Communications Testing in a Power Line Environment

June 17, 2011

Power Line Communication (PLC) devices pose challenges for testing, as a system under test must obtain power from, and communicate over, the same medium. As a result, the transmission medium for the system under test must be connected to the power mains, a noise and interference-filled environment that can make controlled, repeatable testing a challenge.

G.hn - Broadband Home Networking PLC

ITU-T G.9960 (G.hn) is an emerging home networking standard that can operate over phone line, coax, and, perhaps most prominently, power line. G.hn specifies baseband bandplans for all three media types utilizing frequencies from 1MHz to as high as 100MHz. Power Spectral Density (PSD) masks for power line bandplans limit the power transmitted to –55dBm/Hz up to 30MHz, and to –85dBm/Hz for frequencies above 30MHz. It is thus critical to minimize noise and interference up to 100MHz to allow for accurate measurements of signal characteristics such as PSD and Error Vector Magnitude (EVM).

PLC Noise Environment

Power mains is a high-noise and high-interference transmission medium. This interference can corrupt measurements, and produce non-repeatable results. Such noise and interference can fall into several categories, including broadband colored background noise (typically containing a greater PSD at lower frequencies), impulse noise from switching power supplies, and sinusoidal or narrowband interference from harmonics of the 50Hz or 60Hz power and even radio stations. For instance, without proper RF isolation, commercial AM & FM radio stations may appear in G.hn PSD measurements! Also, interference from alien PLC networks must be taken into consideration, especially in a testing environment with simultaneous testing of multiple PLC networks.

Creating a PLC Test Environment

An effective way to create a test environment for a PLC network such as G.hn is to use an isolation transformer and a line filter. The isolation transformer provides ground isolation of the PLC network from the power mains. The secondary winding and the system under test will not be connected to earth ground, thus reducing the risk of shock and eliminating common mode noise referenced to ground. This transformer may also be used to convert from the voltage of the native mains to the voltage of devices in the system under test, allowing for devices from other regions to be tested.

The line filter must remove noise and interference over the frequency range of transmission from the system under test while passing AC power. In the case of G.hn, for example, the line filter must provide minimal insertion loss at 50Hz or 60Hz, while providing high insertion loss from 1MHz to 100MHz. Such filters must be rated for the maximum voltage and current used for the entire system under test. These filters may be carefully designed, or can be obtained commercially. Reasonably priced filters with greater than 60-90dB of insertion loss over the required frequency range with a max current of up to 6A and a max voltage of 250 VAC can be found readily, and should meet the requirements of most PLC systems under test.

Other considerations in the creation of a PLC test environment include coupling and RF isolation. Power cords are unshielded, and can easily be susceptible to coupling from unfiltered lines and RF signals. Power cables for the system under test should be quite short, and should not be placed near power cables connected to the unfiltered power mains. Also, sensitive measurements such as PSD should occur in an RF isolation chamber, as unshielded power cables may act as an antenna and receive radio signals such as those from local FM radio stations.