EMC Awareness
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Radiocommunications Agency EMC Awareness |
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Telecommunications
Interference with ‘plain old telephones’
Brookmans Park broadcast transmitter: local interference
High power motor drives interfere with plain old telephones
Satellite comm’s delay Millennium Wheel erection
| Interference with ‘plain old telephones’ |  |
Description
In 1994 the Federal Communications Commission (FCC) in the U.S.A. was receiving
about 25,000 complaints per year from people unable to use their telephones
because 
of
interference from nearby radio stations. It is believed that this number represents
only a tiny fraction of the actual instances of this type of interference.
The FCC's Field Operations Bureau (FOB) conducted a study, which found that although most residential telephones are susceptible to receiving interference, manufacturers can design telephones to be interference-free.
The transmitting stations most likely to be involved in interference complaints are citizens band (CB), broadcast, and amateurs. Transmitted power was not a significant factor: one-third of the transmitters used under ten watts. The study also found that filters cannot be relied upon to eliminate telephone interference. In two out of three cases in which they were tried during the study, they did not work.
Commentary
All non-linear devices, such as semiconductors (whether discrete devices or integrated circuits) demodulate RF waveforms – a process sometimes called ‘audio rectification’. Analogue devices do not have the noise margins of digital electronics, so this demodulation is often more noticeable in analogue circuits such as those used in what are now known as ‘plain old telephones’.
The telephone lines themselves make good antennas for conducting the strong signals from nearby transmitters into the circuitry of the ‘plain old telephone’. But the wires and printed circuit board traces inside the unshielded plastic telephones, and the ‘curly cords’ to the handsets, make quite reasonable antennas in their own right so filtering the telephone line is unlikely to be sufficient. The inherent susceptibility of unprotected analogue circuits does the rest – yet designing RF protection into such circuits is in fact very easy.
References and links
“Interference Free Telephones”, FCC (Federal
Communications Commission, Washington D.C., U.S.A.), News media information
202/632-5050, May 4, 1994, indexed at: http://www.fcc.gov/Bureaus/Common_Carrier/News_Releases/1994/index3.html,
download from: http://www.fcc.gov/Bureaus/Common_Carrier/News_Releases/1994/nrcc4019.txt
“Telephone Interference”, advice from the FCC’s Consumer &
Governmental Affairs Bureau, at: http://www.fcc.gov/cgb/consumerfacts/ixphones.html
“Radio Transmitters and Telephones, Information for Telephone Users”, RSGB leaflet EMC 05, http://www.qsl.net/rsgb_emc/emcleaflets5.html, or via: http://www.qsl.net/rsgb_emc/emc.html#emcaid or http://www.qsl.net/rsgb_emc (where you will find many other useful leaflets on interference).
Links to Mitigation Techniques
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| Brookmans Park broadcast transmitter: local interference | |
Description
The Brookmans Park Transmitting Station, situated on the A1000 road between Potters Bar and Hatfield in Hertfordshire, played a crucial part in the history of broadcasting in Great Britain. Unfortunately at the same time, its powerful medium frequency transmissions have also plagued its immediate neighbours. With the introduction of electronic push-button telephones more than two decades ago, listening unavoidably to the radio on your telephone became a commonplace occurrence. Since then the problems have not gone away, as these extracts show:
“One local negative effect of the transmitting station relates to the rapid domestic development of the village of Brookmans Park since the Second World War. The originators of the Regional Scheme sought open sites for their transmitting stations away from high-density populations so that local residents would not be swamped by the stronger signals from the more powerful transmitters. …In Brookmans Park, this is an issue that continues to the present day. Many residents employ a series of filters on their televisions and telephones to prevent them from picking up radio programmes. They cannot, however, prevent signals being picked up by domestic wiring, radiators and metal coal buckets! Technology might yet come to their aid, as changing telephones and televisions from analogue to digital appears to remove interference from the radio transmitter…”
A History of Brookmans Park Transmitting Station, Lilian Caras 1982 (revised January 2002)
“Having recently purchased a new video recorder, we have been hearing Indian music through our television, this is obviously coming from the Brookmans Park transmitter…
…Top tip - changing phones from analogue to digital removes all noise from the radio transmitter…
…My computer picks up a radio station, and I disconnect it from the phone line to get a clear line….
…May I sympathise with those of our neighbours who are getting radio transmissions from their appliances ... For years RVC Hawkshead has had to grapple with radio broadcasts coming out of telephones, etc. For example, large-animal clinicians have had signal-interference with their sensitive diagnostic equipment: it's cost RVC a lot of effort and cash to line the current scanning unit … with copper to try to 'screen-out', but the problem is still not resolved…
…Are
any residents hearing increased noise from the radio transmitter? We have had
trouble in the past but luckily haven't heard anything for a year or more. Suddenly
on Saturday February 19th the noise was back and louder than ever… I spoke
to the BBC at Brookmans Park today and they were very sympathetic but could
offer no solutions. They know what it is like as they can hear the same sounds
in their garage playing from the radiator. The most common place for the noise
to come from is radiators, although people have reported sounds from microwaves
etc. ... In the past we have experienced Radio Sunrise which is a station for
the Asian Community and the music was quite distinctive. This time there is
more talking and he thinks it is more likely to be Talk Sport.”
From “Have Your Say”, a bulletin board on the community website www.brookmans.com, 2000-2001.
Commentary
According to the BBC,
“…For a mile or so around the site, therefore, the signal is very strong and any length of wire or pipe will act as a receiving antenna and pick up a strong signal, e.g. telephone lines, loudspeaker wires etc. Unless precautions are taken, which could include careful routing of cables and the fitting of filters, then the equipment connected to the cable can turn this energy into audible sounds…”
See the commentary on the previous example also. High power medium frequency transmitters have long wavelengths which are most efficiently picked up on long wires such as telephone cables and once induced, are hard to filter out.
The reference to singing central heating radiators introduces another problem: when RF energy encounters corrosion in metal structures, the corrosion acts as a crude detector and at the same time converts the detected energy into electromechanical movement, so that sounds (albeit distorted) are produced. Long central heating pipes collect significant RF power. The cure would usually be to dismantle the offending radiator, clean out the corrosion, and reassemble it, ensuring a good, corrosion-free electrical bond with the pipe.
References and links
Brookmans Park Transmission Station, Great North Road, south of Hatfield, Herts, Updated 03 July 2002; http://website.lineone.net/~greenbelt/BP%20Transmit.htm
More examples of interference from nearby broadcasting transmitters can be found in the “Banana Skins compendium”, via a link from www.compliance-club.com or at: http://www.compliance-club.com/archive1/Bananaskins.htm, especially (at the time of writing) numbers: 142, 143, 251 and 252.
Links to Mitigation Techniques
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| High power motor drives interfere with plain old telephones | |
Description
Soon after commissioning two new pumping stations with 6 MW adjustable speed induction motor drives the local power utility and the telephone company received a flood of complaints.
Geographically the complaints came from concentrated pockets spread over an area up to 12.5 miles, and followed the route taken by the 33 kV overhead supply cables that fed the pumping stations.
A payphone over 4 miles away from the power line was noisy enough to be almost unusable, whereas just across the street a householder's telephone was relatively unaffected. Other symptoms included loss of synchronisation on TV sets (rolling pictures) and ringing on the supply to fluorescent lighting circuits.
The problem became a public relations nightmare for all involved, and culminated in questions being raised at Government level. Remedial EMC work was urgently required and was in fact accomplished, although under extreme difficulties because the cost of any downtime of the pumping stations was so high.
Commentary
The drives had been designed to, and met the UK electricity supply industry's G5/3 limits for conducted harmonic emissions into the 33kV line.
But it appears that in some rural telephone networks the coupling from power lines into telephone lines is different from the urban and industrial networks that high-power variable frequency drives are typically used in, hence meeting the G5/3 limits was insufficient.
References and links
“Harmonic filtering of large induction motor variable frequency drives” by M J V Wimshurst of Hill Graham Controls, High Wycombe, U.K., and Allan Ludbrook of Ludbrook and Associates, Ontario, Canada. Presented at the 7th International Conference on Harmonics and Quality of Power (IEEE) at Las Vegas, October 16-18, 1996, pages 354-359 in the Proceedings. Also presented at the "Sixth International Conference on Power Electronics and Variable Speed Drives", Nottingham, UK, 23-25 September 1996, IEE Conference Publication No 429, pp 24-29, from sales@iee.org.uk, http://www.iee.org/shop or http://www.iee.org.uk/Library.
Links to Mitigation Techniques
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| Power harmonic filtering | |
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| Twinkling mobiles | |
Description
Products are available that 'twinkle' when attached or placed in close proximity
to a mobile phone. These products contain one or more Light 
Emitting
Diodes (LED's) that illuminate when the mobile is transmitting. Measurements
have shown that these devices have the potential to cause interference to the
cellular networks and other users of the radio spectrum. In addition, the mobile
phones themselves could experience loss of communications range and the phone
battery may need to be charged more frequently.
Any change in configuration of radio transmitting equipment from that which was originally subject to compliance testing is likely to take it out of specification. This is important on two counts. Firstly, the equipment when used is likely to cause interference and not perform to its true potential. Secondly, where equipment no longer performs to the requisite technical requirements, as is clearly the case when a twinkling antenna is fitted to a cellular telephone handset, the apparatus no longer qualifies for licensing. As a consequence anyone using the apparatus would be committing an offence. The RA recognises that individual members of the public cannot be expected to understand the technical or legal ramifications of what the public would see as the simple replacement of one antenna with another. However, it is not unreasonable for customers to expect that those in business should not be promoting equipment that cannot be legally used.
Commentary
The basic problem here is that the LEDs, powered as they are by the transmitted RF energy, are nonlinear diode devices and so cause intermodulation and harmonic generation from the transmitted frequency. Thus the radiated energy contains frequencies that were not present without the LEDs and which could fall on channels used by other radio services.
References and links
RA Licensing Procedures Manual for Public Wireless Networks Unit, http://www.radio.gov.uk/topics/pmc/document/pwnumanual2/pwnuman2.htm#b1
Links to Mitigation Techniques
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| Intermodulation | |
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| Satellite comm’s delay Millennium Wheel erection | |
Description
The Millennium Wheel on the Embankment in London was supposed to be lifted into position on September 12th 1999, but suffered from a number of delays, one of which was caused by an interference problem.
Satellite communications from the dishes on the roofs of the fleet of media vehicles covering the event interfered with the electronic equipment used to monitor the lifting gear.
Commentary
Assuming that the equipment used to monitor and control the lifting gear complied with all mandatory EMC regulations and their relevant immunity standards, these standards only cover ‘typical’ electromagnetic environments.
Satellite transmitters use dishes to focus the beam, but they aren’t as directional as lasers and there is some ‘spill’ in the general direction that the dish is pointing. Typically one would expect electronic equipment to be under or behind the dish, but in this case the lifting gear was quite high in the air, so when surrounded by a number of satellite transmitters pointing at various communication satellites in different parts of the sky its electromagnetic environment must have been very far from typical.
Also, note that commercial/industrial EMC immunity standards currently only measure immunity to RF fields at up to 1GHz, whereas satellite uplink frequencies are higher than this.
References and links
The Daily Telegraph, Saturday September 11th 1999, page 6.
The Guardian, September 11th 1999.
Links to Mitigation Techniques
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| Circuit design to reduce demodulation | |
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| Shielding enclosures | |
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| Shielding cables | |
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| Filtering | |
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