A Microstrip Lowpass Filter With Wide Tuning Range and Sharp Roll-Off Response

In this brief, a compact lowpass filter (LPF) with wide stopband, very sharp roll-off response and wide tuning range is presented for microwave diode detector applications. It employs two hexagonal-shaped resonators and multiple open-end stubs as suppression cells. The results of the proposed hexagonal-shaped resonator and LC circuits are in good agreement. The 3 dB cut-off frequency for the proposed LPF is 2.2 GHz and the roll-off-rate (ROR) parameter for the proposed LPF is very sharp and equal to 308.6 dB/GHz. Furthermore, insertion loss (IL) and return loss (RL) in the passband region are better than 0.3 dB and 18 dB, respectively. Moreover, the stopband is wide, ranging from 2.34 to 18 GHz, with a 22 dB rejection level. The tuning range of 3 dB cut-off frequency is also wide, equal 0.48 to 1.7 GHz (71%). As an application demonstrator, an envelope detector was designed and fabricated. The proposed detector (with LPF) highly suppresses any harmonics of the fundamental frequency. The proposed LPF and microwave detector is designed, simulated, fabricated and measured. The results of simulation and measurement are in good agreement.