A defected ground structure based adaptive modulator over a UHF RFID frequency bands

In this paper, simpler and cost-effective way of modulator design using defected ground structure is presented. This is motivated from three benefits that can be obtain from defected ground structure; an extensive resonance, size reduction, excellent stopband characteristic. An extensive resonance via a microstrip ground plane configuration, which includes a varactor diode bridging microstrip transmission line with variable capacitance provision that can be controlled, gives on/off switching and its attenuation level (OOK is a 100%, and which less than 100% can be ASK) that plays the key role of the ASK and OOK modulation process. A varactor diode is under control of a Micro Control Unit that can be controlled by user interface software; therefore, user can monitor communication states while each operational sequence of a varactor diode can be controlled. Sharper phase and more precise data modulation can be achieved after several tuning works and practical trials. A modulation index (about 90 to 93%) with stable tag responses over the target bands (860-to-960MHz, centered 912MHz) was verified through the practical measurements on the RFID reader communication using ASK. And when a modulation index was nearly 99% (approximately 98.8%), this is considered to be an OOK. Overall radio communications are with EPC class-1 Gen2 Type-B and Type-C passive tags, respectively.