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5.14 Electromagnetic environment
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).
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.
