EMI/EMC. B.T.P.MADHAV Lecturer, Dept.of ECE KL UNIVERSITY. CHAPTER-I. EMI ENVIRONMENT. EMI / EMC • EMI is defined as the undesirable signal which causes unsatisfactory operation of a circuit or device.
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• EMI is defined as the undesirable signal which causes unsatisfactory operation of a circuit or device.
• EMC is defined as the ability of electronic and communication equipment to be able to operate satisfactorily in the presence of interference and not be a source of interference to nearby equipment.
• EMS Electromagnetic susceptibility (EMS) is the capability of a device to respond to EMI.
These are divided mainly into two types.
I. Natural and
I. Natural EMI sources are again of the following
Terrestrial and Extra-Terrestrial.
These are atmospheric thunderstorms, lightning
discharges and precipitation static.
These are sun-disturbed & quiet, cosmic noise and radio
(i) Annoying Effects
Very often, momentary and random disturbances in
radio and television reception occur.
(ii) Disturbing Effects
Unwanted reset and change of status in settings in
computers and digital equipment is noticed due to EMI.
The malfunctioning of computer key boards are noticed.
(iii) Catastrophic Situations
The burning of electronic components, loss of data,
change of threshold settings, improper or unwanted
operations and sometimes biological hazards occur very
These are of two types
a) Military Standards
These include emission and susceptibility standards. Emission standards specify emission limits in voltage or current, power or field strengths in specified frequency ranges. Susceptibility standards specify conducted spike or radiated field parameters.
b) Civilian Standards
The civilian EMC standards are applicable for equipments used for commercial, industrial and domestic applications. The emission standards are specified to protect the broadcast services from interference. These also take into account the physiological interference effects experienced by human beings.
The advantages are:
1. Compatibility, reliability and maintainability are
2. Design safety margin is provided.
3. The equipment operates in EMI scenario
4. Product life is increased.
5. Higher profits are possible.
EMI CONTROL TECHNIQUES
METHODS FOR EMC
The effective methods to eliminate EMI are
6. Separation and orientation
7. Circuit impedance level control
8. Cable design
9. Cancellation techniques in frequency or time
10. Proper selection of cables, passive components
11. Antenna polarization control
These are used to filter out conducted EMI. The filtering effectiveness is expressed by Insertion loss (IL). It is defined as
GROUNDING is a technique that provides a low resistance path between electrical or electronic equipment and the earth or common reference low impedance plane to bypass fault current or EMI signal.
Lightning stroke current from radio tower
Side flash because of long load grounding lead
Radio equipment cabinet
a) Floating Ground : It isolates circuits from a common ground plane. It may be hazardous some times.
The ground plane is in the form of wire or a conductive rod.
b) Single – point Grounding : It reduces the effects of facility ground currents. This is used to control EMP energy.
c) The multiple point grounding : It reduces ground lead lengths.
Vd = (Vpg - Vng )/2
To obtain noise immunity, the ground loop must be broken. This can be done using transformers, optical couplers etc.
Transient and surge suppressing devices
The gas discharge tube can handle very large transient currents ( >10KA), when the tube is connected between the line and the ground.
2. When the transient EMI voltage in line exceeds the striking voltage of the tube, an arc discharge occurs and the ionized gas produces a low impedance from line to ground to shunt surge current.
Semiconductor Transient suppressors
Semiconductor transient suppression device maintain a constant voltage at a desired level across a device by offering variable resistance when transient voltages are present.
The selection of components can be classified into three categories:
A) Components that affect the RELIABILITY and FUNCTIONALITY .These
Components are marked as RELIABILITY CRITICAL components or components that MODERATELY affect RELIABILITY.
B) Components that affect the EMI PERFORMANCE are marked as EMI CRITICAL and EMI
C) Components that affect the AUDIO PERFORMANCE, e.g. Signal to Noise Ratio are marked as AUDIO CRITICAL and AUDIO MODERATE.
Shielding uses conductive material to wrap up the EMI completely to ground. In this way, electromagnetic energy is kept inside the system. It also gets harder for an external signal to cause EMI into the system. It is useful to both conducting EMI and radiated EMI.
Generally this is an expensive way to protect the sensitive part of the system, and it takes space. It works well for higher frequencies. For clock frequencies or edge rates lower than 100 MHz, EMI is coupled from the clock signal onto the shield and the shield itself does the radiating. In this case, shielding has very little effect.
Good decoupling and careful layout can reduce conducting EMI better than shielding, in most cases. Bypassing or "decoupling" capacitors on each active device (connected across the power supply or ground, as close to the device as possible) help to guide the clock or any other high-frequency signal component directly to ground instead of interfering other signals.
EMC DESIGN GUIDELINES
CHOICE OF PASSIVE COMPONENTS FOR EMC
An inductor or a reactor is a passiveelectrical component that can store energy in a magnetic field created by the electric current passing through it.
An inductor is usually constructed as a coil of conducting material, typically copper wire, wrapped around a core either of air or of ferromagnetic material.
Capacitors are used for charge storage, timing, filtering, blocking, control of rise and fall times and to provide low impedance paths for high frequency signals.
Different Types of Capacitors are ------
Resistors :- ground, the capacitor impedance decreases as the frequency increases.
Film type Noise is in Between wire-wound and composition.
Series Resistors Also among the most important and cheapest of protective elements. Properly selected according to resistance and power dissipation, they can replace more costly elements, with comparable results.
TRANSFORMERS ground, the capacitor impedance decreases as the frequency increases.
Used for voltage and current transformation or level shifting, impedance matching, power transfer and Isolation process
This allows noise coupling through the transformer. This coupling can be eliminated by providing an electrostatic or Faraday shield
Wires of different types are not be bundled together. ground, the capacitor impedance decreases as the frequency increases.
Wire bundles of different types of signals should be physically separated from each other.
Minimum separation is to be 6-8cm.
The area of current loops on PCBs and board interconnections should be minimum.
Maximum loop area should be 4cm2.
Divide larger loops with smaller loops.
Loops and wires should cross at right angles to each other
Distance between twisted pairs should be atleast 1.5 times the twist length.
Multiple ckts with common return should be twisted as group.
Wires between units should follow the most direct route.
CHAPTER-4 ground, the capacitor impedance decreases as the frequency increases.
OPEN AREA TEST SITES
OPEN AREA TEST SITE MEASUREMENTS
Open site measurement is most direct and universally accepted standard approach for measuring radiated emissions from an equipment or the radiation susceptibility of a component or equipment
MEASUREMENT OF RE ground, the capacitor impedance decreases as the frequency increases.
calibrated receiver/ field strength meter
Power line filter
MEASUREMENT OF RS ground, the capacitor impedance decreases as the frequency increases.
Power line filter
Power line filter
1) Electro magnetic environment
2) Electro magnetic scatters
3) Power and cable connections