Specifications

PL 3120/PL 3150/PL 3170 Power Line Smart Transceiver Data Book 137
Figure 4.19 shows additions to the coupling circuit which are required to make it fully functional. The first is an
inductor, L103, connected to the PL Smart Transceiver receive filtering circuitry. The DC resistance of L103 can be up
to 55 ohms. The second consists of diodes, D101 and D102, connected from the transmitter to the amplifier supply rails
to protect the inputs of the PL Smart Transceiver from large (>18V) transients. The characteristics of these diodes are
very important for them to perform their intended function without impairing the operation of the transmit amplifier. All
of the necessary specifications are listed in the “Required Specifications” column of the component tables for each
example coupling circuit included in this chapter. Bypass capacitor C103 also has been added to emphasize the fact that
it is an integral part of the coupling circuit. One of the functions of this capacitor is to protect the V
A
supply line from
excessive overshoot when positive going line surges discharge through diode D101. If the output capacitor of the V
A
power supply meets all of the required specifications for C103 (and it can be located close to the transmit amplifier and
D101) then no additional capacitor is needed as part of the coupling circuit. Alternately a separate C103 capacitor can be
included close to the transmit amplifier and D101, if the V
A
power supply capacitor does not meet these requirements.
To maintain a low impedance signal path, all of the circuit board traces between the output of the transmit amplifier and
the AC mains wiring should be at least 1.3mm (50 mils) wide and less than 15cm (6 inches) long. The corresponding
signal return path should either be a copper plane or a trace that is at least 1.3mm wide and less than 15cm long.
Because positive polarity surge events cause high currents to flow through D101 and C103 back to ground, the circuit
traces from the point where D101 taps into the signal path to the point where C103 connects back to ground are also
critical. They should be at least 1.3mm wide and no more than 2cm (0.8 inches) long. To properly control ripple on the
V
A
supply of the transmit amplifier, the traces between the V
A
input of the transmit amplifier and the point where C103
connects back to ground should also be at least 1.3mm wide and no more than 4cm (1.6 inches) long. The solid bold
lines in Figure 4.20 indicate circuit traces that should be greater than or equal to 1.3mm wide. The dotted-line arrows in
the figure indicate the maximum trace lengths.