Tunable Coaxial Bandpass Filters Based on Inset Resonators

Abstract

A new class of compact, high-Q, tunable coaxial filters is presented in this paper based on a novel inset resonator concept. The tuning concept is based on the displacement of movable resonators inside a properly modified metallic housing which features wide tuning capabilities and stable high Qfactor performance with minimum variation throughout the tuning window. Various prototypes are designed and implemented to demonstrate and validate the proposed concept. A single tunable inset resonator is first designed and measured showing distinctive results of a 43% tuning range, stable high-Q of 4100±4%, spurious-free band up to 3.8•f0, and volume-saving up to 50% when compared to the conventional combline and halfwavelength structures. The design procedure for constant absolute bandwidth (CABW) tunable filters is then presented, and two different tunable inset filters are designed and implemented. Firstly, a manually tunable four-pole filter is demonstrated with the merits of a wide 39.3% tuning range, while maintaining a constant bandwidth of 116 MHz±6%, and a stable high-Q of 1820±6%. Next, an automatically tunable 3rd-order inset filter is designed and measured using high-accuracy piezomotors. Similarly, the measured results exhibit a wide 1.3 GHz tuning range from 2.65 GHz -3.95 GHz with a stable insertion loss that is less than 0.35 dB, a return loss that is better than 15 dB, and a good spurious performance up to 2.8•f0. To our own knowledge, the proposed tuning technique and tunable components represent state-of-the-art tuning range and stable high-Q with minimal variation when compared with similar loaded-waveguide designs.

Widely Tunable TM-Mode Dielectric Filters With Constant Absolute Bandwidth Using Re-Entrant Caps

“…It is known from [42] that the physical inter-resonator (IR) and input-output (IO) couplings should also change with the frequency tuning to maintain a constant ABW and a stable return loss level. In practice, keeping this harmony is a very challenging task, especially for broad tuning ranges.…”

“…The desired design is an octave tunable filter from 2.67 GHz to 1.17 GHz (tuning ratio 2.28:1) with a constant 20-dB bandwidth of 38 MHz. Following the design procedure of CABW tunable filters detailed in [42], the required K 12 × f product and reflection group delay are calculated as 0.063 and 11.2 nS, respectively. Then, one M5 tuning screw and two M2 tuning screws were introduced at both filters to obtain the desired IR and IO couplings, respectively.…”

“…The filter is designed to operate from 2.86 GHz to 1.34 GHz with a constant 20-dB bandwidth of 71 MHz. Similar to the previous example, the design procedure begins from the desired reflection group delay (8.3 nS) and IR couplings (K (12,34) × f = 0.066, K 23 × f = 0.05) calculation based on [42]. Next, the required reflection group delay is realized through the IO feeding structure with the aid of two M3 tuning screws as shown in Fig.…”

“…For instance, a 4-pole 180 MHz K-band reconfigurable TE 311 waveguide filter was presented in [29] with 1.74% of frequency tuning and a Q factor from 3100 to 2600 using dielectric plates. To this end, coaxial and dielectric-loaded waveguides have gained more interest in many tunable filter applications due to their advantages of miniaturized structures, high Q, and good power handling capabilities [3], [30], [31], [32], [33], [34], [35], [36], [37], [38], [39], [40], [41], [42], [43], [44], [45], [46], [47], [48], [49]. For example, a CABW tunable coaxial filter was reported in [3] for future agile 5G Base Transceiver Stations (BTSs).…”

“…The filter operates from 942.5 MHz to 737.5 MHz (24.4%) with a CABW of 23 MHz±8% and a quality factor ranges from 2900 to 2300. Also in [41] and [42], our group recently presented 40% tunable CABW coaxial filters with the least variation of Q factor over the tuning window using inset resonators. To provide more compactness and a higher Q factor than the coaxial filters, a couple of tunable dielectric resonator (DR) filters were proposed [43], [44], [45], [46], [47], [48], [49].…”