To implement critical functions, many highperformance systems rely on fully integrated passive elements with relatively limited performance. Often, such components are combined with higher-quality off-chip devices that implement specific key functions. An alternative may exist in solutions based on microelectromechanical systems (MEMS). At Montreal's McGill University, an integrated and digitally programmable synthesizer that uses a MEMS resonator as its reference has been presented by Frederic Nabki, Karim Allidina, Faisal Ahmad, Paul-Vahe Cicek, and Mourad N. El-Gamal.
The fractional-N synthesizer covers 1.7 to 2.0 GHz. It employs a third-order, 20-b delta-sigma modulator to deliver a theoretical output resolution of ~11 Hz. To maintain a high level of system integration, a fully integrated, on-chip, dual-path loop filter is used. With a supply voltage of 2 V, the phase noise for a 1.8-GHz output frequency and a ~12-MHz reference signal is -122 dBc/Hz at a 600- kHz offset and -137 dBc/Hz at a 3-MHz offset.
Because the MEMS resonator measures just 25 by 14 m, the entire system has a total area of 6.25 mm2. The clamped-beam resonators are fabricated using a CMOS-compatible process. Compared to silicon, they boast higher powerhandling capabilities and operating frequencies because their main structural layer is made of silicon carbide. The resonators also are electrostatically and thermally tunable.
An integrated, high-gain-bandwidth transimpedance amplifier (TIA) is combined with the resonator to generate the synthesizer's input reference signal. The TIA employs automatic gain control (AGC) to mitigate the inherent lowpower- handling capabilities and nonlinearities of the MEMS device. See "A Highly Integrated 1.8 GHz Frequency Synthesizer Based on a MEMS Resonator," IEEE Journal Of Solid-State Circuits, August 2009, p. 2154.