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Tunable RF Front-End Circuits for Advanced Communication Systems /

  • Author(s): Ko, Chih-Hsiang
  • et al.
Abstract

This thesis presents tunable RF front-end circuits for advanced communication systems by using packaged RF MEMS (Micro-Electro-Mechanical-Systems) devices or varactor diodes. First, a low-loss reflective phase shifter using commercial RF MEMS SPDT (Single-Pole-Double-Throw) switches is presented. The phase shifter can provide phase shifting of 0-123.75° with a step of 11.25° at 1.8-2.1 GHz. The measured average loss is ̃0.83 dB and the measured IP3 is > 65 dB. The 4-element dipole phase array is also presented, which can scan up to 9° with a measured gain of 8.6-8.3 dB at 2 GHz. The array is capable of handling 5-10 W of power with no distortion and suitable for base-station applications. The second project presents a 1.7-2.5 GHz asymmetric 4-pole tunable filter using the Cavendish Kinetics RF MEMS capacitors. The MEMS capacitors are fabricated and fully packaged using a 0.18 [mu]m CMOS standard process with integrated high voltage drivers and SPI control logic and with reliability in the billions of cycles. The filter results in insertion loss < 3 dB for 8% FBW (fractional bandwidth), a power handling of at least 25 dBm, a second and third harmonic generation of < -110 dBc at 20 dBm, and an IIP3 > 46 dBm. The measured ACPR (adjacent channel power ratio) for a 5-MHz Wideband CDMA signal is at least 54 dB at 25 dBm input power. The project also discusses the requirements on RF MEMS capacitors in order to achieve high performance filters for wireless systems and the effect of ENIG (Electroless Nickel Immersion Gold) on resonator quality factor and filter loss. Next, an idea for implementing reconfigurable matching networks to realize a tunable diplexer is investigated and demonstrated. The reconfigurable matching networks ensure that the rejection band impedance of every filter is transformed to an open circuit over a wide range of frequencies, allowing two tunable filters to be connected together to form a tunable diplexer without affecting each other. The tunable diplexer is built using Schottky diodes and combline resonators on Duroid substrates, and can operate from 1.4- 2.3 GHz with a closest frequency separation of 110 MHz. Measurements show virtually no difference in the frequency response between a stand-alone filter and a filter placed in the tunable diplexer. The work shows that a wideband tunable diplexer can therefore be realized using tunable bandpass filters and reconfigurable matching networks. Finally, a novel tunable dual-band bandstop filter based on doubly-tuned RF transformers is presented. This design results in two distinct notch frequencies in a single resonator using silicon varactor diodes. The 2-pole tunable dual-band bandstop filters are implemented using PCB transformers and air-coil transformers. With PCB transformers, the lower frequency can be tuned at 513-845 MHz while the higher frequency can be tuned at 715-1151 MHz with a notch level > 16 dB. With air-coil transformers, a single-band 2-pole notch filter with wide rejection bandwidth is achieved. The design results in 604-982 MHz tuning with a 20-dB rejection bandwidth of 27-45 MHz. Also, by implementing a series varactor in a transformer, the separation between two coupled frequencies can be changed. The topology can be easily extended to higher-order filters and design equations are presented

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