Passive devices oFten prohibit higher-performance systems from meeting the more stringent technical requirements of modern applications. At the heart of existing systems are fully integrated passive devices with relatively limited performance (e.g., on-chip inductors). They are often combined with higher-quality offchip devices like surface-acoustic-wave (SAW) filters, which are used to implement critical functions. Yet those off-chip devices also raise power consumption, system size, complexity, and cost. At Montreal's McGill University, a highly integrated 1.7-to-2.0-GHz digitally programmable frequency synthesizer that uses a microelectromechanical-systems (MEMS) resonator as its reference has been developed by Frederic Nabki, Karim Allidina, Faisal Ahmad, Paul-Vahe Cicek, and Mourad N. El-Gamal.
These resonators are both electrostatically and thermally tunable. For a 9-MHz resonator, an 8.4-percent frequency tuning is demonstrated. With the resonator's 100-nm vertical transducer gaps, electrostatic voltages down to 2 V can be used. The fractional-N synthesizer employs a third-order, 20-b delta-sigma modulator to deliver a theoretical output resolution of ~11 Hz. When used with the appropriate feedback loop, it can allow high-output frequency stability. From a 2-V supply, the phase noise for a 1.8-GHz output frequency and ~12-MHz reference signal is 122 dBc/Hz offset 600 kHz from the carrier and 137 dBc/Hz offset 3 MHz from the carrier.
An integrated, high-bandwidth transimpedance amplifier (TIA) teams with the resonator to generate the synthesizer's input reference signal. To minimize any effects on phase noise, the TIA employs automatic gain control to mitigate the low-power-handling capabilities and nonlinearities of the MEMS device. Because this device measures just 25 by 114 m, the whole system can be housed in a small standard chip package. It will therefore reduce the form factor and cost of the system compared to a system that uses an external crystal as a reference. The system also boasts higher power-handling capabilities and operating frequencies than silicon, thanks to the silicon-carbide composition of the main structural layer. See "A Highly Integrated 1.8 GHz Frequency Synthesizer Based on a MEMS Resonator," IEEE Journal Of Solid-State Circuits, August 2009, p. 2154.