Specifications

Chapter 5 – Power Supplies for PL Smart Transceivers
180 PL 3120/PL 3150/PL 3170 Power Line Smart Transceiver Data Boo
k
In some instances it is possible that noise radiated from either the power supply or the supply filter can couple into
the inductors of the coupling circuit of the PL Smart Transceiver. The coupling circuit can then couple this noise
onto the power mains. This problem can be diagnosed by disconnecting the transceiver's coupling circuit and then
analyzing the conducted line noise.
If noise from parasitic coupling is suspected, it can be confirmed by inserting a 10cm (4 inch) twisted wire pair in
series with one of the inductors in question. If the conducted noise spectrum varies by more than a few dB when this
inductor is moved closer to, and farther from, other components, then parasitic coupling might be the source of the
problem.
There is a second, although less likely, potential cause for reduced filter effectiveness. It is possible for the inductive
reactance of the filter components to be canceled by capacitive reactance from the input of the power supply. This
problem is generally seen as narrow band noise which appears to pass through the filter unattenuated. This problem
can be remedied by either damping the unintended resonance, or by adjusting the values of the filter inductor and
capacitor to move the resonance to a non-interfering frequency. Damping can be accomplished by adding resistance
in the range of 200 ohms to 5k ohms in parallel with any undamped power supply filter inductor(s).
Power Supply Output Noise Masks
Products incorporating a PL Smart Transceiver require a 5V (V
DD5
) supply and a second nominally 12V (V
A
) supply.
The amplitude of the noise and ripple on these power supply outputs must be controlled in order to comply with
CENELEC EN 50065-1 or FCC-conducted emission limits, as well as to maintain maximum communication
performance. Noise requirements to accomplish these goals are provided in Figures 5.20 and 5.21. Figure 5.20 shows
the noise limits on the V
A
and V
DD5
supplies for C-band while Figure 5.21 illustrates the A-band limits. In each band
the same limit lines are used to comply with both CENELEC EN50065-1 and FCC conducted emissions. Satisfying
these noise masks ensures that full performance of both operating frequencies is available to overcome unexpected
power line noise in either of the two frequency ranges.
Measurements should be made over the full range of anticipated loads on the supply, because many switching
supplies vary their switching frequency with load. For all CENELEC EN 50065-1 and FCC power supply
measurements a peak detector should be used. The measurement bandwidth for V
A
should be 3kHz. For V
DD5
the
two separate measurements must be made each a using different measurement bandwidth and each having a separate
noise mask. The measurements made using the 3kHz filter should be performed at all frequencies shown in the
graph, while the measurements made using the ≤300Hz filter only need to be performed at frequencies in the
communication frequency range of the PL Smart Transceiver (110kHz-138kHz for C-band and 70kHz-90kHz for A-
band). The video (post detection) bandwidth for all power supply output noise measurements should be 10Hz.
Note that these output noise masks should generally be checked even if the V
A
and V
DD5
power supplies are not
generated by a switching regulator since it is possible for switching loads to introduce noise onto an otherwise quiet
linear supply. An example of one type of device that can inject noise back onto a supply is an IC that includes an
on-chip switching regulator to generate additional internal supply voltages.
Figure 5.19 shows a probe that can be used to measure the noise on the power supply. The twisted wires must be
connected directly to the PL Smart Transceiver power and ground pins, and the coaxial cable must be connected to
the 50Ω measuring equipment. Note that the 1/10 gain of the probe must be taken into account.