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5.14 Electromagnetic environment


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5.14 Electromagnetic environment

  1. 1. Module 5: Digital Techniques and Electronic Instrument Systems 5.14 Electromagnetic Environment
  2. 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. 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. 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. 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. 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. 7. EMI Causes and Effects
  8. 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. 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.