We have experimentally demonstrated the first self-sustaining ultra-high frequency (UHF) NEMS oscillator, using a 428MHz NEMS resonator as its frequency reference.

Active oscillators spontaneously generate self-sustaining periodic signals by extracting power from steady (d.c.) sources. This distinguishes them from passive resonators—which are characterized, in contrast, by a damped response to impulsive stimuli—and makes them invaluable for applications in precision timekeeping, communications and sensing, which require continuous a.c. signals. Or simply and more often in electrical engineers' viewpoint, a radio-frequency (RF) self-sustaining, active oscillator converts d.c. power into RF power, and it functions as an RF signal generator.

Oscillators based upon the mechanical vibrations of crystals such as quartz resonators have long been ubiquitous in electronics, as a result of their excellent stability for frequency control applications. Over the past few decades, there has been considerable incentive to miniaturize such mechanical resonators, in order to integrate them on-chip with electronic components to add frequency-selection and tuning elements. In particular, it is desirable to realize highly accurate and stable clocks or frequency references with integrated, chip-based systems using miniaturized acoustically resonant devices.

Figure 1. Self-sustaining UHF NEMS oscillator. (a) Simplified circuit schematic for the self-sustaining oscillator, which includes the frequency-determining UHF NEMS resonator and the tunable electronic feedback loop. The inset shows scanning electron micrographs depicting the device (right) and its embedding electrical-bridge configuration (left), from which a large resonant response is obtained by efficiently nulling parasitic signals. (b) Open-loop electrical-domain amplitude (blue open circles) and phase (black open squares) signals from the 428 MHz NEMS resonator (referred to the input of the preamplifier). A large coherent response 8 dB above the background is observed. The magenta line is a fit of the signal amplitude to the model for a damped driven harmonic resonator. (c) Output power spectrum of the NEMS oscillator (on a logarithmic scale) as a function of frequency measured with a 100 kHz resolution bandwidth. The inset shows the output power spectrum on a linear scale: the linewidth narrowing can be clearly seen (as compared with Fig. 1b). (d) The clean, stable, sinusoidal time-domain oscillation waveform of the closed-loop NEMS oscillator measured by a high-speed digital oscilloscope.

In one of our recent collaborative efforts, we have demonstrated an autonomous and self-sustaining NEMS oscillator that generates continuous UHF signals when powered by a steady d.c. source. The frequency-determining element in the oscillator is a 428 MHz NEMS resonator that is embedded within a tunable electrical feedback network to generate active and stable self-oscillation. Beyond this demonstration, we have carefully measured and analyzed the frequency stability and phase noise performance of this UHF NEMS oscillator. We have also observed interesting linewidth-narrowing effect in the self-sustaining UHF NEMS oscillator.

We expect the self-sustaining NEMS oscillator technology to have important implications for various real-time sensing applications. Sensors based on NEMS vibrating at high and ultrahigh frequencies (i.e., in the VHF/UHF bands) are capable of levels of performance that surpass those of larger sensors. NEMS devices have achieved unprecedented sensitivity in the detection of displacement, mass, force and charge. To date, however, these milestones have been achieved with passive devices that require external periodic or impulsive stimuli to excite them into resonance. With our prototype UHF NEMS oscillator, real-time mass sensing with a mass sensitivity of ~50zg has been demonstrated, which is enough for detection of single biomolecules of many kinds. Compared to other real-time resonant sensing techniques such as phase-locked loops with external voltage-controlled oscillators, the self-sustaining oscillators provide a comparatively simple means for implementing a wide variety of practical sensing applications. This prototype NEMS oscillator also represents an important milestone in the miniaturization of oscillators based on mechanical resonators. It opens intriguing opportunities for nanomechanical frequency control, timing and synchronization. It may also offer an interesting and generic technique for metrology and some fundamental physics measurements.

Personnel
Caltech NEMS Group: Philip X. L. Feng, Michael L. Roukes
Caltech CHIC Group:  Christopher J. White, Ali Hajimiri

Funding
DARPA/MTO and SPAWAR

References

1. Feng XL, White CJ, Hajimiri A, Roukes ML, "A Self-Sustaining Ultra-High Frequency Nanoelectromechanical Oscillator", Nature Nanotechnology, Vol. 3, No. 6, 342-346 (2008). 

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