By Rahul Parsani , Cypress Semiconductor

Powerline communication (PLC) has gained a lot of traction in recent years as an alternative and cost-effective networking technology that requires no new wires. There are powerline outlets everywhere in homes and office buildings, making it an all-encompassing network and the largest infrastructure which can be used for data communication. However, even today it has remained an engineering challenge to communicate data reliably on the Powerline. The reliability challenge is partly due to the varying nature of the powerline itself and the loads (such as electrical appliances, motors) connected to the powerline.

Studying the powerlines in more detail reveals some interesting facts about the effect of loading on the performance of PLC. A PLC transmit signal (Tx) that reaches the receiver may be too small to be successfully received, mainly due to the impedances of the loads and the powerline.

The impedance of the powerline can be categorized into line impedance and load impedance as shown the Figure 1 below. Z_LINE is a property of the powerline and Z_LOAD is a property of the load connected to the powerline.

Figure 1

Now let us connect two PLC nodes to the impedance model shown above and with the help of some mathematical relations, we will see how the signal levels are related to the various impedances.

Fig. 2

As indicated in Figure 2, Z_TX is the output line impedance of the transmitter and Z_RX is the input load impedance of the receiver. For PLC, the problem arises when the load impedance, Z_LOAD, is low (from sub-Ohms to < 10 Ohms). When Z_LOAD is low, the signal tends to flow to ground through Z_LOAD – following the path of least resistance – causing significant attenuation of the signal. Because Z_LOAD  and Z_RX are impedances in parallel, low values of Z_RX can actually drive the overall impedance much lower than Z_LOAD thereby attenuating the signal further. So, in the interest of successful communication, the value of Z_RX is chosen to be much higher than the usually low Z_LOAD values.

Now, let’s have a look at the effect of load placement on the powerline. In Figure 3 we can see that the load (Z_LOAD) is placed close to Node A.

 

Fig. 3

In such a case, when Node A is transmitting, the voltage divider does not attenuate the signal as much as when Node B is transmitting. This is how it works: Let us consider some impedance values - Z_TXA and Z_TXB are very small (~3 ohms) ,and Z_RXA and Z_RXB are large (~1000 ohms). Also, assume Z_LOAD is 10 ohms and Z_LINE is approximately 50 ohms for longer distances (more than 100 meters). Now when Node A transmits, the voltage at V_A is ~75% [= (Z_LOAD||(Z_LINE+Z_TXB+Z_RXB)/(Z_TXA+(Z_LOAD||(Z_LINE+Z_TXB+Z_RXB))] of the original transmit voltage and the voltage at V_B is ~95% [(Z_RXB+Z_TXB) / (Z_RXB+Z_TXB+ Z_LINE)] of V_A. On the other hand, when Node B transmits, the voltage at V_A is ~15% [(Z_LOAD||(Z_TXA+Z_RXA) / ((Z_LOAD||(Z_TXA+Z_RXA) + Z_TXB + Z_LINE)] of the original transmit voltage. This means that the voltages at both V_A and V_B change depending on which node is transmitting. They are higher when the node closer to the load impedance is transmitting.

Now that we have considered the impedance issues, let us see how we can debug whether the signal attenuation is because of distance or low load impedance. As we have seen before, when the load impedance of the powerline is lowered due to a low-impedance load, the signal at the transmitter (V_TX) itself drops down. When it is a line-impedance or a distance problem, there is a uniform drop in the signal from the transmitter to the receiver. This means if there is a considerable difference between the signal levels at the transmit- and receive-ends, the signal attenuation is due to long distance. If the voltage signal levels at both the ends are not that different but the transmit end itself has a significantly low signal level, we know that it is a load impedance problem. Also, a low transmit signal level along with a uniform drop till the receiver would imply both long distance and low load impedance.

The powerline impedance model helps us in understanding the mysterious behavior of powerline which changes when different electrical loads are connected to it. The network impedance and signal attenuation are affected by the network topology and by other devices connected to the same network at a given time. As a consequence, the maximum communication distance can widely vary from one network to another or even from one time to another on the same network. To battle out these drastic impedance conditions of any powerline network a Powerline Communication solution should have a low transmit impedance and a high receive impedance at the same time to ensure maximum voltage signal transfer between the two communicating ends.

Author Bio

Rahul Parsani is an Applications Engineer for the Powerline Communications group at Cypress Semiconductor. He has a Bachelor's Degree in Electronics and Instrumentation Engineering from the Birla Institute of Technology and Science Pilani, Goa Campus. He can be reached at This e-mail address is being protected from spambots. You need JavaScript enabled to view it