Design of Digitally Reconfigurable Bandwidth and Center Frequency Bandpass Filter

This paper presents design and development of a digitally reconfigurable bandwidth and center frequency bandpass filter for variable data rate communication systems. Bandwidth and center frequency of bandpass filter are reconfigured using varactor diodes present in the lumped tubular filter topology. The proposed filter offers high selectivity with symmetrical pass band over the wide tuning range. Reconfigurable bandwidth and center frequency are achieved by varying the capacitances of series and shunt varactor diodes present in the circuit. Reverse bias voltage of varactor diodes is controlled by digital control circuit. Digital control circuit comprising of a controller and a multi-channel digital to analog converter which enables different combinations of bias voltages required to configure the bandwidth and center frequency of the filter. Fractional bandwidth can be configured from 4 to 20 percent of the center frequency by sending digital control word through serial port interface. Also, center frequency of the filter can be tuned from 140 MHz to 180 MHz. The measured performance of the filter shows good agreement with the simulation.