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In some instances it is possible that noise radiated from either the power supply
or the supply filter may couple into the inductors of the transceiver's coupling
circuit. The coupling circuit may 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") 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 may
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 may be
accomplished by adding resistance in the range of 200 Ohms to 5k Ohms in
parallel with the filter inductor closest to the power supply.
Switching Power Supply Output Noise Masks
The PLT-22 transceiver re5VDC (V
DD5
) and +8.5VDC to +16VDC (V
A
)
supply voltages. 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 achieve maximum communication
performance. Noise "masks" are provided in figures 5.11 through 5.14. Figures
5.11 and 5.12 show the recommended noise (ripple) limits on the V
DD5
supply for
CENELEC EN 50065-1 and FCC compliance, respectively. Figures 5.13 and 5.14
show the recommended noise (ripple) limits on the V
A
supply for CENELEC EN
50065-1 and FCC compliance, respectively.
For each graph, two limit lines are indicated. The solid "single carrier" line shows
the noise limits required to maintain full performance if the transceiver is used
only in its single carrier frequency mode. If the PLT-22 transceiver is going to be
used in its dual carrier frequency mode, then it is recommended that the power
supply noise fall below the dashed "dual carrier" line. Meeting this criteria
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, since many switching supplies vary their switching frequency with load.
For both CENELEC EN 50065-1 and FCC measurements, a quasi-peak detector
should be used. The measurement bandwidths are 200Hz for measurements
below 150kHz, and 9kHz for measurements above 150kHz.
Figure 5.15 shows a probe that can be used to measure the noise on the power
supplies. The twisted wires must be connected directly to the PLT-22 power and
ground pins, and the coaxial cable must be connected to the 50
Ω
measuring
equipment. Note that the 1/10 gain of the probe must be taken into account.
L
ON
W
ORKS
PLT-22 Transceiver's User Guide
5-17
Содержание LONWORKS PLT-22
Страница 6: ...iv Echelon...
Страница 14: ...1 8 Introduction...
Страница 67: ...LONWORKS PLT 22 Transceiver s User Guide 5 7 Figure 5 3 Capacitor Input Power Supply Schematic...
Страница 92: ...6 10 Design and Test for Electromagnetic Compatibility...
Страница 110: ...7 18 Communication Performance Verification...
Страница 114: ...8 4 References...
Страница 118: ...A 4 Appendix A...