EMI/EMC

Contents

Crosstalk In Transmission Lines Transients In Power Supply Lines Surge voltages from lightning Switching Transients Electromagnetic Interference Open Area Test Sites Open Area Test Site Measurements

Crosstalk In Transmission Lines• Crosstalk is the coupling of EM energy from one transmission

line to the another via: - Mutual Inductance(magnetic field) - Mutual Capacitance(electric field)

The circuit element that represents this transfer of energy are the following familiar equations

Mechanism of coupling

dt

dILV mLm

dt

dVCI mCm

The mutual inductance will induce current on the victim line opposite of the driving current (Lenz’s Law).

The mutual capacitance will pass current through the mutual capacitance that flows in both directions on the victim line.

Crosstalk In Transmission Lines

Summary of analysis:

It is clear that superposition of inductive and capacitive coupling is a function of geometrical config., number of conductors and terminal impedances.

Thus the freq. range for which the analysis is valid depends on no. of line conductors, cross-sectional configuration of line and terminal impedances Roi and RLi

Transients In Power Supply Lines

What are transient voltages?

• "Transients or Transient Voltages“ is generally called as "surges" or "spikes".

• Transients are momentary changes in voltage or current that occur over a short period of time.

• This interval is usually described as approximately 1/16 (one sixteenth) of a voltage cycle

• Voltage transients normally last only about 50 microseconds .

• Current transients last typically 20 microseconds according to the ANSI C62.41-1991 which is the standard for transients in facilities operating under 600 Volts.

• Transients may be of either polarity and may be of additive or subtractive energy to the nominal waveform.

Characteristics of the Transient Voltage Waveform

a) Oscillatory transient

b) Impulsive transient

Transients In Power Supply Lines Transient over-voltages (probably as a result of terrestrial

phenomena such as lightning.

Radiation from strong radar/radio/communication transmissions within the vicinity, which are picked up by the power transmission lines.

Sudden decrease or increase in the mains voltage (caused by the switching of low impedance loads).

Burst of high frequency noise (probably due to switching of reactive loads).

Surge voltages from lightning

A nearby lightning strike to objects Lightning ground-current flow The rapid drop of voltage A direct lightning strike to high-voltage

primary circuits. Lightning strikes the secondary circuits

directly.

Switching Transients

• Minor switching near the point of interest• Periodic transients (voltage notching) that occur each cycle during the commutation in electronicpower converters• Multiple re-ignitions or re-strikes during a switch operation• Major power system switching disturbances• Various system faults, such as short circuits and arcing faults

Electromagnetic Interference

Electromagnetic Interference

• Undesired or unintentional coupling of electromagnetic energy from one equipment (emitter) to another equipment (receptor).

The so called source-path-receptor model suggests that electromagnetic interference can be prevented in one of three ways:

Suppress emissions at the source. Interrupt or reduce the efficiency of the coupling

path. Make the receptor immune to emissions.

One of the greatest difficulties in diagnosing interference is determining exactly which coupling path interference is following to the receptor. Coupling paths are typically classified as belonging to one of four general classes.

1. Conductive coupling

Conductive coupling occurs when the path of interference between the source and the receptor is formed by a conducting body.

• By means of a power cord, interface cables, antenna input terminals, ground returns, or

• Unintentional external conductors such as metallic cases or housings.

2. Radiation coupling

Radiation/electromagnetic coupling occurs when the path of interference lies through free space, or some other non-conductive medium.

Radiation coupling usually occurs when distances between the source and receptor are on the order of several wavelengths.

Because of this wide separation, the source is usually not affected by the presence of the receptor.

Radiation fields decay as 1/R for points far away from the source.

3) Induction coupling

Inductive/magnetic coupling is associated with near-fields, in a region where the magnetic field is dominant. Occurs when distances between the source and

receptor are much less than a wavelength. Due to the relatively small separation, the

presence of the receptor can affect the behaviour of the source, referred to as mutual coupling.

4) Capacitive coupling

Capacitive/electric coupling occurs when the source and receptor are less than a wavelength apart.

Here the electric field is dominant. It occurs in the presence of high impedance to

ground and is more predominant at higher frequencies.

Open Area Test Sites & Measurements

Open Area Test Sites & Measurements

Here we are going to study about Open Area Test Site Open Area Test Site Measurements Measurement of RE Measurement of RS Measurement precautions

OPEN -AREA TEST SITE

• The shape and size of the open-area test site will need to be appropriate to avoid scattered signals.

• ANS recommend that Sc ≤ Sd -6dB -----(1)

where Sc & Sd are the scattered signal from obstructions located at the boundary of open-area test site and the direct signal between EUT and the Tx/Rx antenna.

The measurements of radiated emissions (RE) and radiated susceptibility

(RS) of an equipment constitute the two basic EMI and EMC

measurements.

The purpose of radiation susceptibility testing is to determine the

degradation in equipment performance caused by externally coupled

electro magnetic energy.

Open site measurement is most direct and universally accepted standard

approach for measuring radiated emissions from an equipment or the

radiation susceptibility of an equipment.

OPEN AREA TEST SITE MEASUREMENTS

Measurement of RE EUT is switched on

The receiver is scanned over the specific frequency range

It measures electromagnetic emissions from the EUT

It determine the compliance of these data with the stipulated specifications.

calibrated receiver/ field strength meter Power line

filter

EUT

Power source Power

source

Measurement of RS1) EUT is placed in an electromagnetic field created with the help of suitable radiating

antenna.

2) The intensity of the electromagnetic field is varied by varying the power delivered to

the antenna by the transmitter amplifier

3) Performance of EUT are then observed under different levels of electromagnetic

field intensity.

Power line filter

EUT

Power source

TransmitterPower line filter

Power source

Test Antennas

• A convenient approach to illuminate an equipment under test with known field strengths is to used exact half wave length a long dipoles at fixed frequencies.

• This arrangement is superior when compared to connecting a test antenna to a signal source using co-axial cable that might distort the field pattern.

Antenna Type Frequency, MHz

Rod antenna 1 - 30

Loop antenna 1 – 30

Biconical antenna 30 – 220

Dipole antenna 30 - 1000

Log periodic antenna 200 -1000

Conical log spiral 200 – 10000

Wave guide horn Above 1000

Examples of test antennas

Measurement Precautions

1) Electro magnetic environment

According to ANS, it is desirable that the conducted and radiated ambient radio noise

and signal levels measured at the test site with the EUT de-energized, be at least 6 dB

below the allowable limit of the applicable specification.

2) Electro magnetic scatterers

One method for avoiding interference from underground scatters is to use a metallic

ground plane to eliminate strong reflections from under ground sources such as buried

metallic objects.

3) Power and cable connections

The power leads used to energize the EUT, receiver and transmitter should also pass

through filters to eliminate the conducted interferences carried by power lines.