Module 5: Digital Techniques and Electronic Instrument Systems

5.14 Electromagnetic Environment

Electromagnetic Interference An electromagnetic

field consists of an electric field and a magnetic field.

When the energy of such a field disturbs an electrical circuit, the effect is called Electromagnetic Interference.

The disturbance is caused by either electromagnetic induction or electromagnetic radiation.

Types of EMI Narrowband EMI (or Radio-frequency

Interference - RFI) emanates from intended transmissions (radio, TV stations, cell phones etc.).

Broadband EMI (or Radio-frequency Interference - RFI) is unintentional radiation from sources such as electric power transmissions lines.

Conducted EMI is caused by contact of the conductors. Mutual inductance between two radiated electromagnetic fields will result in EMI.

RFI is also called “noise”.

Electromagnetic Compatibility (EMC) EMC is the ability of an equipment to operate

satisfactory in its EM environment, without introducing intolerable EM disturbances to other electrical devices in its environment.

All LRUs are subjected to EMC testing. Two kinds of emissions are tested: Conducted

Emissions (CE) and Radiated Emissions (RE). CE is EM energy that is propagated through

conductors. RE is EM energy that is propagated through free

space.

Causes of EMI Most electrical systems generate noise,

especially motors and generators. Cosmic noise is radiated from space at 12MHz.

Also, electrical discharges are an EMI source. Static electricity, due to the triboelectric effect:

Free electrons accumulated on the aircraft cause EMI.

Lightening Unbalanced Circuits:

Circuits can pickup noise from neighboring circuits. Inductive pickup Capacitive pickup Common Earth Return (Earth loops)

An EMI example The two circuits are

supposed to be isolated. i.e. V1 cannot affect

Vout.

However, if RG is not is not equal to zero, VG is developed.

Now, VG is affected by V1 and affects Vout as well.

A similar problem occurs when both ends of the shield of a cable are grounded.

Noise can be inserted in the shield of the cable, causing EMI.

Thus, only one end of the shield of a cable should be grounded.

However, in aircraft some shields are grounded in both ends, to ensure protection from lightening. (An one-end grounded shield can act as an antenna).

EMI Causes and Effects

EMI Protection Twisted wires:

Immune to EMI: Noise signals developed in one wire by external magnetic fields or radiation, are eliminated by the opposite noise signals developed in the other wire.

Proper cable installation: Grounding only one end of the cable

shielding. Bonding:

All equipment should be bonded together to prevent static electricity build-up.

Static dischargers (wicks). Shielding cages:

Used to protect devices from high intensity radiating fields (HIRF): 10KHz to 18GHz.

EMI Protection Transformers tend

to induce parasite high frequency signals.

A set of capacitor is used to eliminate high frequency noise.

Opto-isolators are a set of led – photodiode which isolate two circuits.