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Mitigation measures for telecommunication installations
PART 2: CASE STUDIES
| Case study # |
1.8 |
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| Title |
Medium-wave AM radio interference, PMR (private mobile radio) radio interference
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| Type of trouble |
Acoustic interference due to demodulation of the RF at semiconductor junctions within the telephone. |
| Source of trouble |
Either a local AM radio transmitter in the MW band, or a private mobile radio transmitter, e.g. from a taxi service.
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| System affected |
Customer's equipment. |
| Location |
Radio station, outdoors. |
| Keywords |
Immunity, emission LF broadcasting, HF broadcasting, amateur radio, citizen band transceiver, common mode chokes, filtering.
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| Version date |
2004-01-01 |
| System configuration |
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A normal PSTN telephone line, generally with (even a small amount, 50-100 m) aerial cable entering the customer premises. (All underground routes tend not to suffer from this type of interference.) Proximity to radio transmitting sites, such as the offices of taxi companies, citizen's band radio or MW AM stations make telecommunications installations particularly vulnerable. The higher the power, the greater the influencing distance. Not all customers on a road are necessarily affected as the coupling depends both on the orientation of the cable with respect to the transmitter and on the equipment the customer is using. Some terminal equipment is more susceptible than others to RF imbalance effects.
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| Searching techniques |
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The detection of such problems is fairly simple: the telephone engineer simply listens to the line and recognizes the noise originates in a radio transmission.
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| Mitigation method/Results/Conclusion |
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Generally, if the transmitter is being used in agreement
with its transmission licence, there is nothing that can be
done to solve the problem at source.
The fitting of small common mode chokes to the incoming
access network cable will normally solve the problem. It is
important to mitigate the interference as soon as possible,
i.e., just before the network termination, such that all
cables within the premises are protected from the
interference by one filter. There are currently many
thousands of these filters fitted to customers' lines working
successfully.
Any filter used should be suitable for the transmission of
DSL services. If they are not correctly designed, they will
introduce too much loss for such services. |
| References |
Rec. ITU-T K.37; Annexes A and B. |
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