A Resonator-Adaptable Oscillator Using Varactor-Loaded Tuned Amplifiers

This letter presents a new monolithic oscillatory circuit that is applicable to different microelectromechanical systems (MEMS)/crystal resonators. The proposed technique cascades tuned amplifiers (TAs) with varactors in their LC-tanks. By adjusting the bias voltage of the varactors, the center frequencies of the TAs can be controlled to change the loop response of the oscillator. Using this approach, the oscillation conditions can precisely be attained for different resonant frequencies. Tunable negative resistance is realized through biasing the MOS transistors in the TAs, so as to compensate losses from the different resonators. The TAs present bandpass characteristics that superbly suppress the overtones, thus improving the oscillator stability. Fabricated in 0.18-μm CMOS technology, a prototype oscillator chip is able to suit a variety of resonators, offering 274- to 565-MHz frequency range, 30-dB dynamic loop gain, and -27 dB/octave roll-off to suppress all undesired frequencies. To the best of the authors' knowledge, the proposed monolithic oscillator is the first of its kind to be applicable to various MEMS resonators at above 100 MHz, which is beneficial for MEMS sensor applications, clock generations, etc.