Tunable tapped combline bandpass filters for RFID systems

Design, simulation and measurement results of a tunable planar bandpass filter with a tapped combline topology using diodes as active devices for varactor effect is given. Tunable planar bandpass filter that can operate from 900MHz to 1.6GHz with an enhanced attenuation characteristics are obtained. Diodes are used with an applied bias voltage to have capacitance variation from 0.84 [pF] to 5.08 [pF]. The proposed method eliminates the need for using different active devices and bias voltages and simplifies the geometry that eases implementation. The simulation and measurement results are compared and found in agreement. The method presented in this paper can be integrated in RFID systems to improve the common leakage problem in TX-RX due to poor isolation in circulators.