High linearity 1.5-2.5 GHz RF-MEMS and varactor diodes based tunable filters for wireless applications
The dissertation presents tunable banpass filters in the 1.5-2.5 GHz frequency range targeted for wireless applications. The tunable filters are designed for size miniaturization, good linearity and constant absolute bandwidth characteristics while maintaining low insertion loss. The improved linearity has been demonstrated using back-to-back varactor diodes and using RF MEMS devices. The constant absolute bandwidth characteristics was achieved using a novel corrugatedcoupled lines approach and also using a localized capacitive compensation concept. In the improved linearity varactor diode design, two miniaturized tunable filters with two zeros were developed at 1.4-2.0 GHz on. The filters were built using single and back-to-back varactor diodes and compared for linearity characteristics. The single diode filter has a 1-dB bandwidth of 5 ± 0.5% and an insertion loss of 2.5-1.8 dB. The back-to-back diode filter has a 1-dB bandwidth of 4.9 ± 0.5% and an insertion loss of 2.9-1.25 dB (resonator Q of 56-125). A detailed Volterra series analysis is done on the back-to-back diode including the effect of the bias network and diode mismatches. The measured IIP 3 for the back-to-back diode tunable filter is 22-41 dBm depending on the bias voltage and is 13-15 dB better than the single diode design. The power handling capabilities of both designs is explored using large signal S21 measurements. To our knowledge, these planar tunable filters represent state-of-the-art insertion loss and linearity characteristics performance with varactor diodes as the tuning elements.
In the corrugated coupled-lines design, miniaturized fixed and tunable microstrip bandpass filters were developed t 1.4-1.9 GHz. The novel approach uses microstrip corrugated coupled-lines concept to synthesize a coupling coefficient which maintains a nearly constant absolute bandwidth across the tuning range. In addition, a miniaturized 2-pole varactor tuned filter is demonstrated with a frequency coverage of 1.44-1.89 GHz and an insertion loss < 2.92 dB with a constant 1-dB bandwidth of 70±4 MHz across the tuning range. In addition, a 3-pole combline 4.7% fixed filter at 1.94 GHz shows a 3:1 resonator spacing reduction over the conventional approach, with an insertion loss of only 1.1 dB. This technique will allow the design of miniaturized small bandwidth fixed and tunable microstrip filters.
In the localized capacitive compensation design, the approach was used to design 3-poles combline tunable filter with an electrical length of 42° at the mid band. The frequency coverage of the tunable filter is 1.4-2.2 GHz with a 1-dB bandwidth of 157±7 MHz across the tuning range. Detailed design equation as well as the design procedure were presented.
In order to get substantial improvement in linearity and achieve a high resonator Q, high Q RF MEMS devices are used to demonstrate tunable filters with constant absolute bandwidth for the 1.5-2.5 GHz wireless band. The filter design is based on corrugated coupled-lines and ceramic substrates (εr = 9.9) for miniaturization, and the 3-bit tuning network is fabricated using a digital/analog RF MEMS device so as to provide a large capacitance ratio and continuous frequency coverage. Narrowband (72±3 MHz) and wideband (115±10 MHz) two-pole filters result in a measured insertion loss of 1.9-2.2 dB at 1.5-2.5 GHz, with a power handling of 25 dBm and an IIP3 >> 33 dBm. The filters also showed no distortion when tested under wideband CDMA waveforms up to 24.8 dBm. The designs can be scaled to higher dielectric-constant substrates to result in even smaller filters.