Echelon LONWORKS PLT-22, Руководство пользователя

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Power Line Communications
The PLT-22 transceiver employs sophisticated digital signal processing techniques, a
transmit power amplifier with a very low output impedance, and a very wide (80dB)
dynamic range receiver to overcome the signal attenuation and noise inherent in
power mains communication. Maintaining the full communication capability of the
PLT-22 transceiver requires careful selection and implementation of the mains
coupling circuitry external to the PLT-22 transceiver. This section gives an overview
of the sources of signal attenuation as a basis for understanding choices in selecting
and implementing mains coupling circuits.
Attenuation is the difference between the signal level at the output of the power line
transmitter and the level of that same signal at the input of the intended receiver.
While attenuation is technically defined as the ratio of power levels, it is referred to
in this document as the ratio of the transmitted signal voltage (unloaded) to the
voltage of that same signal at the receiver input. A voltage ratio is more convenient
to measure since power measurements require knowledge of the circuit impedance
which, in the case of the power mains, varies with both location and time.
In power mains communications the attenuation of transmitted signals spans a wide
range and is most conveniently denoted in decibels (dB), where voltage attenuation is
defined in dB as 20log10 (
V
transmit
/
V
receive
). Thus 20dB of attenuation means that the
signal was reduced by a factor of 10 by the time it arrived at the receiver, 40dB of
attenuation corresponds to a factor of 100, 60dB a factor of 1000, and so on. A PLT-22
transceiver is capable of reliably communicating on a low-noise line, such as a
dedicated twisted wire pair, when the transmit signal is attenuated by as much as
80dB (a factor of 10,000). Thus a signal transmitted at 7Vp-p (2.5Vrms) may be
received when reduced to less than 700µVp-p (250µVrms).
To better understand the sources of attenuation in a power mains network, it is
helpful to look at a simplified model of a power distribution network. This example
is based on an installation having one power distribution panel and two phases of
mains power. While many applications for power line communication employ more
phases, different topologies, voltages, and wire types, this example illustrates some
of the key issues affecting the successful application of the PLT-22 transceiver.
Figure 4.1 depicts the path that a power line communication signal might traverse,
starting from a wall socket and passing through the building's electrical wiring and
circuit breaker panel, across power phases, and ultimately to another wall socket.
Each socket in the power network may power a device that generates noise and loads
the transmitted signal. For clarity, neutral and earth wires have not been shown.
4-2 Coupling Circuits