Specifications

PL 3120/PL 3150/PL 3170 Power Line Smart Transceiver Data Book 171
An example of a system that would benefit from an even greater input impedance would be one in which 100 or
more PL Smart Transceiver-based devices were connected to a long twisted pair cable. In this case the input
impedance of the power supply could limit either the maximum transmission distance and/or the maximum number
of devices that could be connected to the cable. To maximize communication distance on dedicated twisted pair
wiring where the system impedance is approximately 100 ohms, a minimum power supply input impedance of 500
ohms is recommended. For extreme cases with >100 nodes on dedicated lines longer than 1000m, a power supply
input impedance of 2000 ohms might be needed to maximize communication distance. Table 5.9 shows the
appropriate inductor value by application.
Table 5.9 Recommended Inductor Value vs. Application
Application
Network Impedance
at Communication
Frequencies
Inductor Impedance
at Primary Communication
Frequencies
C-band
Inductor
Value
A-band
Inductor
Value
Single building AC
mains
1-20 ohms
≥100 ohms ≥150µH
≥220µH
Inter-building mains
distribution
1-50 ohms
≥250 ohms ≥330µH
≥470µH
Dedicated cable
≤100 nodes
≤100m
50-100 ohms
≥500 ohms ≥680µH
≥1mH
Dedicated cable
>100 nodes
>100m
50-100 ohms
≥2000 ohms ≥2.4mH
≥3.9mH
There is one further constraint on the value of the inductor. When the inductor is combined with the input
capacitance of the switching supply, the LC resonant frequency should be at least one octave away from the
communication frequency range (110kHz-138kHz for C-band and 70kHz-90kHz for A-band). The unintentional
series resonance between the inductor's reactance and the power supply's capacitive reactance can produce a low
impedance at communication frequencies if this frequency range is not avoided.
One way to reduce the size and cost of an inductor used to raise a power supply's input impedance is to purposefully
parallel resonate it at the communication frequency with a capacitor, as shown in Figure 5.11. If this option is
chosen, resistive damping must be included so that impulse noise from the power line does not excite excessive filter
ringing, which could degrade the reception of weak signals. The parallel resistor has been selected to optimize
receive impedance and impulse noise damping. Note that even though only a 100µH inductor is shown in Figure
5.11, this circuit built with C-band values provides a series impedance of ≥200 ohms from 125kHz to 138kHz due to
the parallel resonant effect. When built with A-band values the series impedance is ≥150 ohms from 70kHz to
90kHz. Also, note that the inductor must be rated for the peak AC current drawn by the power supply.