Sub-octave-tunable microstrip notch filter

A frequency-agile narrowband microstrip bandstop filter technology able to maintain near constant absolute bandwidth while tuning over a frequency range of nearly an octave is described. The technology is demonstrated by a reconfigurable notch filter designed to selectively remove unwanted spectral content prior to final amplification in a transmitter's output module. The six-resonator planar microstrip notch filter uses hyperabrupt GaAs varactor diodes to tune the operating frequency over a 92% range, from 480 MHz to 925 MHz. The filter maintains 3MHz-wide stopband attenuation of 34dB to 64dB, absolute 3dB bandwidth of less than 84 MHz, 3dB bandwidth variation of less than 24%, and low stopband reflection over the full tuning range.