White Paper-Smart Grid – Power Line Communication

2.1 PLC vs. EMC  (Power line communication verses Electro-magnetic compatibility)

Electromagnetic Compatibility (EMC) regulation normally limits interfering emissions from electronic devices but most EMC standards have a lower frequency limit of 150 kHz. This means that especially the European PLC frequency band is largely uncontrolled when it comes to conducted emissions of connected equipment.

3.0 Recognizing the interference

So what does interference look like? The frequency of an interfering signal is the most common parameter leading to the identification of the interfering source. Thus, an interference problem can often be categorized by its frequency characteristics.

It should be noted that whether the interfering signal is in-band or out-of-band, the signal is almost certainly coming through the antenna, down the cable, and into the affected receiver. Therefore, a spectrum analyzer connected to the operating system antenna will serve as a substitute measuring receiver which will display and help identify unwanted signals. Remember that the system’s band pre-selection filters are inside its receiver, so many out-of-band signals are naturally present at its antenna input connector.  Interference generally only affects receiver performance. Although it is possible that a source of interference can be physically close to a transmitter, the characteristics of the transmitted signal will not be affected. Thus, the first step in recognizing if interference has corrupted a receiver is to learn the characteristics of the signal that the affected system is intended to receive.

By analyzing the frequency domain using a spectrum analyzer the signal frequency, power, harmonic content, modulation quality, distortion and noise or interference can easily be measured. If interference is overlapping the intended receiver signal, it will be relatively obvious on the spectrum analyzer display.

In figure 2 the transmitted spectrum of a PLC system is displayed. The actual signal is the saddle shape pointed out with the red arrow.

A displayed interference “fingerprint” contains important identification characteristics. As mentioned above different causes of interference will typically have a different kind of spectrum.

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