AN4444 datasheet(40/68 Pages) STMICROELECTRONICS

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Reference design standard tests

AN4444

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DocID025925 Rev 2

Reference design standard tests

9.1

Input impedance

The input impedance of a power line communication node is another critical point.

According to the network impedance measurements carried out in some European

distribution networks (Italy, Germany and France) the following characteristics can be

associated to the impedance of a typical low voltage (LV) power line:



Typical impedance magnitude is around 5





Nearly 90% of measured values range between 0.5 and 10





The impedance value depends on the measurement point



The measured value changes over time.

The reasons for these characteristics can be described as follows:



The LV distribution network has a “tree” structure, with many branches and sub-

branches acting as parallel impedances



Several electronic devices connected to the LV network offer a very low impedance,

mostly because of the EMI input filters installed at their mains connection



The type and number of electronic loads connected to the mains network varies over

time.

For all these reasons, particular attention must be paid to the impedance of the ST7590 line

coupling circuit. Specifically:



In receiving (idle) mode, the coupling impedance must be high enough to make the

power line source impedance negligible and to minimize the mutual interference

between different PLC nodes connected to the same network



In transmitting mode, the coupling impedance must be very low inside the signal

bandwidth but high enough for out-of-band frequencies.

According to such requirements, the EN50065-7 standard document fixes the following

constraints for the PLC node operating in the A band (see Part 7 of Section 14: Normative

references on page 63):

In Rx mode, minimum impedance is:



 from 3 to 9 kHz



 between 9 and 95 kHz only inside the signal bandwidth (free for frequencies

outside the signal bandwidth), which means 50

minimum from 42 KHz to 89 kHz for

the ST7590 device



 from 95 to 148.5 kHz.

In Tx mode, minimum impedance is:



Free in the range 3 to 95 kHz



 from 95 to 148.5 kHz.

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