ZyXEL Communications PLA-470 V2 - V3.0.5 Guía de instalación descargar pdf (Pagina 39)

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harmonic noise, composed of multiple frequencies used by electrical equip-

ment connected to the network and which are multiples of the line frequency

(for example, 50 Hz yields harmonics of 300 Hz, 600 Hz, and so forth).

Overall, the noise is quantified by the signal-to-noise ratio, or SNR, generally

measured in decibels (dB).

In addition to noise on the power line medium, devices connected to the electri-

cal network, or disconnected but located in proximity to the wiring, create a mea-

surable level of perturbations on the transmission channel. This rather complex

technical subject is known as EMC (electromagnetic compatibility).

From the EMC point of view, every powered electrical device generates electri-

cal perturbations, which may be conducted (transported on the electrical wiring) or

inducted (emitted in the radio environment of the device).

Numerous working groups in Europe (Cenélec) and internationally (IEC) have

established rules that specify authorized limits for the perturbations produced by

each category of electrical equipment, including PLC equipment. Also, telecommu-

nications standards organizations in Europe (ETSI) and internationally (ITU) are

studying perturbation thresholds in order to optimize the transmission channel, and

signal processing techniques to obtain the best performance from PLC. The IEEE is

also working on these issues to optimize the physical layer of the OSI model.

The ISRIC (International Special Radio Interference Committee) Working

Group 3 has set allowable limits for perturbations from PLC electrical devices in the

150 kHz to 30 MHz band.

The EMC perturbations received and provoked by PLC are the subject of

numerous other projects and studies. Their objective is to standardize emission lev-

els of each device and to obtain a transmission channel that works efficiently with

this level of emissions.

Attenuation

In the same way that a radio signal’s power is and attenuated function of the dis-

tance traveled by the waves, or a DSL signal is attenuated as it travels on the PSTN’s

copper pairs, the electrical signal loses power as a function of the distance traveled.

This characteristic of electrical wiring must be taken into account when imple-

menting a PLC network. In Chapter 8 (Table 8.10), we will study in detail the choice

of parameters that offers the best performance for a PLC network. They vary greatly

as a function of the range and attenuation of the signal.

Variations in impedance on the electrical network provoke effects such as

multipath, giving rise to “notches” or amplitude peaks in the PLC signal, which may

be considerable at certain frequencies. In the home, signal attenuation on electrical

wiring is on the order of 20 to 60 dB, depending on the topology and content of the

wiring network.

The minimum attenuation for the meter and circuit breakers combined is 30 dB

for a system based on frequencies above 20 MHz. For frequencies below 20 MHz,

the average attenuation is approximately 50 dB. However, a good PLC coupler can

reduce the attenuation to 10 to 15 dB for certain frequencies.

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ZyXEL Communications PLA-470 V2 - V3.0.5 Guía de instalación Pagina 39

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