CMU MEMS Laboratory Publication Abstract


in M.S. Thesis, August 2004, Carnegie Mellon University, Pittsburgh, PA.
Low Power Wide Tuning Range LC-VCO using RF MEMS Passives
V. K. Saraf
The thesis describes the motivation, design, layout and test of a voltage controlled oscillator (VCO), targeted for use in a dual frequency-hopped receiver configuration to be used for portable applications. The key requirements of this architecture are integration, low power and wide tuning range. Micromachined passives enable low power operation through increased quality factors (Q) and wide tuning range through discrete reconfiguration. Integration is achieved by using foundry provided metal interconnect layers for building these passives. This is the first time both micromachined inductors and capacitors have been integrated on-chip using standard CMOS processing.

Micromachining leads to reduced substrate coupling capacitance causing an increase in inductor self-resonance, resulting in an increased Q at higher frequencies. This allows usage of bigger inductors and smaller capacitors, leading to low power operation. The degradation in phase noise due to usage of small capacitors is offset by the high Q achieved for the tank. The decreased power (2.75 mW from a 2.5 V supply), combined with higher operating frequency (2.8 GHz) and acceptable phase noise (-122 dBc/Hz at 1 MHz offset) results in an attractive figure of merit of 187. This is the best performance achieved till date for a VCO using MEMS passives as well as for integrated VCOs operating above 2 GHz, with the exception of VCOs using bondwire inductors and SOI technology.

Additionally, the VCO also achieves a wide tuning range through micromachined reconfigurable capacitors. The capacitors can be discretely tuned relaxing the requirements of the continuously tunable lossy non-linear varactor. The advantage of using the low parasitic linear MEMS capacitor in parallel with the tank is that it allows an extension of the tuning range without any degradation of phase noise or the requirement of any additional mixed-signal control. A tuning range of 700 MHz is obtained from 2.1 GHz to 2.8 GHz with the same phase noise (-122 dBc/Hz) at both these frequencies.

© 2004 Carnegie Mellon University, Department of Electrical and Computer Engineering.
Full paper (PDF) (opens in new window).

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