AN4068 datasheet(49/63 Pages) STMICROELECTRONICS

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AN4068

Application ideas

Doc ID 022923 Rev 2

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Application ideas

10.1

3-phase architecture

The ST7580 modem can be used to communicate on a 3-phase network. This is especially

required for low-voltage substation nodes (concentrators), collecting data from several

energy meters all along the three phases of the distribution network.

In the example scheme of Figure 40, the line coupling circuit allows the signal to divide into

the three phases via capacitive coupling. That structure has been designed to keep similar

impedance on each phase, therefore optimizing the signal distribution between the phases.

A critical point regarding this solution may be the total impedance that the ST7580 power

amplifier is required to drive, which is the result of the three phases in parallel. For

concentrator nodes, however, the impedance per phase is likely to be considerably above

the driving limit of the power amplifier, as all the electrical devices supplied by the power line

are placed at a certain distance from the substation.

Figure 40.

Scheme of principle for non-switched 3-phase architecture

In the switched coupling scheme of Figure 41, a more complex circuit is shown, being the

coupling to each phase selectable via opto-switches.

Only one phase at a time can be used to transmit. The received signal, however, can be

picked up from either one phase at a time (J1 closed, J2 open) or any phase at the same

time (J1 open, J2 closed). Both solutions can work well: the first solution offers the

advantage of reducing crosstalk between the three phases, while the second allows the

whole network to be listened to at the same time. The choice depends on electrical and

performance tests as well as on specific protocol requirements.

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