1. Module 5: Digital Techniques and
Electronic Instrument Systems
5.14 Electromagnetic Environment

2. 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.

3. 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”.

4. 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.

5. 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)

6. 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).

7. EMI Causes and Effects

8. 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.

9. 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.