----Module 6

EMC

 
The Definition of ElectroMagnetic Compatibility:
"You can watch "Oprah" with your Television Set sitting on top of a working PC" 
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As practiced by many: designing for, and understanding EMC has been, and remains an ART more than a Science. However, it needn't be that way; using what you have learned in the proceeding pages, this page will attempt to remove some of the mystery from this subject--but NOT all!

The greatest insights remain to be discovered in Practice.
 

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WHAT IS EMC?
ElectroMagnetic Compatibility: With the proliferation of electronic systems in every aspect of our daily lives, there inevitably comes the problem of compatibility. Listening to the news on AM radio while using an electric razor should not be a problem, as it was in days gone by.
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WHY DO WE NEED EMC?
If EMC design practices are adhered to by both the razor and the radio manufacturers, then listening to the news on an AM radio, while using an electric razor, presents no problem.

Also, worldwide governmental regulations prohibit electronic products from emitting or being susceptible to, Electro-Magnetic Interference.

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HOW DO WE IMPLEMENT EMC?
Emission, Susceptibility, and Path are the three constituents of EMC, with Emission being the one causing the most incompatibilities, while yielding the greatest number of solutions. Susceptibility on the other hand, is more subtle in its effect and its solutions. Finally, the Path can be the arbiter of both.
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The Dichotomy of EMC:

What is Good Practice for the Circuit isn't necessarily Best for EMC.
 

What is Good Practice for EMC isn't necessarily Best for the Circuit.
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DISCUSSION:
EMC can be approached in two ways: before the fact— designing for EMC; or after the fact—patch work/clean up, Band Aids. Of course, designing in EMC safeguards ahead of time is always best; however, sometimes even the "best" designs aren't enough, and require some Band-Aids. Thus the "ART" aspect of EMC.
With the proliferation of electronic systems in every aspect of our daily lives, there inevitably comes the problem of compatibility. Listening to the news on AM radio while using an electric razor shouldn't be a problem, thanks to EMC design practices on the part of the razor manufacturer. 

The three constituents of EMC are [unwarranted] Emissions, [inappropriate] Susceptibility, and the [unintended] Path between them. The electric razor's motor brushes arcing is a case of unwarranted Emissions; and the AM radio's picking up the noise through the Path(s) (power line, and/or through the air),  is the unnecessary Susceptibility.

TOP

Block Diagram Depicting the EMC Paradigm

Path consists of Radiated and Conducted energy.
     1. Radiated (electromagnetic field) 
     2. Inductively coupled (magnetic field) 
     3. Capacitively coupled (electric field) 
     4. Conducted (electric current) 
Inductively coupled (magnetic field) 
Inductively coupled, (magnetic field) 
   
Capacitively coupled (electric field) 
Capacitively coupled W/Shield (electric field) 
   
Microstrip Transmission Line Characteristic Impedance affected by Groundplane proximity: Z = High Microstrip Transmission Line Characteristic Impedance affected by Groundplane proximity: Z = Low
Near Field, Far Field Radiation
In dealing with a Radiation Source --be it an Antenna or a Circuit Board:
-
The dominant energy in the Far Field is Electric.
--
Plane Wave radiation is how radio works
377 ohms is the impedance of free space
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The dominant energy in the Near Field  is Magnetic.
-
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In any high frequency signal the dominant radiation component under ~1/6 wavelength from the source, is Electromagnetic; this is referred to as Near Field radiation.

Any signal beyond ~1/6 wavelength, is Electrostatic, and is referred to as Far Field radiation; sometimes called the Plane Wave. Plane Wave radiation is how radio works and that the 377 ohms is the impedance of free space.
 

Who are the Culprits we are trying to Control?

EMI
Electro-Magnetic Interference
An electrical disturbance in a system due to natural phenomena, low-frequency waves from electromechanical devices or high-frequency waves (RFI) from chips and other electronic devices. Allowable limits are governed by the FCC.
RFI
Radio Frequency Interference
High-frequency electromagnetic waves that emanate from electronic devices such as chips and other electronic devices. Allowable limits are governed by the FCC.
TVI
Television Interference
High-frequency electromagnetic waves that emanate from electronic devices causing Interference to Television Reception.
Radiated
Radiated EMI is most often measured in the frequency range from 30 MHz to 10 GHz (according to the FCC).
Emission Sources:
Clocks, clock lines, data lines; switching power supplies, 
Susceptibility:
Clock lines & data lines poorly laid out, improperly terminated;
Solutions:
Balanced transmission lines, proper terminations, groundplanes, shielding, limited rise & fall time drivers
Solutions:
Shielding, layout, filtering, groundplanes, differential line receivers, 
Conducted
Conducted EMI is most often measured in the frequency range of several kHz to 30 MHz (according to the FCC).
Emission Sources:
Power supplies (switching), power rails, motors, relays, 
Susceptibility:
A.C. power cord poorly filtered, power rails poorly decoupled, 
Solutions:
Good bypassing & decoupling practices, layout, groundplanes, shielding, 
Solutions:
Good bypassing & decoupling practices, layout, groundplanes, shielding, power line filtering
 
Solutions to EMI
Spread Spectrum, clocks reduce measured interference. Spread spectrum is where frequency hopping and bandwidth spreading reduce measured interference. LINK--V--

Line Drivers with controllable Rise and Fall Times.  LINK--V--
 

Regulatory:  Residential, Commercial, Industrial, and Military
 
--Entity Standards
USA / FCC Part 15, subpart J 
Canada CSA
Japan VCCI
European EU  (European Union) 89/336/EEC  EN specifications:
Electronic Equipment Spec. Industrial, scientific and medical equipment EN55011 Broadcast receivers and associated equipment EN55013 Electrical motor-operated and thermal appliances for household and similar purposes, electrical tools and similar apparatus EN55014 Electrical lighting and similar apparatus EN55015 Information technology equipment EN55022
Military MIL-STD-461/462
Aviation  DO-160
Belcore  GR1089
Automotive SAE, GM, Ford
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Measuring EMI, RFI
Click Image for Zoom
Spectrum Analyzer
(EMI Receiver)
Field Strength FCC: Class A & Class B
  Class A limits industrial, commercial, or business use. 
  Class B limits are more stringent and intended for residential use.

Test Setup

.Anechoic Chamber

The materials used in the walls and Ceiling of an Anechoic Chamber are such that there is little or no reflection of Electro Magnetic Waves. This is accomplished by the geometry and the absorptive nature of these materials. 

   
Transmission Lines
One source of interference can be from improperly terminated or defective transmission lines. Standing Waves on a coax can cause the shield to radiate like an antenna.
 Transmission Line Reflections due to improper Termination Impedance 
Transmission Line Termination Effects
(proper termination, open circuit, short circuit)
TDR 
Time Domain Reflectometry measurements

Time Domain Reflectometry (TDR) measurements have been long used to find transmission line faults.

Applying a fast pulse to a transmission line, while observing the resulting reflections within the transmission line, one can deduce the nature of any faults or discontinuities, as well as, the distance to those faults--assuming one knows the velocity factor of the transmission line dielectric material (insulating material).
 

Power Distribution in a Standing Wave
(SWR) Standing Wave Ratio
Data Buses & Clock Lines
Buses are sometimes required to span long distances across a board, which can lead to Crosstalk. This can be ameliorated by allowing adequate spacing between traces (see fig. B). However, there can be instances where this either doesn't work or there is not enough board area available; in which case interleaving ground returns may be required. (see fig. C & D)
Bus Layout
A) Simple 16 bit bus, close spacing; B) Wide Spacing; C) Interleaved Ground traces, 1 ground per 2 signal; D) 1 ground per signal
System Clocks are often the single greatest source of EMI
SIngle Ended
Balanced Pair
Balanced Triplet
Notice: Series Termination 
(SMD chip resistors)
Notice: Parallel Termination 
(SMD chip resistors)
* SMD: Surface Mount Device
Clock Line Drivers and Receivers on PCB Layout, 
using Single Ended, Balanced Pair, and Balanced Triplet lines, with Series and Parallel Termination
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Microstrip & Stripline transmission lines allow optimum operation of very fast logic, especially ECL (emitter coupled logic) logic. An important resulting benefit of this is reduced EMI.

Less Traveled Ways of EMI Reduction

Line Drivers that have Programmable/Controlled Rise and Fall Times 
To reduce EMI, some Line Drivers use internal slew rate limiting. The figure shows an FFT plot when transmitting a 150 kHz data stream. As may be seen, the slew limiting attenuates the high frequency components. EMI is therefore reduced, as are reflections due to improperly terminated cables. The objective is to control the level of emissions, both conducted and radiated. Conducted emissions are assumed to predominate below 30 MHz, while radiated emissions predominate above this frequency.
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Spread Spectrum System Clock
All digital clocks generate unwanted energy in their harmonics. Conventional digital clocks are square waves with a duty cycle that is very close to 50 %. Because of the 50/50 duty cycle, digital clocks generate most of their harmonic energy in the odd harmonics, i.e.; 3 rd, 5 th, 7 th  etc. It is possible to reduce the amount of energy contained in the fundamental and harmonics by increasing the bandwidth of the fundamental clock frequency. Conventional digital clocks have a very high Q factor, which means that all of the energy at that frequency is concentrated in a very narrow bandwidth, consequently, higher energy peaks. Regulatory agencies test electronic equipment by the amount of peak energy radiated from the equipment. By reducing the peak energy at the fundamental and harmonic frequencies, the equipment under test is able to satisfy agency requirements for Electro-Magnetic Interference (EMI). Conventional methods of reducing EMI have been to use shielding, filtering, multi-layer PCBs etc. The SM532 uses the approach of reducing the peak energy in the clock by increasing the clock bandwidth, and lowering the Q; this can also be thought of as "Frequency Dithering."
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PCB Layering: Signal, Power, Groundplane, & Shielding
Better for the Circuit
Better for EMC
Optimum for Both
Shield Layer
Note the single grounding (reference) point.

Analog & DIgital Powerplanes

Currents in either power plane can couple NOISE to the other; the coupling mechanism is mostly magnetic.
Separation
Power Supplies
Power supply and distribution are common sources of Emissions, as well as, Paths for Conducted and Radiated EMI.
Linear Supplies
Linear power supplies, if designed and laid out properly, offer little or no EMI problems
Layout, Bypassing & Decoupling are critical to this. 
Switching Supplies
Switching power supplies are potently a large source of  EMI. 
Proper Layout, Bypassing, Decoupling, and Shielding are very critical to the ultimate success of Switching Power Supplies.
---------------------Notice the Ground Returns
REMINDER: Near Field, Far Field Radiation
In dealing with a Radiation Source --be it an Antenna or a Circuit Board:
-
The dominant energy in the Far Field is Electric.
--
Plane Wave radiation is how radio works
377 ohms is the impedance of free space
-
The dominant energy in the Near Field  is Magnetic.
-
-
In any high frequency signal the dominant radiation component under ~1/6 wavelength from the source, is Electromagnetic; this is referred to as Near Field radiation.

Any signal beyond ~1/6 wavelength, is Electrostatic, and is referred to as Far Field radiation; sometimes called the Plane Wave. Plane Wave radiation is how radio works and that the 377 ohms is the impedance of free space.
 

Shielding as a method of EMI control
RF Emitter inside a Shielded Enclosure
Example of "Ideal" Shielding: no leakage
Shield Reflecting
Shield Reflecting, with some "Parasitic" Radiation
 
Driven Shield Radiating
Currents in Shield create "Antenna Effect"
Leakage of Apertures in Shielded Enclosure: Diffraction
Apertures act as "Point Sources"
(e.g., "Slot Antennas")
 
Something
to
Remember
The area of an Aperture is less important than its length—as in seams or joints. The rule of thumb is the maximum dimension of an aperture should not be greater than 1/20 the wavelength of the highest frequency of interest. Such apertures can act as "Slot Antennas."
Some  Apertures I have Known:
 
  • CRTs
  • Meter bezels
  • Lamps/LEDs 
  • Switches
  • Connectors 
  • Control Knobs (e.g., volume, etc.)
  • Sheet metal enclosures
  • Cable Entrances
 . 

Shielded Cable

Avoid extraneous currents in the coax shield: maintain shield isolation.
By definition: the Ideal shield has No current flowing thru it, and is "grounded" or referenced, at only one point.

In the case of shielded cables, the shield is typically referenced or grounded at both ends. 

In the case of coax cables the shield acts as both the shield and the return current path for the signal carried on the center conductor. However, with shielded balanced differential pair (sometimes called "twisted pair")  the shield is just that, a shield. Often this shield can be grounded at either end; however, in the case of EMI suppression, grounding both ends is prudent. 

Though coax and shielded twisted pair are used for both audio and higher frequency signals, there can be differing effects on how the shield is treated. The most difficult case is that of Video, where both very low to high frequencies (~ 10 Hz to >4 MHz) are involved.

Audio run on coax is subject to "ground loop" noise, i.e., dissimilar ground potentials (especially 60 Hz) which can cause a noise current to flow in the shield, inducing that noise into the protected center conductor. As the frequency of interest increases and the coax gets longer the ground loop problem is diminished; mainly due to the increased inductive reactance (higher Z) of the coax shield.

When long runs are required and/or the environment is noisy, it is best to use shielded twisted pair; the reason being the differential pair's inherent rejection of common mode noise (CMR).

An extreme case might find a remedy in transformer coupling or even optical Isolation.
 

EMI Protective Arsenal 
Metal Foil Tape
conductive adhesive, and solderable
Conductive Paint
copper, nickel, silver, etc. 
Conductive Plastic Cases
Bronze Finger Stock
for access panels, doors, drawers, etc.
Conductive Conformal Gasket Material
for access panels, etc.
Conductive Caulking
Custom conductive gasket
Solderable & snap-in metal shields
 "O" Ring
Conductive ATV
 Connector gaskets
 Compressible gasket

Additional Shielding Aids
Conductive paint, conductive gasket making silicone (ATV), and assorted conductive gaskets.

Power Line and Data line EMI Suppression using Ferrite
Common Mode Noise Suppression
Common Mode Xformers
Common Mode Xformer
Common Mode Xformer

Two Section A.C. Line Filter
A.C. Line Filter
Two Section
A.C. Line Filter
A.C. Line Filter 
built in Receptacle
Assorted EMI Suppression Devices using Ferrites--
Assorted Ferrites 
Ferrite Snap-in 
Balun, Ferrite Core
Tubular & Toroidal Ferrite
Ferrite Beads
Axial Leads
Multilayer Ferrite  Chip Beads
SMT Ferrite Beads
"DB" Data-Connector
Flat Cable Ferrite Cores
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Summary
Transmission Lines:
Be careful of high SWR (Standing Wave Ratio) on ALL transmission lines, be they twisted pair, coax, PCB traces, etc.; if not terminated correctly, or if there are discontinuities, e.g., unterminated stubs, damaged cable, etc., they can radiate.

Properly terminated, differentially driven and received Shielded Twisted Pair offer the best performance.
 

Bypass & Decouple:
Bypass ALL active devices!
Remember that bypass capacitors are self resonant.
Connect capacitors using the shortest leads!

Voltage regulators offer the best decoupling
Caution: voltage regulators WILL generate parasitic oscillations if not properly bypassed!

Groundplane:
Every signal should have its own ground return!

Shielding:
By definition: real shields have No current flowing thru them, and they are "Grounded," or referenced, at only one point!

Enclosures:
Leakage Apertures
Diffraction: Openings or Apertures in Enclosures Appear as Point Sources, and Radiate their Venomous Exhaling.

Area of the Apertures is less important than the longest dimension, e.g., Seams and Joints; greater than ~ 1/20 wavelength of the highest frequency of interest. They start looking like Slot Antennas. 

  Some Apertures:
   CRTs 
   Lamps/LEDs
   Switches 
   Connectors
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EMI REDUCTION CHECK LIST

A. Suppression of Emitter EMI

 Enclose noise sources in a shielded enclosure.

 Filter all leads leaving a noisy environment.

 Limit pulse rise & fall times.

 Relay coils should have surge damping.

 Shield and/or twist noisy leads.

 Ground both ends of shields used to suppress radiated interference (shield should be insulated).

B. Reducing Noise Coupling

 Twist low-level signal leads.

 Place low-level leads near chassis (especially if the circuit impedance is high).

 Twist and shield signal leads (coaxial cable may be used at high frequencies).

 Shielded cables used to protect low frequency, low-level signal leads should be grounded at 
 one end only (coaxial cable may be used at high frequencies with shield grounded at both ends).

 Insulate shield on signal leads.

 When low-level signal leads and noisy leads are in the same connector, separate them and 
 place the ground leads between them.

 Carry shield on signal leads through connectors on a separate pin.

 Avoid common ground leads between high-level and low-level equipment.

 Keep hardware grounds separate from circuit grounds.

 Keep ground leads as short as possible.

 Use conductive coatings in place of nonconductive coatings for protection of metallic surfaces.

 Separate noisy and quiet leads.

 Ground low frequency, low-level circuits at one point only (high frequencies and digital logic 
 are exceptions).

 Avoid questionable or accidental grounds.

 For very sensitive applications, operate source and load balanced to ground.

 Place sensitive equipment in shielded enclosures.

 Filter or decouple any leads entering enclosures containing sensitive equipment.

 Keep the length of sensitive leads as short as possible.

 Keep the length of leads extending beyond cable shields as short as possible. 

 Use low impedance power distribution lines.

 Avoid ground loops in low frequency, low-level circuits.

 Consider using the following devices for breaking ground loops: 
    Isolation transformers
    Common-mode chokes
    Optical couplers
    Differential amplifiers
    Guarded amplifiers
    Balanced circuits
    Hybrid ground

C. Receiver Noise Reduction

 Use only necessary bandwidth.

 Use frequency selective filters when applicable.

 Provide proper power supply decoupling.

 Bypass electrolytic capacitors with small high frequency capacitors.

 Separate signal, noisy, and hardware grounds.

 Use shielded enclosures.

 When using tubular capacitors, connect outside foil end to ground.

D. Controlling Digital System Emissions

 Minimize ground inductance by using a ground plane or ground grid.

 Locate bypass capacitors next to each IC in the system (use shortest leads possible).

 Use the smallest value decoupling capacitor that will do the job.

 Use a bulk decoupling capacitor to recharge the individual IC decoupling capacitors.

 Clock signal loop areas should be kept as close to zero as possible.

 All cables should be treated to minimize their common-mode current.

 All unused inputs on logic gates should be connected to either power or ground.

 I/O drivers should be located near where the cables leave the system.

 Use the lowest frequency clock, and slowest rise time that will do the job.

 Keep clock circuits and leads away from the I/O cables.

 

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Copyright 2000 2001   webmaster@williamson-labs.com
Last update 6-13-2001
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