Abstract
This paper presents a compact tunable bandpass filter (BPF) based on a switched varactor tuned resonator with a wide frequency-tuning range and a constant absolute bandwidth (ABW). The proposed switched varactors are adopted to overcome the limited tunability of varactor diodes by switching the stubs. The pass-band can be easily tuned by controlling two resonant frequencies of each state. After the separation of the two resonant frequencies, the ABW of the filter is maintained constant by tuning the external coupling of the tapped-line input and output matching capacitors. To verify the proposed structure, a two-pole tunable BPF has been fabricated and measured. It exhibits a low in-band insertion loss of 1.4-2.9 dB, maintaining a constant 3-dB ABW of 75 MHz with a large tuning range from 255 to 725 MHz (1:2.85). The proposed two-pole filter shares the resonator for the low- and high-band states, and therefore the overall size is only 0.2λ g ×0.14λ g . In addition, this two-pole tunable filter is extended to a four-pole tunable one as an estimation of the measured results by using the simulated results. They have the wide frequency tuning ratios of 2.85 and 2.92, with the low insertion losses, respectively.
Highlights
IN recent wireless communication systems, tunable filters with a wide frequency tuning range are increasingly required
DESIGN THEORY AND ANALYSIS Fig. 2 presents the configuration of the proposed tunable bandpass filter (BPF) designed by using switched varactors
Min: Widely Tunable Compact BPF Based on a Switched Varactor-Tuned Resonator TABLE 1
Summary
IN recent wireless communication systems, tunable filters with a wide frequency tuning range are increasingly required. To increase the tuning range, several configurations have been proposed such as a switchable filter bank, zero-value coupling and switchable resonator. The switches, and their loss, can be eliminated by zero-value coupling [18], [19], but there are still multiple filters/resonators to cover low and high bands, resulting in a large overall filter size. Min: Widely Tunable Compact BPF Based on a Switched Varactor-Tuned Resonator. Compact owing to the use of proposed switched varactors, and can cover a wide frequency range continuously. The filter was designed to be switched between low-band (open stub-loaded varactor), and high-band (short stub-loaded varactor) states with a single resonator, resulting in a compact size. To achieve a wide frequency tuning range, we firstly propose and utilize the concept of the switched varactor. We summarize the design procedure of the proposed tunable BPF
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