Specifications

Chapter 4 – Coupling Circuits
132 PL 3120/PL 3150/PL 3170 Power Line Smart Transceiver Data Boo
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Power Line Coupling Basics
Injecting a communication signal into a power mains circuit is normally accomplished by capacitively coupling the
output of a transceiver 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 communication signal of the transceiver. Figure 4.14 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 (Z
0
Transmitter) shown in Figure 4.13. 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 provide a relatively high impedance at the communication frequencies of the PL
Smart Transceiver. The inductor impedance can be considered part of the receiver input impedance (Z
i Receiver
) shown in
Figure 4.13. 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.
Figure 4.14 Basic Mains Coupling Circuit
A key factor affecting the type of coupling circuit to be used is the wiring style of the power distribution system to which
the coupling circuit will be connected. Wiring topologies vary from application to application, e.g., homes versus
commercial buildings, as well as from country to country. Wiring styles can be divided into two major categories: wiring