Impedance Matching with Low-Cost, Passive Components for Narrowband PLC Mloyiswa P. Sibanda † , Petrus A. Janse van Rensburg † , and Hendrik C. Ferreira ‡ † Department of Electrical Engineering ‡ Department of Electrical and Electronic Engineering Walter Sisulu University University of Johannesburg P.O. Box 1421, East London, 5200, South Africa P.O. Box 524, Auckland Park, 2006, South Africa Phone: +27-43-702-9262, Fax +27-43-702-9226 Phone +27-11-489-2463, Fax +27-11-489-2357 e-mail: msibanda@wsu.ac.za; pvanren@wsu.ac.za e-mail: hcferreira@uj.ac.za Abstract —The impedance variation in power-line networks is a well known problem in power-line communications (PLC). These variations arise when different electric loads are connected and, or disconnected to the power line network at different times. This causes a problem in trying to achieve maximum power transfer, hence the need for a matching circuit. Previously we investigated transformerless power-line coupling circuits, with a view of simplifying the design and minimizing costs. Therefore, we used only passive L-C components. The second step of this project is to design an accompanying adaptive impedance matching network. The main objective is to improve on mismatch power-losses, while at the same time it must be an economical solution. In this paper we outline basic impedance matching procedures and the necessary power-line environment considerations. The design of some typical passive impedance matching circuits for narrowband PLC is also discussed. We further investigate the suitability of such a network in a low- voltage residential power-line network. Keywords — Coupling circuits, passive components, filters, impedance matching. I. INTRODUCTION HE fluctuating network impedance in power-line networks is a well documented hindrance to power-line communications. These impedance variations are caused by different electric loads connected and disconnected to the power line network at different times. Therefore mismatch losses typically account for a large portion of the attenuation experienced between transmitter and receiver. Further, accurate impedance matching is practically impossible and has thus been neglected as a sub-field of power-line communications. Also, the coupling interface together with impedance adaptation components make up the physical boundary between sensitive modem equipment and harmful power voltages and currents, and therefore pose many challenges to the designer. Fig. 1 shows simulated results of how impedance levels would impact on the received signal through a band-pass coupler, evaluated in [1]. These curves show an increasing attenuation and distortion of the signal as the power-line impedance drops, thus aggravating mismatch-losses. Fig. 1. Simulated mismatch losses between transmitter and power line in the Cenelec band. As the power-line impedance drops from 50 ohms to 25 ohms, 12.5 ohms and 5 ohms, the relative dB-loss at 110 kHz is ≈ -3 dB, ≈ -5 dB, and ≈ -13 dB. (Here the reference level is -6 dB) In [1], the design of an economic, passive, transformerless coupler was described, however impedance matching was not considered. As a second stage of this project, impedance matching is explored in this paper. Although the final goal is to achieve automated impedance adaptation, this paper only looks into the design and validation of passive L-C impedance matching circuits, and explores practical implications for PLC. Generally, impedance adaptation circuits for PLC leverages the impedance transformation properties of some transformer, often the coupling transformer itself, see e.g. [2]-[6]. Also, an adjustable inductor of some sort is often used to fine-tune the process. In [7] however, a 1:1 coupling transformer is used (no impedance adaptation), rather a bank of coupling capacitors are switched between to facilitate impedance adaptation. Take note that these coupling capacitors reside on the power-line side where power voltages prescribe expensive capacitors. In this paper, all impedance matching circuitry is utilized on the modem-side of the coupler, therefore economical, low- spec components may be used: The L-C bandpass coupler described in [1] is used as the first line of offense to filter 50- Hz power voltages and currents, thereby relieving the impedance matching circuitry, allowing it have low-voltage and low-current specifications with substantial cost implications. T 2011 IEEE International Symposium on Power Line Communications and Its Applications 978-1-4244-7750-0/11/$26.00 ©2011 IEEE 335