No doubt that wireless communication has changed our daily life dramatically in the past decade. From simple data service, to voice service, to today’s multimedia service, the needs for more advanced functions and real time access to information is driving the technology to have multi-frequency, multi-standard, and multi-mode operation in a single hand-held device which should be kept compact and cheap. Traditional fixed frequency transceiver can’t satisfy this new trend. A reconfigurable, or tunable, transceiver has to be designed, in which tunable components like tunable capacitors (varactors), tunable inductors and tunable resistors are critical. Among them varactors are of most importance due to their wide and easy use in the circuits. Nowadays various varactor technologies exist: semiconductor varactor, micro-electro mechanical system (MEMS) and ferroelectrics.
Ferroelectrics are an interesting material whose permittivity can be varied under external static electric field. By using it as the dielectric in parallel capacitors, tunable capacitors can be achieved. Ferroelectric varactors have the merits like high capacitance tunability (~ 3:1 under 15 V), high quality factor (~100 @ 2 GHz), fast tuning speed (pico-second range), high power handling capability, low tuning voltage (5 V to 15 V) and etc. So it’s a very promising candidate for future tunable microwave circuits application.
Our research focuses on one such material called Barium Strontium Titanate (BST). We work from the material deposition, device characterization to circuits implementation, in detail: characterizing thin film ferroelectrics and varactors in microwave frequency range, design and fabrication of various tunable microwave circuits like tunable filters, phase shifters, tunable matching circuits and tunable FBARs.
1. Two Port Measurement Technique
It is important to characterize thin film ferroelectric material itself first, especially at microwave frequency range. Here we proposed a simple yet accurate method to extract the permittivity and dielectric loss tangent of the thin film BST, excluding all the parasitics from the metal conductors. Three major structures were fabricated on the sapphire substrate, which are BST varactor, short standard and long standard. Two port scattering matrixes of them were measured by the vector network analyzer. Then these two port s-parameters were converted to ABCD parameters to get the complex impedances of them. By doing simple algebra among these impedances, one can extract the pure dielectric information of the thin film BST.
2. Tunable Microwave Filter
A tunable microwave filter was designed and fabricated. The building block was a capacitively loaded ring resonator which exhibited an additional zero close to the pole. This makes the filter to have a deeper roll-off skirt compared with traditional half-wavelength, quart-wavelength resonator based and combline filters. The designed center frequency was 1.8 GHz, and the fractional bandwidth was 5%. The calculated BST varactor capacitance was 0.75 pF. The measured insertion loss was about 6 dB which was due to the low quality factor of BST varactor (~40). The center frequency can be tuned from 2.0 GHz to 2.4 GHz under 10 Volts, which corresponds to 20%. During this tuning range, the fractional bandwidth of 8% was maintained.
