Specifications

PL 3120/PL 3150/PL 3170 Power Line Smart Transceiver Data Book 189
Table 6.1 EMC Suppression Capacitor Value vs. Application
Application
Network
Impedance at
Communication
Frequencies
Capacitor
Impedance at
Primary
Communication
Frequencies
A-band
Capacitor
Value
C-band
Capacitor
Value
Single building
AC mains
1-20 ohms ≥250 ohms ≤4700pF ≤4700pF
Inter-building
mains
distribution
1 - 50 ohms ≥500 ohms ≤3600pF ≤2200pF
Dedicated cable
≥100 devices
≥100m
50 - 100 ohms ≥1000 ohms ≤1800pF ≤1200pF
Dedicated cable
>100 devices
>100m
50 - 100 ohms ≥2500 ohms ≤680pF ≤470pF
Under no circumstances should a capacitor >4700pF be used directly between line and neutral because it will
result in excessive signal attenuation.
Another common method of EMC suppression, the addition of ferrite beads, is unacceptable if high impedance beads
are placed anywhere in the transmit signal path. Most ferrite beads have an impedance of several ohms at 100kHz. The
impedance of any element placed in series with the transmit signal or return path must be less than 0.5 ohms at
communication frequencies, as described in chapter 4.
There is, however, one means whereby a higher impedance ferrite bead can be used to reduce common mode high
frequency emissions without affecting the transmit signal. If both the communication signal and its return conductor
(i.e., Line and Neutral for L-to-N coupling or Line and Earth for L-to-E coupling) pass through the same bead in a
common-mode fashion, the bead will not add any series impedance to the transmitter. This is true because the signal
currents in the two conductors produce opposite polarity (canceling) flux in the ferrite bead’s core. Common mode noise
of equal polarity on both conductors will produce additive flux in the ferrite bead’s core and will thus be attenuated.
Figure 6.1 illustrates both acceptable and unacceptable topologies for high-impedance ferrite beads.
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