AN4068 datasheet(34/63 Pages) STMICROELECTRONICS

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Reference design standard tests

AN4068

34/63

Doc ID 022923 Rev 2

Reference design standard tests

8.1

Input impedance

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

According to the network impedance measurements carried out in certain 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 ST7580 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 [5]:

●

Tx mode:

–

free in the range 3 to 95 kHz

–3

Ω from 95 to 148.5 kHz

●

Rx mode:

–

Ω from 3 to 9 kHz

–

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

outside the signal bandwidth)

–5

Ω from 95 to 148.5 kHz.

Figure 20 and 21 show the input impedance magnitude vs. frequency measured in

transmission and reception mode.

The impedance magnitude values prove that the ST7580 reference design is compliant with

the EN50065-7 requirements. At the same time, the line interface gives an efficient signal

coupling both in transmission and reception.

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