Echelon LONWORKS PLT-22, User Manual

Page: 38 / 128

Transmitter
Wiring
Phase-to-Phase
Receiver
Power Line Signal Return Path
Transmitter Z o Z Z
Wiring
Z
Load
Z
Load
Z
Load
Z Receiver Z i
Figure 4.2 Power Mains Attenuation Model
This model illustrates that minimizing the series impedances and maximizing the
line-to-return path impedances reduces the attenuation of the transmitted signal.
Coupling Techniques
Power Line Coupling Basics
Injecting a communication signal into a power mains circuit is normally
accomplished by capacitively coupling a transceiver's output to the power mains. In
addition to the coupling capacitor, an inductor or transformer is generally present.
The coupling capacitor and the inductor or transformer together act as a high-pass
filter when receiving the communications signal. The high-pass filter attenuates the
large AC mains signal (at either 50Hz or 60Hz), while passing the transceiver's
communication signal. Figure 4.3 below shows a basic mains coupling circuit. The
value of the capacitor is chosen to be large enough so that its impedance at the
communication frequencies is low, yet small enough that its impedance at the mains
power frequency (50Hz or 60Hz) is high. The impedance of the capacitor can be
considered as part of the transmitter's output impedance (Zo
Transmitter
) shown in
figure 4.2. Keeping the impedance of the coupling capacitor low minimizes the signal
injection loss caused by the voltage divider formed between the output impedance of
the amplifier and the mains loading (Z
Load
).
The value of the inductor is chosen to have a relatively high impedance at the PLT-
22 transceiver's communication frequencies. The inductor impedance can be
considered part of the receiver input impedance (Z
i Receiver
) shown in figure 4.2.
Keeping the inductor impedance high helps minimize any signal loss at the receiver
due to the voltage divider formed by the wiring impedance and the receiver input
impedance.
4-4 Coupling Circuits