A Compact X-Band Tunable Bandpass Filter Module Using a Pair of Microstrip Composite Bandpass Filters in Cascade

In this paper, a compact X-band tunable Bandpass Filter (BPF) module which employs a pair of passive microstrip composite-BPFs in cascade on RT/Duroid 6010 substrate is reported. The passive microstrip composite-BPF was realized by combining four open-circuited stubs of a microstrip X-band BPF with four inductive elements of a stepped impedance low-pass filter. Placement of one YIG/GGG layer upon each of the passive microstrip composite-BPFs enables the resulting composite-BPFs to function simultaneously as an X-band BPF and a tunable Band-Stop Filter. By tuning the bias magnetic fields to 1780 and 2670 Oe, 2450 and 3250 Oe, and 1780 and 3 250 Oe on the pair of the composite-BPF, respectively, the passband of the module was tuned toward the high end, the low end, and the center frequencies. The corresponding center frequencies of 12, 7.9 and 10 GHz, minimum 3 dB bandwidth of 0.6, 0.4, and 0.5 GHz, respectively, and an insertion loss of 2.5 dB were measured. Compared to the BPF module reported recently, reductions of 43.5% in circuit area and 52.8% in insertion loss were achieved. The minimum 3 dB bandwidth was narrowed by 40%, 63.6%, and 58.3%, and its tuning ranges were extended from 3.7, 3.6, and 3.5 GHz to 4.1, 4.3, and 4.2 GHz, respectively. This newly realized tunable BPF module has demonstrated superior features such as more compact device structure, smaller device dimensions, lower insertion loss, and improved frequency tunability and selectivity.