AN4068 datasheet(54/63 Pages) STMICROELECTRONICS

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FAQs and troubleshooting

AN4068

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Doc ID 022923 Rev 2

signal level. A signal coupler (for example, a capacitive coupling) between the two

sides of the distribution transformers may be required.

●

Q: What method of coupling is preferred for a medium-voltage and low-voltage mains

line: capacitive or inductive?

–

A: For an MV line, capacitive coupling is preferable for narrow-band PLC. In the

case of an LV line, having an unpredictable actual line impedance because of the

number of electrical devices connected on it, the solution should be an L-C series

resonant circuit tuned at channel frequency, designed to have low Q even with

very low line impedance (5

Ω and below).

●

Q: Is it possible to detect the channel quality through the ST7580 device?

–

A: Yes. Using the ST7580 estimators for SNR, always available for the external

host, it is possible to evaluate the channel quality over the physical link between

transmitter and receiver.

●

Q: Why use zero crossing synchronization?

–

A: Zero crossing synchronization is not mandatory for the power line

communication, however it has several advantages.

For instance, it can improve the communication immunity against line noise, since

most of the electrical equipment generates noise on the power line in

correspondence with the mains voltage peak. Zero crossing synchronization

allows the establishing of the link between the transmitter and receiver during the

time with the minimum time-dependent noise.

Zero crossing synchronization is also needed for 3-phase communication. In a

case where one node must communicate with nodes that are connected on other

phases of the mains network, zero crossing synchronization understands which

phase a certain message is coming from via delta-phase calculation.

●

Q: What is the minimum signal-to-noise ratio that the ST7580 can manage?

–

A: A bit error rate (BER) of 10-3 is used as reference. In this condition, the ST7580

can have good receiving performance with a signal-to-noise ratio down to 3-4 dB

for B-PSK coded mode.

●

Q: What could be the main sources of harmonic distortion in the ST7580 transmitted

signal?

–

A: Generally, harmonics can increase because of:

High output current, due to low line impedance

Saturation of magnetic components in the line coupling circuit, due to either poor

dimensioning of the saturation current or to 50 Hz residual current

Capacitive load applied to the power amplifier output

Insufficient margin to the supply rails (low VCC or high output voltage).

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