Design of Frequency Agile Circuits Using Barium Strontium Titanate Varactor

Abstract

Multi-standard and multi-band radio are the next milestone in the ever developing radio industry. Frequency agile circuits are an enabling technology which allows us to achieve this goal. It is a low cost and efficient technology that provides the necessary flexibility required to implement multi-standard and multi-band radios. One important circuit component which comes easily to one's mind for frequency tunability is a varactor; with so many constraints of high tuning range, fast tuning speed and low loss on varactor, new technologies have come up to full fill these requirements. Among various varactor technologies, Barium Strontium Titanate (BST) has shown promise in the implementation of various tunable circuits like filters, voltage controlled oscillators (VCO), and tunable matching networks with good performance.

This thesis concentrates on the use of BST for implementation of frequency agile circuits. Two circuits which form an integral part of any RF transreciever are a VCO and a low noise amplifier(LNA). These circuits were chosen to demonstrate the usability of BST varactor as a tunable element. The VCO implementation shows how a BST varactor can be integrated with modern integrated circuit process like IBM 0.18 um SiGe BiCMOS process. A low power VCO with 14% tuning at flow = 11.27 GHz and a phase noise of -104.5dBc @ 1MHz offset has been designed and fabricated. Special interface technique like Flip Chip has been used to interface the BST varactor to the SiGe VCO core circuit. The second circuit implementation is a tunable matching network for a low noise amplifier. The tuning range achieved was 140 MHz(885MHz-1.025GHz) (14.65%) for deltaV bias= 80V. The input return loss was better than 14.5dB throughout the tuning range and the output return loss was better than 13dB for the same. The gain remained almost constant over the tuning range with a change of 1dB nominally being 19.4dB. The IIP3 was better than -3.55 dB over the tuning range with little variation in IIP3 over tuning range. The measured minimum noise figure was better than 2.88dB over the entire tuning range.