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EMI-EMC Overview Of EMI-EMC
Design For EMI-EMC
EMI-EMC For Testing & Measurement
EMI-EMC Standards
EMI-EMC for Layout
Agenda
• What is EMC ? • Basic EMI-EMC Tests• EMC Spectrum• Elements of EMC Situation• Statement of problem • Electronic design in a systems approach• EMI-EMC and Signal Integrity • Board Level Grounding • Ground for Grounding • Board level EMC and mixed signal design
Electro Magnetic Compatibility
E M I + E M S = E M C
From a designer’s point of view, EMC phenomena have to be considered in two different ways:
• How the environment may affect the design(susceptibility)EMS.• How the design may affect the environment(interference)EMI.
Traditionally, the only government regulations have been on the emission side: An electronic device is not allowed to emit more than a certain amount of radio frequency energy to avoid disturbing radio communication or operation of other electronic equipment.
Most countries in the world have regulations on this topic
What is EMC ?
EMI-EMC TESTS • Radiated Emission
• Conducted Emission
• Radiated Susceptibility
• Conducted Susceptibility
• Electro Static Discharge
• Electrical Fast Transients
• Surge Testing
• Power Frequency Magnetic Field Testing
• Line Voltage Fluctuations
• Harmonics and Flicker
The radiated coupling path will be more efficient in the higher frequencies while a conducted coupling path will be more efficient in the lower frequencies.
The EM Spectrum
Audio Microwave Infrared
Visible
UltraViolet
X Ray GammaRay
Frequency - 300kHzWavelength - 1km
300MHz1m
300GHz1mm 0.7-0.4um 0.03um 0.3nm
Domain ofEMI/EMC10kHz - 40GHz
Radiowave
Intra-System and Inter-System EMC
Intra-System EMCMotherboard
Plug-in card
Inter-Systems EMC
EMC Phenomena That affect the electro-magnetic environment can be divided into two characteristic groups
1. Transient Interference Sources2. Continuous Interference Sources
Electromagnetic Compatibility & Product Design Approach
EMC for Test & Measurement
EMC for Systems & Installation
EMC for Board design
Circuit Design Signal Integrity
Board design(Layout)
DFM D-EMC
DFT
Elements of an EMI Situation– Source "Culprit"– Coupling method or Medium "Path"– Sensitive device "Victim"
SOURCE
VICTIM
Statement of the problem and Design Requirements
Tools that help
•Question the customer
•Differentiate Needs and Wants
True needs
Needs as reflected to problem statement
Analog design octagon
Electronics design in a systems approach1. Standards (DO254, FAA, MIL-217, ASTM, CE, FCC, TUV, UL,IEC,CISPR)2. Protocol (CAN, RS-232, ETHERNET, SPI, TTP)3. Topology (Point TO POINT, multi-drop, star, mesh, bus)4. Physical layer (UTP, TP, STP)5. Noise margin (cm, dm, THD, lvc, ttl, cmos, Differentials, Single Ended)6. Voltage levels (rs-232,rs485,lvds,ttl,cmos,can,se, Diff)7. Bandwidth (Amplifiers, DAC, ADC, ETHERNET)8. Data rate (RS232,CAN,RS485, ttp)9. Distance (Trace length, Trace width-1mil,2mils,50mils,100Mils, cable length)
10. Analog transmission of analog signals (example ?)11. Analog transmission of Digital signals (example ?)12. Digital transmission of analog signals (example ?)13. Digital transmission of digital signals (example ?)
14. Signal integrity (Layer stackup, trace width, spacing, termination….)15. Power integrity (PS Layout, PSTopology, SWINCHING,BODE,LOOP ANALYSIS)16. Data integrity (15V& 3.3VCMOS, PARALLEL,SERIAL,SPEED,CPU,CLOCK ,PWB)17. DATAconversion (ADC,DAC,COMPARATOR)18. Analog signal conditioning (Amplifiers, ADC, DAC, SENSORs, transducers)19. Digital signal processor or Controller (Fixed point, floating point…)20. Grounding (Protection, Shielding, reference, mother earth, zero, AGND, DGND, Chassis, Field, Aruguments, rules, ground
bounce, Equippotential, bonding,EMC grounding, plane, neutral, isolated, non isolated, Pulldown, safety, symbols, Return path, differential Gnd, common mode Gnd, pspice ground)
Mixed signal design in a systems approach
Low voltage interfacesGrounding in Mixed signal SystemsDigital and Power Isolating Techniques Power supply noise reduction & filteringDealing with Logic design and its noise
Single Ended Transmission-Switching Levels
Differential Transmission-Switching Levels
Differential Mode & Single-ended Mode
DM: Interference signal in two lines are oppositely directed and thus no ground current path is required
CM: Interference signal in two lines are unidirectional and return through ground
Differential TransmissionBelow figure shows the electrical schematic diagram of a differential transmission circuit in which noise sources VN and VG add to each signal line and are common to both signals. The differential receiver measures the difference between the two lines and rejects the common voltage of the signals. If used with closely coupled lines, the complementary signals cancel each other’s electromagnetic fields resulting in high immunity and low noise emissions. This immunity to external noise influence and the low radiated emissions make differential signaling a good choice when relatively high signaling rates and long distance are required in electrically noisy, or noise-sensitive applications. Differential signaling comes with the additional cost of the line driver, receiver, and interconnection over the cost of single-ended transmission.
Since ground noise is also common to both signals, the receiver rejects this noise as well. The twisted pair cable used in these interfaces in combination with a correct line termination—to avoid line reflections—allows very high data rates of more than 10 Mbps and a cable length of up to 1200 m. Most recent standards allow up to 2.5 Gbps.
Advantages and DisadvantagesAdvantages of Single-Ended Transmission
The advantages of single-ended transmission are simplicity and low cost of implementation. A single-ended system requires only one line per signal. It is therefore ideal for cabling, and connector costs are more important than the data transfer rate, e.g. PC, parallel printer port or serial communication with many handshaking lines, e.g. EIA-232. Cabling costs can be kept to a minimum with short distance communication, depending on data throughput, requiring no more than a low cost ribbon cable. For longer distances and/or noisy environments, shielding and additional ground lines are essential. Twisted pair cables are recommended for line lengths of more than 1 meter.
Disadvantages of Single-Ended Transmission
The main disadvantage of the single-ended solution is its poor noise immunity. Because the ground wire forms part of the system, transient voltages or shifts in voltage potential may be induced (from nearby high frequency logic or high current power circuits), leading to signal degradation. This may lead to false receiver triggering. For example, a shift in the ground potential at the receiver end of the system can lead to an apparent change in the signal, sufficient to drive the input across the thresholds of the receiver, thus increasing its susceptibility to electromagnetic fields.Crosstalk is also a major concern especially at high frequencies. Crosstalk is generated from both capacitive and inductive coupling between signal lines. Capacitive coupling tends to be more severe at higher signal frequencies as capacitive reactance decreases. The impedance and termination of the coupled line determines whether the electric or the magnetic coupling is dominant. If the impedance of the line is high, the capacitive pickup is large. Alternatively, if the line impedance is low, the series impedance as seen by the induced voltage is low, allowing large induced currents to flow. Single-ended transmission is much more susceptible to external noise and the radiation of EMI is increased compared to differential systems. These problems will normally limit the distance and speed of reliable operation for a single-ended link.
Advantages & Disadvantages of Differential Transmission
Advantages of Differential Transmission
Differential data transmission schemes are less susceptible to common-mode noise than single-ended schemes. Because this kind of transmission uses two wires with opposite current and voltage swings compared to only one wire for single-ended, any external noise is coupled onto the two wires as a common mode voltage and is rejected by the receivers. This two-wire approach with opposite current and voltage swings also radiates less electro-magnetic interference (EMI) noise than single-ended signals due to the canceling of magnetic fields.
Disadvantages of Differential Transmission
The Differential data transmission is expensive and the high data-rates that are possible with differential transmission require a very well-defined line impedance and correct line termination to avoid line reflections. For this method of transmission twisted pair cables instead of less expensive multi-conductor cables are recommended.
Block Level Representation of EMI-EMC
ESD, Transient & SURGE ! & EMC gapDon’t get confused by the similarities between 4 kV ESD testing, 4 kV fast transient burst testing and 4 kV surge. The voltages are the same, but the energy behind them is totally different. Dropping a small rock on your foot may hurt, but you will still be able to walk. Dropping a large rock from the same height will most likely cause severe damage to your foot. Doing this 250 times per second will reduce your shoe size permanently. When the surge boulder falls, you'd rather be somewhere else.
Surge immunity test is the mother of all transient test, It tries to emulate what happens when lightning hits (near) the power network, and the energies involved are high
Electrical & Physical Parameters and EMC bridge
Board level ground
Board level ground cont…
Board level ground cont… Single point grounds, with regards to noise, are very undesirable because of the series connection of all
the individual circuit grounds. At high-frequencies the inductances of the ground conductors increase the ground impedance. A single-point ground is preferable below 1 MHz. Between 1 and 10 MHz a single point-ground can usually be used, provided the length of the longest ground conductor is less than one twentieth of a wave-length to prevent emissions and to maintain a low impedance.
Multi-point grounds, have very low ground impedance and should be used at high frequencies and in digital circuitry. The low impedance is due primarily to the lower inductance of the ground plane. The connection between each circuit and the ground plane should be kept as short as possible to minimize their impedance. Multipoint grounds should be avoided at low frequencies since ground currents from all circuits flow through a common ground impedance . the ground plane.
A hybrid ground, is one in which the system grounding configuration appears differently at different frequencies . a single-point ground at low frequencies, and a multi-point ground at high frequencies. When different types of circuits (low-level analog, digital, noisy, etc.) are used in the same system or on the same PCB, then each must be grounded in a manner appropriate for that type of circuit. The different ground circuits should be tied together, usually at a single point.
Ground for Grounding The term “signal grounding” encompasses two primary and complementary objectives, namely signal voltage reference and signal current return path. In systems and facilities, a ground structure is typically intended to function as a common signal reference structure and to provide a near perfect voltage reference for the signal.
The term “reference” implies voltage consideration rather than current (open-circuit voltage can exist, but no current can flow through an open circuit); designers thinking in terms of voltage may be tempted to ignore the need for adequate currentreturn paths.
For high-frequency signals, “ground” is a concept that does not exist in reality. Signal ground can better be defined as a low-impedance path for the signal’s current to return to the source.
The concept of equipotential reference defines an ideal objective of the grounding system, whereas the concept of current return path characterizes what the ground actually is.
Ground for Grounding Cont…Safety Grounding is intended for preclusion of hazards due to power faults or lightning strikes, which could set a facility ablaze and constitute a safety hazard to equipment and personnel.
EMI Grounding is intended for controlling common mode EMI current drainage from cable shields and suppression devices as well to serve as an “image plane” for conductors routed adjacent to them.
Signal Grounding essentially constitutes a “functional” or “technical” ground, intended to provide an equipotential signal voltage reference between components of the system and serve as a path for signal current return, particularly in unbalanced or single-ended interfaces. With the exception of electrical safety considerations and certain issues related to electrostatic shielding, connections of electronic circuits to the ground play no other role than to provide the signal and EMI current returns (whether desired or unintentional)
Ground for Grounding
Real-world ground structures are non-ideal in nature. Some potential difference always exists; thus, all reference conductors should be assumed to carry current, whether intended or unintended. The extent to which potentials in the ground system can be minimized and ground currents reduced will determine the effectiveness of the ground
Grounding
Radiated & Conducted Energy
Impedance
• Resistance, Capacitive reactance and inductive reactance all act to oppose the flow of current
• Resistance R opposes current flow because of resistivity of the conductor from which it is made
• Capacitive reactance XC opposes current flow because of charge present on the plates of the capacitor
• Inductive reactance XL opposes current flow because of electromagnetic field within the inductor
• The term that covers all these kinds of opposition to current flow is IMPEDANCE
EMI Sources and Paths in a PWB
Transmission Lines
Differential Mode Radiation & Controlling
Common mode radiation and ground
Digital Logic Current Flow
Positive Pulse Waveform
Fourier and Frequency Domain of Digital Signals
In Figure above, the bandwidth contains 99% of the spectral energy of the signal. The spectrum of the square wave in Figure is also its Fourier series. Fourier theory states that a periodic signal can be expressed in terms of weighted sum of harmonically related sinusoids.
A square wave has an AC component during the transition times and a DC component during the steady state.
The AC current contains all of the frequency components of the square wave. In addition to the fundamental frequency, a digital signal also contains harmonic frequencies which are integer multiples of the fundamental frequency. For example, a digital signal with a fundamental frequency of 10 MHz has harmonic frequency components at 20, 30, 40, . MHz
Measuring Common Mode Currents
Equation gives the electric field in dBµV per meter for a short wire (relative to wavelength) in free space due to the spectral amplitude of current In. Use this equation to estimate the electric field emissions due to CM current.
Measuring common mode currents cont..
Table below shows the maximum CM current that can flow on a single wire to just meet the limit for radiated emissions. To find and measure the maximum CM current move the current probe along the harness length while monitoring the current with a spectrum analyzer.
Solving Equation above for the current gives Equation below:
Radiated Emissions Near field and Far field
Above equation predicts the maximum electric field in the far field from a small loop. It is accurate when the loop perimeter is less than one-quarter wavelength, and approximate for larger loops. In the near field multiply Equation above by Equation mentioned below
Radiated emissions design exampleTable below shows the radiated emissions at 1 meter from a PCB circuit with the following values:Area = 5.0x10-4 meter2 (5 cmx1 cm)Fundamental frequency = 10 MHzImax = 10 mARise time = fall time = 5 ns (typical high-speed CMOS)
PWB Design Flow
EMI Control techniques at sourceImportant techniques to control EMI at source are Proper Grounding: single point, multi point or hybrid grounding depending upon the frequency of operation
Shielding: Metal barrier is used to suppress coupling ofRadiated EM energy into the equipment.
EMI Filtering: used to suppress conducted interference on Power, signal and control lines.
PCB Layout: Proper PCB design from the early design stage is required
Mixed signal design example
EMI-EMC FOR LAYOUT CONT….PART 2
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