EMC Awareness
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Radiocommunications Agency EMC Awareness |
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Office and IT equipment
Laptop interferes with aircraft navigation
| Laptop interferes with aircraft navigation |  |
Description
The pilot of a Brasilia EMB120 experienced interference with VHF navigation reception in May 2000. A passenger in seat XA was operating a laptop computer, and normal reception was restored when she switched it off.
A similar incident happened on a different EMB120 at row X and with the same make of laptop computer. This was at a different navigation beacon and needles on the MFD (multifunction display) “went crazy” and were “spinning in circles”. In both cases, they were spinning in circles on both duplicated navigation channels.
This pilot does not yet limit the use of passenger electronic devices (PEDs), but whenever he experiences navigation problems he now immediately checks to see if any passengers are operating laptops.
Commentary
Radiated emissions from digital processing products (such as computers) occur at discrete frequencies – the harmonics of its internal ‘clock’ oscillators. If these emitted frequencies coincide with the frequencies used by various navigational instruments, such as the VOR (VHF Omni-directional Ranging) beacon, they could cause interference. Closer to the product, the emitted fields are more intense.
Much of the electronic equipment for the avionics instruments is located under the floor of the aircraft’s passenger compartment, which may not provide any shielding. Certain seats in the passenger cabin will be closer to certain avionic equipment, and/or may be closer to antennas located outside the aircraft. So PEDs may be more liable to cause interference when they are used by people sitting in those seats.
One would usually expect equipment that meets emissions standards such as required for CE, FCC or VCCI marking not to cause interference, but not all equipment complies with these standards, and sometimes a ‘bad batch’ could get manufactured which had higher emissions than expected.
However, radiated emissions test standards measure using antennas in the ‘far field’ and avionics equipment aircraft’s external antenna could be in the ‘near field’ – or at least significantly closer than is assumed by the test standards. For example, CISPR 22 (EN 55022) measures emissions at 10 metres distance and does not guarantee freedom for interference for domestic radio receivers that have antennas closer than 10 metres to the product tested.
References and links
NASA Aviation Safety Reporting System (ASRS) report dated 1st May 2002, download from: http://asrs.arc.nasa.gov/report_sets/ped.pdf. This example is from No. 475267 in that report.
Links to Mitigation Techniques
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| PCB layout for EMC |  |
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| Circuit design for emissions control | |
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| Filtering with CM cable-mounted chokes |  |
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| Filtering | |
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| Shielding enclosures | |
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| Shielding cables | |
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| Bluetooth and Wi-Fi | |
Description
The co-existence of Bluetooth and Wi-Fi in the 2.4GHz Industrial Scientific and Medical (ISM) band was discussed at the recent Wireless Symposium in San Jose. Because the simultaneous operation of these two systems can interfere with each other, the search is on for ways to improve their performance when they are in proximity.
Commentary
These two technologies are headed for significant growth and so their co-existence at a given site is more likely and has become a significant topic of analysis and discussion.
Since they operate in the same (narrow) frequency band they can cause co-channel interference for each other. But neither was designed with specific mechanisms to combat the interference from the other.
Bluetooth uses a frequency-hopping scheme and assumes it will hop away from congested channels (this is how it avoids interference from microwave cookers) – but this may not be possible when Wi-Fi is present. Wi-Fi (IEEE 802.11b) assumes that if it fails, it is because two Wi-Fi stations tried to transmit at the same time.
References and links
“Living in a Wireless World” by Sherrie Steward, Editor, Compliance Engineering, March/April 2001, page 10, www.ce-mag.com/archive/01/03/toc.html.
Links to Mitigation Techniques
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| Preventing co-channel interference | |
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