Monday, April 9, 2012

Simulating Power Line Communication Signals

Power line communication or power line carrier (PLC) is getting a lot more attention these days since it is used in many of the new green energy electronics such as smart grid devices, solar inverters, and home automation. PLC is communication technique that uses the power wiring in buildings or grid power transmission lines as its communication channel. In this post we will look how you can easily generate complex PLC signals for test purposes with a low cost function / arbitrary waveform generator (FG/AWG). For a more general overview on PLC click here.

Generating communication signals for testing typically requires costly test equipment like a signal generator for the carrier and a FG/AWG for the baseband. For PLC signals we can skip the costly signal generator and just use a modern FG/AWG. There are two main reasons we can skip the signal generator for simulating PLC signals. The first one is the carrier signal for PLC is typically less than 1 MHz so it falls well within the bandwidth capabilities of a FG/AWG. The second is modern FG/AWGs have advanced features for creating complex signals. These features include:

  • Large waveform memory for storing not only arbitrary waveforms, but also arbitrary signals.
  • Arbitrary waveform sequencing, which is analogues to a playlist on an MP3 player. It allows you to seamlessly combine multiple waveforms from memory to create a complex signal. Click here for more on waveform sequencing.
  • Optional second independent channel for creating an I and Q signal.
  • Advanced modulation capabilities such as waveform summing, modulating an arb with an arb, and for two channel FG/AWGs the ability to modulate the signal from one channel with the other channel.
Let's look at a couple of examples using Agilent's 33522A FG/AWG. Here is a simple example just using built-in waveforms. In the below screen shot from the 33522A, a BPSK signal with a 135 KHz carrier was created. For the baseband a built-in waveform known as a pseudo random bit stream (PRBS) was used. The PRBS waveform just delivers a close to random stream of 1s and 0s at a chosen bit rate, for this example 5.5 kbps was used. Of course an arbitrary waveform made up of real data could have been used for the basedband as well.

For the second example lets look at something a little more complicated. In this example a QPSK signal with a frequency hopped spread spectrum carrier was created using Matlab. The bit rate of the digital data was 10 Kbits/s. The signal lasts for 15 ms and consists of >500,000 data points. The waveform was transferred to the 33522A via a USB stick and a CSV file. Below is a screen shot of a portion of the signal.

In this example we used the FG/AWG's large waveform memory to output a large arb file (greater than 500K points) to create a 15 ms signal segment with the baseband modulation and the frequency hopping already in signal. This frees up the FG/AWG's modulation capabilities for other purposes such as simulating communication channel noise. Below is an example of using the modulation function to add some channel noise. The noise signal was a sharp pulse signal representing a large load transient on the power line. This was done on the 33522A by using the "sum" modulation feature. The source of the pulse noise signal was channel two on the 33522A.

In this post we talked about using a low cost function / arbitrary waveform generator for creating PLC signals. There are two main reasons why a FG/AWG makes a great solution for simulating PLC signals compared to a signal generator. The first is the carrier frequency of PLC signals is well within the capabilities of a FG/AWG. Second modern FG/AWGs have features like a large waveform memory, waveform sequencing, and advanced modulation capabilities. If you have any questions related to this post please email me and if you have anything to add use the comments section below.


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