Echelon LONWORKS PLT-22, User Manual

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Noise at the Power Supply Input
In order to achieve maximum communication performance and to comply with
the conducted emissions specifications of CENELEC EN 50065-1 and FCC Part
15, the switching power supply input must not conduct excessive noise onto the
power mains. A switching power supply contains an oscillator that operates at a
frequency between 10kHz and several MHz. Depending on the power supply
design, significant energy at the switching frequency fundamental and/or its
harmonics may appear at the input (line side) of the power supply. If the
amplitude of the signal is large enough in sensitive frequency ranges, the
performance of the PLT-22 transceiver may be degraded.
While the PLT-22 transceiver is designed to operate reliably with a combination
of attenuation and significant switching supply noise, noise from the switching
power supply closest to the receiver will have the most deleterious effect on
communication performance. Noise from more distant sources will be attenuated
before reaching a receiver, reducing its effect on reception.
Switching Power Supply Frequency Selection
Selection of an appropriate operating frequency for a switching power supply will
minimize the effect of switching noise on the PLT-22 transceiver's
communication performance. By choosing an operating frequency such that the
fundamental and harmonics of that switching supply avoid the transceiver's
communication frequencies, the amount and cost of additional circuitry to filter
interference from the switching supply can be minimized. Table 5.4 below lists
the optimal switching frequency ranges. The switching supply should be
designed such that the fundamental switching frequency falls within the stated
ranges under all line, load, environmental, and production conditions.
Table 5.4 Optimal Switching Power Supply Fundamental Operating Frequencies
Optimal Frequency Ranges
80kHz-110kHz
>155kHz
Switching Power Supply Input Noise Masks
The noise "masks" presented below show the maximum noise allowable at the
input of a switching power supply such that optimum performance of the PLT-22
transceiver is achieved and the appropriate conducted emissions specification is
met. Figure 5.7 defines the noise mask for CENELEC EN 50065-1 conducted
emissions compliance. Figure 5.8 defines the noise mask for FCC-conducted
emissions compliance. The limits assume that the PLT-22 transceiver is used in
conjunction with one of the recommended coupling circuits shown in Chapter 4.
Measurements of a particular power supply should be made by connecting the
supply to the artificial mains network as specified in subclause 8.2.1 of CISPR
Publication 16, second edition. Measurements should be made over the full
range of anticipated loads on the supply since many switching supplies vary their
switching frequency with load. Two different limits are shown for the CENELEC
5-12 Power Supplies for the PLT-22 Transceiver