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Chapter 4. Electrical Wiring > Noise Environment

4.4. Noise Environment

4.4.1. Noise Sources

Without electromagnetic shielding, power line cables are sensitive to ingress noises from radio frequency devices and other electromechanical equipment. Partially unbalanced wiring resulting from two-phase appliances and three-way switches makes the noise pickup a little worse. On the other hand, many household appliances, except light bulbs, make severe quasi-stationary or transient noise. Some of them are synchronized to the 60-Hz AC cycle, and others are impulses. Many of them can also come from nearby neighbors sharing the same transformer. From time to time, the noise level from a nearby appliance can exceed the transmit signal level. Information needs to be spread in both time and frequency domains to establish reliable communication links. Information spread in the time domain can be realized using error correction coding techniques while broadband signal should be designed in frequency domain.

Some field measurements have shown that a stationary background noise exists at a level of between –150 and –140 dBm/Hz [10] that is 20 to 30 dB above the thermal noise floor of about –170 dBm/Hz. Above the background noise, ingress noise from radio sources can be observed. Some ingress noises are leakages from nearby electrical or industrial equipment and can be characterized by a broad bandwidth and a moderate intensity of about 10 dB above the background noise level. Other ingress noises are from radio stations. Radio station ingresses are characterized by a narrow bandwidth and a high intensity. Clusters of radio stations can be identified within particular bands from ingress noise measurements. Depending on its distance, a radio station ingress noise can be 30 to 40 dB above the background noise. Their intensities also vary depending on the time of the day—with the worst ingresses occurring during the late night and early morning when the ionosphere provides the best conditions for radio transmission. Depending upon the proximity, noises from appliances can be very disturbing. A blender with its motor running can raise the noise floor to –80 dBm/Hz, which is 60 to 70 dB above the background noise. Brush motors of appliances such as a blender or a hair dryer generate nonsynchronized noise, while appliances using brushless motors can generate noises synchronized to 60 Hz AC when some TRIACs are in action. Another example of a synchronized noise source is the light dimmer.


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