Specifications

Chapter 4 – Coupling Circuits
136 PL 3120/PL 3150/PL 3170 Power Line Smart Transceiver Data Boo
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Figure 4.18 Simplified Coupling Circuit with Resonant Inductor
An important design constraint on L102 is that its DC resistance must be a small fraction of an ohm because it is in the
transmit signal path and effectively part of the transmitter's output impedance. Fortunately low-cost inductors with DC
resistance of 0.3 ohms or less are widely available.
Capacitors C101 and C102 should be of metalized film construction in order to minimize their equivalent series
resistance and provide adequate surge immunity.
It is critical that no additional series impedance be added in the signal path between the PL Smart Transceiver
transmit amplifier and the power mains (or in the return path from the power mains to the ground pins of the PL
Smart Transceiver) unless verified to be less than 0.5 ohms between 70kHz and 138kHz.
To illustrate the importance of maintaining a low impedance signal path, consider the example of a ferrite bead with an
impedance of 9 ohms at 100kHz added in series with the line. In this case the signal injected into a 1 ohm power line
would be reduced by a factor of 10. Under typical conditions, the end product would still function, however,
communication margin and reliability over a full range of power line environments would be severely compromised.
Ferrite beads, unless carefully selected to be low impedance at 100kHz, offer too much series impedance. Most devices
built with the PL Smart Transceiver do no need ferrite beads in order to pass EMC regulations. If, due to other noise
generating circuitry ferrite beads are required, then refer to the end of Chapter 6 for a discussion of acceptable
topologies.
The impedance of series circuit protection elements must also be kept very low. Low current fuses (<2A) and protection
devices that can be reset generally add unacceptable series impedance to the signal path.
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