An electronically tunable multi-passband filter using one varactor diode is implemented based on transmission line stubs method for passing several most favourable applications through multiple operating bands. In this paper, the filter is designed on Rogers RT/Duroid 5880-substrate and its input and output ports are terminated by 50 Ω microstrip feed line. The filter passbands consist of low pass filter (LPF) with tunable cut-off frequency which can reach to 0.94 GHz, then several tunable bandpass filters (BPF) that can cover the following frequency ranges BPF1 (1.94 3.33 GHz), BPF2 (3.83 4.23 GHz), BPF3 (4.53 5.56 GHz) and BPF4 (6.83 7.48 GHz) with insertion loss (IL) of |S21| ≤ 3 dB. The designed filter is the binomial type with 3 elements that are implemented in three shunt stubs with the middle stub has been shorted. A parametric study was conducted for the optimum location for the varactor diode and an external DC biasing circuit also introduced to produce the required reverse biasing for the varactor and its effect was considered. The demonstrated filter is investigated using the high-frequency structure simulator (HFSS). The measured scattering parameters S11 (reflection coefficient) and S21 (transmission coefficient) results show good agreement with the simulated values.
Key words: Tunable filter, Reflection coefficient, Transmission Coefficient, Varactor diode, Low pass filter, Band pass filter, bandwidth, Multiple pass bands, group delay, external quality factor, mutual coupling.
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