supply units (UPS) are of reputed make with sufficient back up time such that the operations are carried out without break, data backed up, and system shut down safely. In case of any unanticipated power breakdown, there are arrangements to protect data and timely back up. An individual PC is in control of the monitoring system, which with the help of sensors and associated power supply system, ascertain whether it is time for the UPS to act or alarm to set off etc. Measures to adhere to EMC: The first ever EMC issue was from the natural source, namely, lightning. Later with massive generation, transmission and distribution of electrical energy equipment, short circuit became common. Circuit breakers were installed in power stations, Fuses in buildings and for appliances. Gradually, Miniature circuit breakers came into existence. Radio signals of different frequencies started giving fresh problems. Switching devices in automobiles, refrigerators, television, radio etc were a source of cause for ‘transient interference’. When Radio Frequency (RF) emission levels rose alarmingly high, it became necessary to designate certain frequency bands for industrial, scientific and medical (ISM) purposes. Sideband, Harmonic emissions, Broadband, Broadcast signals etc initiated the development of international regulatory, standards and laws. With the advent of Digital circuitry, with fast switching speeds (increasing Emissions) and low circuit voltages (increasing Susceptibility), EMC became an important issue. Miniature circuits were susceptible to Electrostatic Discharge (ESD) damage. Mobile communications and broadcast channels occupied large amount of airspace. It became necessary to keep cross channel interference to the minimum. ISM bands are being used for mobile communications making use of error correction methods to guarantee full and continuous reception. EMC deals with unintentional generation, propagation and reception of electromagnetic energy pertaining to ‘unwanted effects’, namely, electromagnetic interference (EMI) induced by it .EMC must safeguard against interference effects. For that it goes after two issues – Emission issues: They are with respect to the creation of unwanted electromagnetic energy by any ‘Source’ and the steps necessary to reduce it. The energy should be prevented from escaping to the outside environment. . Susceptibility issues (also known as ‘Immunity issue’) pertain to the proper operation of electrical items, referred to as ‘Victims’, in the presence of electromagnetic disturbances. Reduction in interference and noise is attained by taking care of both the issues. The coupling path between source and victim may be considered to increase its attenuation. EMI: The noise can be natural or man made. When a source emits a certain range of frequencies continuously, Continuous Wave is created. Audio frequency (Frequencies up to 100 kHz): The Sources are: Power supply units, wiring, substations, transmission lines, loud speakers, power amplifiers etc. Radio frequency interference: The sources are: Radio and television receivers, wireless, radio frequency transmissions, and high frequency circuit signals, medical, scientific and industrial equipment. Broadband sources include CDMA mobile telephone systems, solar activities, and spark gaps as in ‘arc welding’ machines. Transient disturbance or Electromagnetic pulse develops when the source lets out pulse energy of short duration. This is usually of broadband, though it excites a Narrow band response in the victim. Sources are broadly divided into Isolated and Repetitive events. Sources of Isolated events include: Electrostatic discharge, Lightening electromagnetic pulse, nuclear electromagnetic pulse, non- nuclear electromagnetic pulse, power line surges, switching actions of relays, solenoids, motors etc. Sources of repetitive events include: Electric motors, gasoline engine ignition systems and digital circuit switching actions. Coupling: Noise source and victim (receptor) are normally hardware devices. The basic coupling mechanisms are radio active, conductive, capacitive, inductive or magnetic. In radio active coupling the source and victim are separated by more than a wavelength, which amounts to a large distance. They act as radio antennas – The electromagnetic waves emitted by the source intercepted by the victim. . In conductive e coupling, there is direct contact between source and victim, just as in wire, cable, transmission line etc. In common mode coupling, noise appears in the same direction in both conductors. In differential mode coupling, noise appears in opposite directions on he two conductors. In capacitive coupling, a varying electrical field exists between two conductors which are positioned a wavelength apart bringing rise to a change of voltage in the gap. In inductive coupling or magnetic coupling, a varying magnetic field develops between two parallel conductors positioned less than a wavelength apart. A change in voltage occurs along the receiving conductor. The induction can be electrical or magnetic. Electrical induction is referred to as capacitive coupling and magnetic induction as inductive coupling. EMC control: It is essential to control the damaging effect of EMI. Here are the ways. Characterization of the problem: It requires the understanding of the source and signal of the interference, the coupling path to the victim, the nature of the victim both in terms of its malfunctioning and electrically. Set the standards for emission and susceptibility levels. Design and test for standard compliance. EMC design: The cause for noise is due to the quick changes in current and voltages. The noise is spread through the coupling mechanism. Good EMC design practice apply equally to source and victim as breaking a coupling path at start or end of the path mean the same. An improvement in design reduces emission and susceptibility. Grounding: This provides a low impedance path, shielding the EMI from the victim. The n methods of grounding include, Star earthling for audios, ground planes for RF signals, shielded lines and housings. The following are also important: Impedance matching and balanced differential signal and return paths for transmission lines, avoiding of loops for circulating current, unbalanced cable impedances, poor earthling etc in case of Antennas, decoupling of cable entries using RF chokes RC elements etc. The following are the ways to reduce emissions: Avoid unnecessary switching operations, separate the noisy circuit with lot of switching activities from the rest of the design, install harmonic wave filters and use the spread spectrum method to avoid high peaks. To reduce susceptibility, provide transient absorbers, circuit breakers, trip switches and fuses. EMC testing consists of emission testing and susceptibility testing. Emission testing: An oscilloscope is used to know the pulse waveform in the case of pulse emissions. Emissions are measured for radiated field strength, using antennas as transducers. Emissions must be measured in all directions around the device under test (DUT). Conducted emission along cables and wiring has also to be done. Spectrum analyzers or EMI Test Receivers which accommodate bandwidth and detectors of international EMC standards are used to measure emissions levels across a wide band of frequencies. To suppress the effects of out-of-band signals, Pre-selector filters can be used on the front end of the receiver. For conducted emission, Line Impedance Stabilization Networks (LISN) is the transducers. Susceptibility testing: It involves a source of RF pulse energy and an antenna directing the energy at the device being tested. A pulse generator and a transformer to inject the test signal are used for testing of conducted voltage and current susceptibility. Various power line disturbances like surges, lightning strikes, switching noise etc against the DUT are tested by Transient immunity test. Similar tests after being conducted on battery and signal lines in the case of motor vehicles. A Piezo spark generator will perform the Electrostatic discharge testing. RF testing is mostly done in RF anechoic chamber. Computational Electromagnetic Simulations are used to test virtual models. System Analyses: EMC is the capability of electrical and electronic items to function in the destined environment as per the design requirements, in safe conditions without causing any sort of damage to the outside world due to the electromagnetic interference. Neglect of this may lead to hazards. With the rapid growth of electronics in the day to day lives of humans, compatibility has become all the more important. The government organizations worldwide are bringing laws prohibiting the products from emitting and being susceptible to EMI. EMC constitutes Emission, Susceptibility and Path. Though emission is the main culprit, it is also the provider of largest amount p of solutions. Susceptibility can neither grasp its effect nor find any solution. Path can be the judge for both. Path consists of conducted (electric current) energy and radiated (electromagnetic field) energy. In the case of radiation, Electric is the dominant energy in the Far Field, while Magnetic is the dominant energy in the Near Field. Radio works because of Plane wave radiation, also known as Plane wave radiation or Far field radiation Electrostatic radiation. The signal is beyond 1/6 wavelength Free space impedance is 377 ohms. The signal under 1/6 wavelength is electromagnetic and is known as Near Field Radiation. We have to control the following: 1. EMI: This is a natural phenomenon and is an electrical disturbance due to Low frequency waves from electromechanical devices and high frequency waves (RFI) from chips. 2. Radio frequency interference: High frequency electromagnetic waves from chips and other electronic products. 3. Television Interference: High frequency electromagnetic waves from electronic devices causing disturbance to TV reception. Conducted EMI: (up to 30 MHZ frequency) The sources of emission are Power supplies, motors, relays etc. Causes of susceptibility are poor filtering of power cord, poor decoupling of power rails etc. Good circuit design incorporating decoupling, bypassing, filtering, grounding etc and putting the same into practice will solve the problem. Radiated EMI: (30 MHZ to 10 GHZ frequency ranges.) The sources are switching power supplies, Clocks, Clock lines, Data lines etc. Susceptibility is the poor laying out and improper termination of clock lines and data lines. The solution lies in balanced transmission line lay-out, proper filtering, shielding, termination and grounding. Differential line receivers, Limited rise and fall time divers can also be installed. The walls and ceiling of an anechoic chamber should be of such a material that there is no reflection of electromagnetic waves. An improperly terminated transmission line can be a source of interference. Transmission line faults can be determined by Time Domain Reflectometry measurements. Cross talk result when buses span long distances. The remedy is by providing enough space between the buses. If it is not possible, provide interleaving ground returns. It is ideal to maintain shield isolation. Avoid unwanted currents in the coaxial shield. The ideal shield has no current flowing through it, and is grounded at only one point. In the case of shield cables, the shield is grounded at both ends. For coax cables, the shield also acts as the Return current path for the signal carried by the center conductor. In twisted pairs, the shield acts merely as a shield and can be grounded at either end, but in view of EMI suppression, grounding at both ends is advisable. In noisy environment and long runs, shielded twist pair is preferred as common mode noise (CMR) is straight away rejected. At times even transformer coupling is being adopted. Additional Measures: The above measures are good enough to ensure EMC. However, if the purpose is not fulfilled the following steps can be adhered to: 1. The loop areas connected with high frequency power and signal current should be kept at the minimum. For digital circuit designers, the signal is in terms of voltage. But for EMC engineers it is in terms of current. The designer should note that signal current always take the path of least impedance and that it returns to their source and that the current paths are loops. At low frequencies (meaning KHZ frequencies and below), the currents spread out and are difficult to trace. The paths of least impedance and least resistance tend to be the same. At MHZ frequencies and above, the paths can be easily traced because at high frequencies, the path of least impedance is the path of least inductance, which is the path that minimizes the loop area. 2: Never split or cut or gap the signal return plane. It is ideal to provide one solid plane for return of all signal currents. When a low frequency signal interferes with the circuit board, provide a separate layer to return it to the source. 3. Avoid high speed circuitry between connectors. This is a simple factor, but if neglected, will give nightmares. A high caution is necessary for locating the connectors. At low MHZ frequencies, wavelength are one meter long or more. Any antenna on the PCB is small and inefficient. But cables and other devices connected to the board can be efficient antennas. Signal currents returning through solid planes create small voltage differences between two points, proportional to the current. Generally this voltage is negligible. However, a potential difference of a few milli-volts develops between the connectors in the case of high speed circuitry. This is sufficient to induce current in the attached cables which will cause increase in radiated emission. 3. Take control of signal transition time. The signal will have a good amount of power in the lower harmonic frequencies, but not in the upper harmonics, which is controlled by regulating the transition times in digital signals. For EMC, longer transmission times are recommended but not so long as to cause thermal problems. However, long transitions result in signal integrity. The recommended transition time is about 20% a bit period, which will ensure a nice waveform and reduce problems due to emissions and cross talk. The rise and fall times in digital logic can be controlled by installing a resistor in series at the output of the device. It is not good to filter a single ended signal by restricting the flow of current in the return path. For example, it is not advisable to have a low speed trace over a gap in return pane, to filter out high frequency noise. Reference Electromagnetic compatibility, available at, http://www.bing.com/reference/semhtml/Electromagnetic_compatibility?src=abop&fwd=1&qpvt=electromagnetic+compatibility&q=electromagnetic+compatibility (accessed 9, March 2010) ( (EMC, Available at, http://ec.europa.eu/enterprise/sectors/electrical/emc/(accessed 9, March 2010) The Clemson University, Electromagnetic compatibility, available at, http://www.cvel.clemson.edu/emc/tutorials/definitions.html(accessed 9, March 2010) EMC, available at http://www.williamson-labs.com/480_emc.htm(accessed 9, March 2010)(,20108899“(( The Clemson University, Why you should be cautious about using EMC design rules, available at, http://www.cvel.clemson.edu/emc/tutorials/guidelines/Word_of_Caution.html(accessed 9, March 2010)(,20108899“(( Most important EMC Design guidelines, Available at, http://www.cvel.clemson.edu/emc/tutorials/guidelines/Important_Guidelines.html (accessed 9, March 2010). What is EMC, (updated feb 10 2010) Available at, http://www.electronics-project-design.com/EMC.html(accessed 9, March 2010)(,20108899“(( Read More