Electromagnetic Interference (EMI) is caused by undesireable radiated electromagnetic fields or conducted voltages and currents. The interference is produced by a source emitter and is detected by a susceptible victim via a coupling path. The coupling path may involve one or more of the following coupling mechanisms: 1. 2. 3. 4.
Conduction - electric current Radiation - electromagnetic field Capacitive Coupling - electric field Inductive Coupling - magnetic field
Conducted noise is coupled between components through interconnecting wires such as through power supply and ground wires. Common impedance coupling is caused when currents from two or more circuits flow through the same impedance such as in power supply and ground wires. Radiated electromagnetic field coupling may be treated as two cases. In the near field, E and H field coupling are treated separately. In the far field, coupling is treated as a plane wave coupling. Electric field coupling is caused by a voltage difference between conductors. The coupling mechanism may be modeled by a capacitor. Magnetic field coupling is caused by current flow in conductors. The coupling mechanism may be modeled by a transformer. Some typical external noise sources into a radio receiver include radiated electric field coupling from: high-voltage power lines, broadcast antennas, communications
transmitters, vehicle ignition systems and electric machinery. Most conducted coupling from external sources occurs through the ac power lines. Typical radio interference to other equipment includes radiated electric field coupling to: TV sets, broadcast receivers, telephone lines, appliances, and communications receivers. Most conducted coupling to other equipment occurs through the ac power lines. The most common methods of noise reduction include proper equipment circuit design, shielding, grounding, filtering, isolation, separation and orientation, circuit impedance level control, cable design, and noise cancellation techniques. Electromagnetic radiation involves electric (E) and magnetic (H) fields. Any change in the flux density of a magnetic field will produce an electric field change in time and space (Faraday's Law). This change in an electric field causes another change in the magnetic field due to the displacement current (Maxwell). A timevarying magnetic field produces an electric field and a time-varying electric field results in a magnetic field. This forms the basis of electromagnetic waves and timevarying electromagnetics (Maxwell's Equations). Wave propagation occurs when there are two forms of energy and the presence of a change in one leads to a change in the other. Energy interchanges between electric and magnetic fields as the wave progresses. Electromagnetic waves exist in nature as a result of the radiation from atoms or molecules when they change from one energy state to another and by natural fluctuations such as lightning. The technology of generating and processing electromagnetic waves forms the basis of telecommunications. Electromagnetic Effects (EME) includes many electromagnetic environmental disciplines such as Electromagnetic Compatibility (EMC), Electromagnetic Interference (EMI), and Electromagnetic Pulse (EMP). Electromagnetic Interference (EMI) is electromagnetic energy that adversely affects the performance of electrical/electronic equipment by creating undesirable responses or complete operational failure. The interference sources may be external or internal to the electrical or electronic equipment and they may propagate by radiation or conduction. This discipline includes Radio Frequency Interference (RFI), the term which was originally used to describe most electrical interference. EMI is usually divided into two general categories to help in analyzing conducted and radiated interference effects: narrowband and broadband. Narrowband Emissions - a narrowband signal occupies a very small portion of the radio spectrum. The magnitude of narrowband radiated emissions is usually expressed in terms of volts per meter (V/m). Such signals are usually continuous sine waves (CW) and may be continuous or intermittent in occurrence. Communication transmitters such as single-channel AM, FM and SSB fall into this category. Spurious emissions, such as harmonic outputs of narrowband communication transmitters, power-line hum, local oscillators, signal generators, test equipment, and many other man made sources are narrowband emissions. Broadband Emissions - a broadband signal may spread its energy across hundreds of megahertz or more. The magnitude of broadband radiated emissions is usually
expressed in terms of volts per meter per MHz (V/m/MHz). This type of signal is composed of narrow pulses having relatively short rise and fall times. Broadband signals are further divided into random and impulse sources. These may be transient, continuous or intermittent in occurrence. Examples include unintentional emissions from communication and radar transmitters, electric switch contacts, computers, thermostats, motor speed controls, thyratron circuits, ignition systems, voltage regulators, pulse generators, arc/vapor lamps, and intermittent ground connections. They may also result from galactic and solar noise, lightning electromagnetic pulses, and by radio frequency pulses associated with electrostatic discharges. Electromagnetic Compatibility (EMC) is the ability of electrical or electronic equipment/systems to function in the intended operating environment without causing or experiencing performance degradation due to unintentional EMI. It is recommended that the performance be tested or qualified to insure operation within a defined margin of safety for the required design levels of performance. The EMI source minus the coupling mechanism path losses should result in an emission level that is less than the victim's susceptibility threshold minus a predetermined safety margin. The goal of EMC is to minimize the influence of electrical noise. Electronic equipment can malfunction or become totally inoperable if not designed to properly minimize the effects of interference from the internal and external electromagnetic environments. Proper equipment and system designs are also necessary for minimizing potential electromagnetic emissions into the operating environment. It is important that electronic equipment designs ensure proper performance in the expected electromagnetic environment, thus maintaining an acceptable degree of Electromagnetic Compatibility (EMC). EMC Regulations affecting the U.S. are provided primarily by the following three agencies: 1. The Federal Communications Commission (FCC) The FCC regulates the use of all licensed radio and wire communications in the USA. Three sections of the FCC Rules are applicable to non-licensed electronic equipment: Part 15 (RF devices), Part 18 (industrial, scientific, and medical eqpt.), and Part 68. All have sections governing the control of interference. Part 15 generally covers RF Devices capable of emitting by radiation or conduction, any electromagnetic energy in the 10 kHz to 3 GHz frequency range. Measurement techniques may be done in an open area test site. Part 18 refers to devices that use radio waves for industrial, scientific, or medical purposes. 2. The U.S. Department of Defense (DoD) Most of the Military Standards (MIL-STD) relating to EMI/EMC are more stringent than the FCC limits set for commercial electronic equipment. These generally cover the frequency range of 30 Hz to 40 GHz and measurement techniques require a special shielded room. Tests are required for radiated emissions (RE), conducted emissions (CE), radiated susceptibility (RS), and conducted susceptibility (CS). 3. International Special Committee on Radio Interference (CISPR) CISPR, a technical committee of the International Electrotechnical Commission
(IEC), is considered as part of the international harmonization effort for EMI/EMC. CISPR's primary responsibility is at the higher end of the IEC frequency range, starting at 9 kHz and extending upwards. It prepares standards that offer protection of radio reception from interference sources such as electrical appliances of all types; the electrical supply system; industrial, scientific and electromedical RF; broadcast receivers (radio and TV) and information technology (IT) equipment (ITE). Europe uses the CISPR test limits and the FCC has adopted some of its limits since the U.S is a member of CISPR. One or more of the following types of EMI/EMC Tests are applicable to commercial and military electronic equipment as determined by the intended application: Conducted Emissions (CE), Radiated Emissions (RE), Conducted Susceptibility (CS), and Radiated Susceptibility (RS). The Emissions Tests (CE & RE) record any undesirable emissions from the test article. This data is plotted against the applicable specification limits. The Susceptibility Tests (CS & RS) determine the test article's ability to operate in the typical operating environment. The test article is exposed to electromagnetic signals at the levels and frequency ranges required by the applicable specification. This web site offers information which may be used to design, evaluate, and troubleshoot electronic equipment for EMI, EMC. The site resources include Radio Frequency Interference (RFI) topics, circuit design guidelines; typical EMI/EMC test descriptions, EMI/EMC reference documents; document Search Engine access; and EMI/EMC related links for additional information. Radio Frequency Interference (RFI) Have you ever wondered why the interference that you receive on the radio communications bands can sometimes disrupt the intended signals?� Interference probability is based on the potential power transfer densities involved due to the proximity of equipment and antenna systems; the various transfer mechanisms, and equipment performance. The electromagnetic transfer mechanisms may vary depending on modes of operation, propagation conditions, and other variables.� The propagation paths that exist for signal transfer from the transmitters to a receiver within the RF environment of a radio communications band can be numerous.� Antenna-to-antenna coupling parameters may vary depending on antenna gain, directivity, beam width, side lobes, polarization, separation, propagation conditions of the path (path loss), etc.� � The receiver characteristics which influence performance include noise, dynamic range, sensitivity, selectivity (RF, IF), desensitization, adjacent signal susceptibility, intermodulation, cross modulation and spurious response susceptibility.� Once a particular type of interference is determined to be likely, any analysis should be limited to its most predominate effects.� The following types of interference are applicable to Radio Frequency (RF) communications equipment. 1.0 Receiver Co-Channel Interference This is defined as undesired signals with frequency components that fall within the receiver�s RF passband and are translated into the Intermediate Frequency (IF) passband via the mixer stage.� The interfering signal frequency is equal to the sum of the receiver�s tuned frequency and one half of the narrowest IF bandwidth.� These signals are amplified and detected through the same process as the desired signals; therefore, a receiver is very susceptible to these emissions even at lower levels.� Results:� Receiver desensitization, signal masking, distortion.�
2.0 Receiver Adjacent Signal Interference This is defined as undesired signals with frequency components which fall within or near the receiver�s RF passband and are translated outside of the IF passband via the mixer stage.� These signals must be of sufficient amplitude to produce non-linear effects within the receiver�s RF amplifier or mixer stages.� Some of the resulting non-linear response signals may be converted to the IF passband frequency via the mixer stage where they are amplified and detected through the same process as the desired signals.� These become similar to cochannel interference signals at this point. The undesired emissions which are translated outside of the IF passband may still pass through the remaining receiver stages, if at high enough levels to survive the out-of-passband attenuation.� They may then be processed by the detector.� The predominant response for this case is desensitization.� Results:� Non linear effects in the RF or mixer stages producing receiver desensitization, intermodulation and cross modulation.� 3.0 Receiver Out of Band Interference This is defined as undesired signals with frequency components that are significantly removed from the receiver�s RF passband.� High level signals may produce spurious responses in the receiver if mixed with local oscillator (LO) harmonics to produce a signal falling within the IF passband.� The spurious responses result from the mixing of an undesired signal with the receiver�s LO.� The amplitude of these responses is directly proportional to the level of the undesired signals prior to mixing with the LO.� The spurious responses in a receiver usually occur at specific frequencies.� Any other out of band signals are attenuated by the IF selectivity.� Results:� An undesired response created by the mixing of an undesired signal with the LO.� The undesired signals which mix with the LO and are capable of being translated to the IF stages are the spurious response frequencies.� These frequencies and their interference power levels are a function of the receiver�s susceptibility to these responses.� 4.0 Transmitter Fundamental Emissions The transmitter�s fundamental output signal includes characteristics of the power distribution over a range of frequencies around the fundamental frequency.� These are determined by the base-band modulation characteristics and are represented by a modulation envelope function.� The primary parameter associated with the modulation envelope is the transmitter�s nominal bandwidth (3dB).� This may be derived from the transmitter modulation characteristics (by Fourier analysis), measured, or from the manufacturer�s specifications.� The power distribution in the modulation sidebands may be represented by a modulation envelope function showing the variation of power with frequency.� 5.0 Transmitter Harmonic Emissions The main concern with a transmitter�s harmonic emissions is the undesired signal outputs which are harmonically related to the fundamental signal rather than to other oscillator circuits.� The relative power associated with the harmonic emissions may be modeled using data for the particular transmitter type.� However, since harmonic output power can vary considerably from one transmitter to another for the same type and model, it should be represented statistically.� Harmonic emission models may be derived from statistical summaries of measured data or from manufacturer�s equipment specifications.� Transmitter spurious emission models for prediction of frequencies above the fundamental are based on harmonic emission levels.� The modulation envelope must be represented for harmonics as was done for the fundamental.�
6.0 Transmitter Noise Transmitter noise includes the output spectrum that is a result of the thermal noise generated in the driver and final amplifier stages as well as the synthesizer noise from lower level stages.� This is a broad-band noise; however, it usually does not cover the immediate modulation sidebands.� The level may be specified as the power per bandwidth as a function of frequency (dBm/Hz).� 7.0 Transmitter Intermodulation These are the undesired signals that result from the local mixing of a transmitter�s output emission with that of another transmitter.� The mixing usually occurs in the non-linear circuits of a transmitter whose antenna receives a high level of RF from another transmitter antenna in close proximity.� The mixing products are radiated by the transmitter�s antenna as possible cochannel or adjacent signal interference signals. 1. What is EMI?
A) Electromagnetic Interference (EMI) is defined as electromagnetic energy
from sources external or internal to electrical or electronic equipment that adversely affects equipment by creating undesirable responses (degraded performance or malfunctions). EMI can be divided into two classes: continuous wave (CW) and transient. 2. What is EMC? A) Electromagnetic
Compatibility (EMC) is defined as an electrical system's ability to perform its specified functions in the presence of electrical noise generated either internally or externally by other systems. The goal of EMC is to minimize the influence of electrical noise. 3. Why is EMI/EMC testing required for commercial and military equipment? A) EMI/EMC testing is designed to ensure that electrical/electronic equipment
will perform properly in its expected electromagnetic environment, thus maintaining an acceptable degree of Electromagnetic Compatibility (EMC). 4. What are some basic test categories and their associated requirements? A) The following EMI Tests are performed as determined by the equipment's
intended application: Conducted Emissions (CE), Radiated Emissions (RE), Conducted Susceptibility (CS), and Radiated Susceptibility (RS). The Emissions Tests (CE & RE) record any undesirable emissions from the test article. This data is plotted against the applicable specification limits. The Susceptibility Tests (CS & RS) determine the test article's ability to operate in the typical operating environment. The test article is exposed to electromagnetic signals at the levels and frequency ranges required by the applicable commercial or military specification.
5. Where are EMI/EMC tests performed? A) There are many EMI/EMC testing labs
throughout the U.S. and many other countries. Shielded room test chambers and open area test sites (OATS) may be used depending on the test requirements document. 6. What is an EMI Receiver? A) An EMI Receiver is a tunable,
sensitive voltmeter used to measure electric and magnetic field strengths. Most are similar to specialized spectrum analyzers, but are characterized by having preselectors, several detector functions, a housing shielding effectiveness of at least 90 dB and other unique additions. The frequency coverage of high-end receivers is typically 30 Hz to 22 GHz. Measurement bandwidths are variable to cover broadband and narrowband measurements. 7. What is RFI? A) Radio Frequency
Interference (RFI) is considered as part of the EMI spectrum, with interference signals being within the radio frequency (RF) range. This term was once used interchangeably with EMI. 8. What equipment should be furnished as part of an EMI/EMC test? A) A typical EMI/EMC test should include the test article and all
associated cables, interfaces, power supplies, software, and other support equipment as needed to simulate the actual operational configuration. 9. What is an emitter?
A) In EMI applications, this term applies to unintentional radiators, particularly
those that are the source of interference. Otherwise, the term refers to the intentional radiators such as transmitter antennas. 10. What is a receptor? A) A device that receives
conducted or radiated electromagnetic emissions. In EMI applications, a receptor has the potential for being susceptible to undesired interference. It is considered to be a victim if it is susceptible to EMI from the emissions received. 11. What is crosstalk? A) Crosstalk results from
the coupling of conducted emissions between two pairs of wires, one pair carrying emissions from a source and the other pair connected to a susceptible device. In a printed circuit board (PCB), crosstalk involves interaction between signals on two different electrical nets. The one creating crosstalk is called an aggressor, and the one receiving it is called a victim. Often, a net is both an aggressor and a victim.
12. What is susceptibility? A) The inability of equipment/systems
to perform without degradation in the presence of an electromagnetic disturbance. Susceptibility is often characterized as a lack of immunity. The threshold of susceptibility is the level of interference at which the test article begins to show a degradation in performance. This is often frequency-dependent. 13. Why should EMI/EMC testing be done at a certified Test Lab? A) In industry, it has been common for EMI/EMC tests to be conducted
in Open
Area Test Sites (OATS) and other non-shielded manufacturer facilities. However, due to the increased use of electronic office equipment and wide-area communication systems such as cellular phones, pagers, and high powered radio signal sources, it has become more difficult to conduct accurate emissions tests using OATS. The increased ambient emissions from the external sources preclude accurate results in many of these test environments. Indoor shielded chambers, such as a certified Test Facility or Lab, provide a good environment for EMI/EMC testing because they attenuate ambient emissions from the surrounding area. 14. What is ambient level? A) The nominal level of radiated
and conducted electromagnetic signal and noise existing at a specified location. This is usually considered to be a function of the entire electromagnetic environment including atmospheric noise and interference generated from within the measuring set-up.ips
Tips for Electronic Printed Circuit Board Design Introduction This information is presented as guidelines to the preliminary design and development stages of electronic circuits for the purpose of preventing potential electromagnetic interference (EMI) and electromagnetic compatibility (EMC) problems.� The tips are representative of good printed circuit board (PCB) design practices and are recommended as a checklist for evaluating and selecting EMI/EMC software modeling tools.� The EMI simulation of circuit boards requires the evaluation of many details such as clock frequencies, switching rates, rise/fall times, signal harmonics, data transfer rates, impedances, trace loading and consideration of the types and values of the various circuit components.� The physical layout of the PC board and its
associated metallic components are important considerations.� Special attention should be given to the placement and characteristics of signal source components, vias, traces, pads, board stack-up, shielded enclosures, connectors and cables.� For example, as signal frequencies and clock/switching rates increase, PC board trace characteristics can become similar to those of transmission lines and radiators.�� A PC board trace or component can become an efficient antenna at a length as small as one twentieth of a wavelength.� EMI/EMC problems may be approached at the component, PC board or enclosure levels.� However, it is much more efficient to deal with these problems as close to the source or susceptible victim as possible.� Therefore, it is important to consider these tips as guidelines for PCB design and layout so that problems may be identified and prevented prior to actual fabrication of the equipment. General (1) EMI controls should be applied at the circuit and box levels prior to addressing EMI at the interconnected and system levels. (2) Digital circuits are more likely to be the source of emissions due to the handling of periodic waveforms and the fast clock/switching rates.� Analog circuits are more likely to be the susceptible victims due to higher gain functions. (3) The source or susceptible victim of most EMI problems is typically an electronic component.� Although active components are usually the sources of EMI, passive components often contribute to it, depending on the signal frequencies and component's characteristics. � For example, an inductor can become predominantly capacitive due to the high frequency parasitic coupling between windings.� A capacitor can develop parasitic series inductance due to its internal inductance and external lead inductance at high fundamental and harmonic frequencies.� (4) EMI problems involving an active component can be the result of the device's output transferring the emissions or its input providing the path for susceptibility.�� However, at high frequencies the active component may become a direct radiator or receptor of EMI.� Also, the component�s power and ground connections can provide paths for both emissions and susceptibility. (5) Although common mode currents are usually small compared to differential mode currents, they can be the main cause of radiated emissions. (6) Emissions and susceptibility that are typical in single layer, free wired (using power and ground traces instead of planes) PC Board design, can be greatly improved by using multi-layer PC boards with power planes.� High capacitance between a forward signal and its return path (ground plane) provides containment of the electric field.� Low inductance of the paths provides for
magnetic flux cancellation.� Although not always realistic in a PCB stack-up design, a trace should be spaced one dielectric layer away from its associated return path and the voltage and ground planes should be as closely spaced as possible. (7) PCB stack-up design is important in containing the electromagnetic fields, while providing for additional bypassing and decoupling of the power bus and minimizing bus voltage transients.� Some of the benefits of multi-layer PC board design with power planes are:� a. The power planes, if properly designed, will provide an image plane effect.� Since the return currents in the power planes are equal and opposite polarity to the associated signal currents, their electromagnetic fields will tend to cancel.� Power planes can also reduce the loop areas of signal and power traces, resulting in a decrease of EMI emissions and susceptibility.� b. A ground plane can lower the overall ground impedance, thus reducing high frequency ground bounce.� Also, the impedance between the ground and voltage planes is lowered at the high frequencies and this reduces power bus ringing.� (8) Clocked IC�s with rapid output transitions can be very demanding on voltage and current distribution components such as the power supply, power bus, and power planes.� The inductance of the power bus can prevent the rapid energy transfer needed to meet the quick output transitions and fast rise times.� This can be improved with the placement of decoupling capacitors at the IC�s power pins.� The capacitors must be properly selected in their frequency response to deliver the energy needed at the IC�s output frequency spectrum.� However, as the number of decoupling paths increase, so do the number of voltage drops across them and this can result in power bus transients along with the associated common mode emissions.�� This problem can be minimized with proper power plane design in the area of the IC�s.� The power plane acts as an effective high frequency capacitor, and consequently, as an additional energy source needed for cleaner IC outputs.��� PCB Layout (1) Use multi-layer PC boards rather than single-layer boards whenever possible. (2) If a single layer board must be used, a ground plane should be utilized to help reduce radiation. (3) Top and bottom ground planes can help reduce radiation from multi-layer boards by at least 10 dB.
(4) Segmented PC board ground planes are useful for reducing cable radiation due to common mode currents. (5) Power and return planes should be located on opposite sides of a multi-layer PCB.� Effective power planes are low in inductance.� Therefore, any transients that may develop on the power planes will be at lower levels, resulting in lower common mode EMI. (6) Connection of the power planes to high frequency IC power pins should be as close to the IC pins as possible.� Faster rise times may require connections directly to the pads of the IC power pins. (7) Analog and digital circuits are susceptible to interaction when located in close proximity to each other.� These should be located on different layers of the PC board whenever possible.� If the circuits must be located on the same layer, they should be separated into analog and digital areas with proper isolation layout. (8) High frequency traces, such as those used for clock and oscillator circuits, should be contained by two ground planes.� This provides for maximum isolation.� The reactance of a trace or conductor can easily exceed its dc resistance as frequency increases.� If this trace is run close to its ground plane, the inductance can be reduced by about one third.�� (9) Additional EMI preventive measures for clock/oscillator traces include the utilization of guard traces grounded to the ground plane at several locations. The shielding of clock and oscillator components with foil or small metallic enclosures may also be needed. (10) Overall circuit cross-talk increases by a factor of two whenever the clock rate is doubled.�� EMI radiation and cross-talk may be reduced by minimizing the PC board trace height above the ground plane.� (11) PC board edge radiation may be the result of traces being located too close to the board edge.� This can be minimized by keeping traces at a distance of at least 3 times the board thickness away from the board edge.� (12) PC board trace stacking should be avoided if possible.� Otherwise, it should be limited to one trace height in order to reduce radiation, cross-talk and impedance mismatches. (13) Parallel traces are often susceptible to cross-talk.� These should be separated by at least 2 trace widths for cross-talk reduction. Decoupling, Bypassing and Filtering
(1) EMI filters can be used as a shunt element to divert electrical currents from a trace or conductor; as a series element to block a trace or conductor current; or they may be used as a combination of these functions.� Selection of the filter elements should always be based on the desired frequency range and component characteristics.� A low pass filter can be useful for reducing most high frequency EMI problems.� It incorporates a capacitive shunt and series resistance or inductance.� However, at frequency extremes, the capacitor can become inductive and the inductor can become capacitive causing the filter to act more like a band-stop filter.�� The filter design type should be based on the overall impedance at the circuit�s point of application for proper match.� A Tfilter design is effective for most EMI applications and is ideal for analog and digital I/O ports.��� (2) Capacitors may be used for signal filtering and power source decoupling within their high frequency performance characteristics.� However, their internal and external inductance can limit performance at high frequencies.� Ceramic capacitors are recommended for the high frequencies, particularly those in the GHz range.� A capacitor providing a reactance of less than 1 Ohm at the frequency of concern should suffice.�� Capacitor lead and trace lengths must be short at the high frequencies in order to prevent the addition of inductive reactance. (3) PC board bypass capacitors used at high frequencies (greater than 100 MHz) should utilize surface mount technology (SMT) with vias close enough to the mounting pad to minimize or eliminate the traces.� The via holes should be large (greater than .035 inch in diameter) and the PC board should be thin enough to bring power and ground planes near the body of the capacitor (less than .030 inch thick).�� Proper design layout of the bypass capacitors can greatly reduce the power and ground circuit noise by lowering the overall effective inductance of the capacitors. (4) Wire wound ferrite inductors may be used for EMI emissions and immunity filtering at lower RF frequencies.� These can supply from about 1 microhenry to 1 millihenry of inductance.�� However, they can become a capacitor above their resonant frequency and are useless in the most common EMI frequency range of 50 MHz to 500 MHz.� Ferrites and ferrite beads are recommended for higher frequency applications where they become lossy and act more like a resistor.� Select a ferrite impedance of about 100 to 600 Ohms at the frequencies of concern. (5) Shielded I/O cable connectors equipped with bypass capacitors or filter pins should be used whenever possible. (6) I/O filters should be inside of the I/O connector (as with filter pins) instead of on the PC board.
(7) I/O bypass capacitors should be mounted at the I/O connector instead of on the PC board. (8) I/O ferrites should be mounted inside of the I/O connector instead of on the PC board. (9) A snap-on ferrite bead at the I/O cable connector can provide 3 to 5 dB of common mode absorption. (10) Multiple ferrites may be used to reduce radiation by up to 10 dB depending on their characteristics at the frequency of interest. (11) Ferrite beads are available in high-Q resonant and low-Q non-resonant (absorptive) types.� The low-Q beads are recommended for digital circuits and filtering applications.��� (12) External cable or I/O connector filters can provide for a common mode rejection of greater than 10 dB. Cables and Connectors (1) Cables should be grouped according to their function such as power, analog, digital, and RF. (2) Separate connector assemblies should be used for analog and digital signals. (3) Analog and digital connectors should be located as far apart as possible. (4) Analog and digital signal pins should be separated by unused grounded pins when sharing the same I/O connector. (5) Individual pins should be used inside the I/O connector for each signal return so that all return circuits remain separated. (6) Connector crosstalk may be reduced by using separate power and ground pins for each signal and by reducing the circuit�s loading and current flow.��� (7) Cable shields should be grounded to equipment housing at the I/O points. (8) Shielded I/O cables are most effective if grounded at both ends. (9) Cable common mode currents should be removed at the equipment�s metal housing prior to internal connections.
(10) Cables should be routed close to ground planes, shielded structures, and cable trays. Grounding (1) Use ground planes instead of vectorial traces. (2) Ground traces should be as short and thick as possible. (3) Decouple signal and RF circuit grounds. Common Printed Circuit Board Design Problems Introduction It is important that software analysis tools address the following major problem areas of PCB design. The fast clock speeds and rapid edge rates needed in many PCB designs require proper management of board level design, layout and signal interconnects in order to minimize EMI. High speed switching has the capability of producing electromagnetic waves that generate resonance, power/ground bounce, simultaneous switching noise, reflections, and coupling between traces and power/ground planes. The effects of improper ground plane design can hinder the performance of high-speed clocks and synchronous busses. Most major sources of emissions appear due to signal related parameters such as the clock or pulse repetition frequency, signal edge rate, and signal ringing due to impedance mismatches. The greater the performance characteristics of semiconductor components in a circuit design, the more electromagnetic interference that may occur. Therefore, the design of a PCB must eliminate any antennas that are capable of radiating electromagnetic energy. Loops of signal and the corresponding ground return lines carrying high frequency signals must be minimized. Typical PCB Design Problems The synchronous activity of a large number of clocked devices causes the current switching events to occur simultaneously. The PCB traces connected to integrated circuit (IC) input/output (I/O) pins can form effective antennas that radiate noise, coupling it to the external cables. The synchronous nature of a circuit design can cause glitches and emissions from the power supplies that provide voltage to the electronics and clocked circuits. Oscillator circuits can also become a source of EMI when the oscillator swings rail to rail, producing harmonics due to the squaring of the output sine wave. Single layer boards are much more likely to cause EMI problems than multi-layer boards. Two layer and four layer boards generally have a minimal of radiation in comparison, if properly designed. A common problem at the system level is the
radiation due to cables interconnecting the PCB with external support equipment or peripherals such as displays, external processors, and keyboards. In many cases, there may be only one ground wire between the PCB and external peripherals. The inductive ground conductor provides the return for all of the RF energy carried to the external equipment through the multiple wiring. Any impedance in the ground wire will prevent some of the RF energy from returning to the PCB through the normal ground path. This radiated RF energy may be coupled back through undesirable paths including some of the external equipment. The emissions may cause EMI in other parts of the system, as well as to EMI/EMC testing facilities. Differential mode noise is potentially generated by every signal in electronic and electrical equipment. It travels down its trace to the receiving device and then back along the return path, resulting in a differential voltage between the two conductors. A circuit design should be based on minimizing the magnitude of this current, its frequency content, and the signal rise and fall times. PCB loops formed by the signal and its return can be reduced through proper design techniques utilizing more returns and by using signal/return twisted pair wiring. Common mode noise or common impedance noise results from an electrical current or signal traveling down the signal and return lines simultaneously. For this type of noise, there is no differential voltage between the signal and its return. In this case, the impedance that is common to the signal and the return paths causes the source noise voltage. Common mode noise is often present in cables due to the PCB signal connections and returns forming a common impedance. This type of noise may be reduced through the use of proper PCB design techniques by reducing the common node impedance or by placing a ferrite bead around the cable. Crosstalk is another type of interference caused by noise or undesired signals being coupled from one source to another. Crosstalk can propagate into connecting cables and greatly increase the chances of EMI. The PCB design techniques for addressing susceptibility are somewhat similar to those for emissions. Loop areas of greater size will tend to receive larger signal levels. They are also capable of radiating EMI at higher levels. Ground bounce, or common impedance coupling of circuits, can cause circuit operational failure and the associated EMI. For example, if a ground path has high impedance, it may cause a driving circuit�s reference voltage to shift, resulting in its input to the microcomputer to be outside of the device�s required switching range for normal operation.