Echelon LONWORKS PLT-22, User Manual

Page: 43 / 128

Transmit Amp
Receive Front End
PLT-22
C1
C2
L1
L2
Figure 4.7 Simplified Coupling Circuit with Resonant Inductor
An important design constraint on L2 is that its DC resistance be kept very low since
it is in the transmit signal path and effectively part of the transmitter's output
impedance. Low-cost inductors with DC resistance on the order of 0.2 Ohms are
widely available.
Capacitors C1 and C2 should be of metalized polyester construction in order to
minimize equivalent series resistance and provide adequate surge immunity.
!
It is critical that no additional series impedance be added in the signal path
between the TXOUT pin of the PLT-22 transceiver and the power mains (or
in the return path from the power mains to the ground pins of the PLT-22)
unless verified to be significantly less than 1 Ohm near 130kHz. If, for
example, a ferrite bead with an impedance of 9 Ohms at 130kHz were added then 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. For the same reason, the impedance of series circuit protection
elements must also be kept very low. Low current fuses (<2A), protection devices
that can be reset, and ferrite beads generally add unacceptable series impedance to
the signal path.
L
ON
W
ORKS
PLT-22 Transceiver User’s Guide 4-9