Applications of On-Chip Coplanar Waveguides to Design Local Oscillators for Wireless Communications System

Abstract

On-chip distributed transmission line resonators in CMOS technology have become the interest of research subjects recently (Ono et al. 2001; Umeda et al., 1994; Kanaya et al., 2006; Wolf, 2006) because of their size which becomes more compact, as the frequency of application increases. Among the various transmission lines, coplanar waveguide (CPW) has more engineering applications (Toyoda, 1996; Civello, 2005) because it is easy to fabricate by LSI technology since the signal line and ground plane exist on the same plane so that no via holes are required for integrating active components such as transistors on Sisubstrate (Toyoda, 1996). The applications of the CPW were reported for many on-chip LSI components. The CPW was exploited as an inductor and used to design a conventional-type matching circuit for LNA (Ono et al., 2001) in microwave-band frequency, and they are most popular in monolithic microwave integrated circuit (MMIC) (Umeda et al., 1994). However, the application of CPW lines as an inductor takes larger space than the conventional spiral inductors (Umeda et al., 1994). Some of the present authors have also implemented the onchip CPW impedance-matching circuit for a 2.4 GHz RF front-end (Kanaya et al., 2006) and for 5GHz band power amplifier (Pokharel et al., 2008) using impedance inverters. In designing the matching circuits using impedance inverters and quarter wavelength resonators realized by on-chip CPW (Kanaya et al, 2006; Pokharel et al., 2008) the size of the matching circuits becomes compact thus reducing the chip area by about 30% than using spiral inductors for 2.4GHz-band applications and 40% for 5 GHz-band applications. However, the applications of on-chip CPW resonators in designing other components such as a voltage-controlled oscillator (VCO) have not been reported yet. A conventional VCO consists of a LC-resonator to produce an oscillation at the frequency band of interest, and this LC-resonator may be replaced by a CPW resonator. Such possibilities are investigated in this paper. In a conventional VCO, the performance such as phase noise of the VCO depends on the quality (Q) factor of the LC resonator. Usually, a spiral inductor is used in 17