Specifications

PL 3120/PL 3150/PL 3170 Power Line Smart Transceiver Data Book 135
.
Figure 4.17 Simplified Coupling Circuit with DC Blocking Capacitor
Given the attenuation model presented earlier in Figure 4.13, one critical design constraint is that the impedance of the
series combination of C101 and C102 must be very low at the communication frequencies of the PL Smart Transceiver.
The impedance of these capacitors, along with the PL Smart Transceiver transmit amplifier’s output impedance,
corresponds to “Z
0
Transmitter” in Figure 4.13. Because the equivalent load impedance of the power line can in some
cases be on the order of 1 ohm, and because the output impedance of the PL Smart Transceiver transmit amplifier is less
than 1 ohm, the impedance of these capacitors at communication frequencies should be less than 1 ohm so that they do
not add significantly to “Z
0
Transmitter”. While the values of C101 and C102 could be set high enough to meet this goal,
doing so would significantly increase the cost of the high-voltage capacitor C101. Because C102 is connected only to
low voltage, and thus is lower cost for a given value, its value can be set higher relative to the value of the high-voltage
capacitor C101.
A simple and cost-effective way to achieve low transmit impedance with modest size capacitors is to add inductor L102,
as shown in Figure 4.18. This inductor forms a series-resonant circuit with C101 and C102, and its value can therefore be
chosen to optimize coupling at the communication frequencies of the PL Smart Transceiver while minimizing the cost of
C101 and C102. Different values of C101 and L102 are needed for A-band and C-band operation to optimize the
performance of a coupler in its respective band. The component values listed with the example coupling circuits
documented in this chapter include values optimized for each application and band of operation.