Varicap-tuned narrowband filters with extended rejection band based on U-shaped microstrip resonators

Alexander V. Zakharov; Mykhailo Ye. Ilchenko

doi:10.3103/S0735272717090011

Authors

Alexander V. Zakharov Igor Sikorsky Kyiv Polytechnic Institute, Ukraine https://orcid.org/0000-0002-1222-1623
Mykhailo Ye. Ilchenko Igor Sikorsky Kyiv Polytechnic Institute, Ukraine https://orcid.org/0000-0002-3459-4949

DOI:

https://doi.org/10.3103/S0735272717090011

Keywords:

tunable filter, U-shaped resonator, varicap, tuning range, rejection band

Abstract

A resonance equation for the first unwanted resonant frequency of tunable microwave U-shaped loop-type resonator with variable capacitance has been derived. This frequency does not depend on the variable capacitance value; it is invariable and proportional to λ-type oscillations. The ascertained feature makes it possible to determine the ratio of the first unwanted resonant frequency to the frequency of the main resonance and control the rejection bandwidth of tunable filters with U-shaped resonators. A narrowband (2%) varicap-tuned filter with extended rejection band has been built and its experimental data are presented. The experimental four-resonator filter could be tuned in the frequency range 225–400 MHz. The filter rejection band in terms of the attenuation level of –40 dB was located in the frequency range 420–1290 MHz, i.e., f_max/f_min = 3.07. Varicap-tuned filters with microstrip U-shaped resonators are shown to have a good potential for their practical use.

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