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