Specifications

Chapter 6 – Design and Test for Electromagnetic Compatibility
188 PL 3120/PL 3150/PL 3170 Power Line Smart Transceiver Data Book
EMI Remedies
If a device does not pass conducted emission regulatory limits it is helpful to know whether the failing frequencies are
from a switching power supply (fundamental or harmonics), communication harmonics, or other frequencies from other
digital circuitry. If a failing frequency is related to the power supply, refer to the sections of Chapter 5 that cover power
supply noise and mitigation techniques.
If the device under test fails at a harmonic of the communication frequency (i.e., an integer multiple of 131.6kHz for C-
band or 86.2kHz for A-band) then the most common reason for failure is overload of the measuring instrument as
described in the previous section. The second most common cause of a failure at a communication harmonic is the use
of a switch mode power supply with an impedance raising inductor (described in Chapter 5) that saturates and thus does
not provide the proper impedance. Another source of failure at communication harmonics is the use of coupling circuit
components that do not meet the specifications listed as “Required Specifications” for each example coupling circuit of
Chapter 4.
If the unit under test is found to exceed the applicable conducted noise limit at frequencies other than those related to
power line communication or a switching power supply, it is likely the result of unintentional coupling of noise from
various other digital circuits. If this occurs, improvements in grounding and printed circuit layout are often required. It is
also possible for the coupling circuit components to pick up stray fields from nearby circuitry and then conduct it onto
the power mains. Refer to Chapter 4 for a discussion of how to avoid stray field pickup in coupling circuits.
In some instances conducted emissions above 500kHz can be adequately reduced by the addition of a small value
capacitor (e.g., 470pF) either across the AC mains or from the line conductor to ground. While devices using the PL
Smart Transceiver have been demonstrated to pass CENELEC and FCC limits without an additional capacitor,
variations in node design and layout might require the addition of this small value capacitor. If a capacitor is added
across the line it should be an X2 safety-rated type for maximum surge reliability. If capacitors are added from either
line or neutral to earth, they should be Y safety rated. Alternately, this capacitor can be added across coupling circuit
inductor L101 (see Figure 4.2) or across the line-side winding of transformer T101 (see Figure 4.1). If this option is
chosen, either a metallized film capacitor of ≥250VDC, a ceramic 1000VDC capacitor, or a Y-type capacitor should be
used for surge reliability. Note that this extra capacitance should only be added to the line side of the coupling
transformer and not to the transceiver side of the transformer.
Adding capacitance in the above locations reduces the input impedance of the device and could therefore cause an
increase in communication signal attenuation. The maximum value of capacitance which can be added without
significantly affecting attenuation depends on the application. Table 6.1 shows the maximum value of added capacitance
by application.