Integrated lowpass and C spectrum band pass filter using miniaturized stub based model

Abstract

A stub-based model for a miniaturized lowpass circuit integrated with C spectrum bandpass response is developed in this brief. The proposed integrated filter structure has two sections, which consist of upper stub based lowpass model with 3 dB cut-off frequency of 3.03 GHz and lower inverted T shaped (ITS) stub structure having bandpass response within C band. The symmetrical stub sections along with an H shaped transmission line increases the lowpass suppression response. The passband of 5.56 GHz to –7.12 GHz is realized in the stopband region of the lowpass response. The final design provides a normalized circuit size of 0.037 λg2 with stopband noise rejection up to 25 GHz after the upper cut-off of the bandpass response. The in-band return loss of 20.6 dB is achieved using the half of quarter wavelength model of ITS. The fractional bandwidth of 24% is observed with out-band suppression of 25 dB. The lowpass and bandpass circuit geometry is optimized and explained individually. The proposed structure is fabricated and measured which is in admirable agreement with the simulated results. This compact stub-based lowpass-bandpass filter prototype can be used for C band satellite uplink communication applications.