- Author(s): Liu, Ruonan
- Advisor(s): Nguyen, Clark T.-C.
- et al.
In today’s connected world, our lives depend heavily on information obtained from networks of sensor nodes and mobile devices. Billons of mobile devices are in communication over these networks at any given time. The trillion sensor vision calls for low-power, low-cost and small-sized wireless sensor nodes, posing ever-increasing constraints on the power consumption of wireless networks. Radio Frequency (RF) Microelectromechanical Systems (MEMS) offer one path towards nano-watt wireless communications.
This dissertation presents the first demonstration of an all-mechanical wireless receiver that employs a micromechanical resonant switch (“resoswitch”) to consume zero quiescent power while listening and ~30nW while actively receiving. Here, high-Q mechanical resonance offers frequency selectivity, while mechanical impact switching provides amplification. The mechanical receiver successfully detects and demodulates (in an On-Off-Keying (OOK) fashion) Frequency-Shift-Keying (FSK)-modulated input signals down to -68dBm, a promising sensitivity for low-speed and low-power wireless applications. Because this receiver consumes no power while listening in standby, it obviates the sleep/wake cycles often used by low power sensor networks to save energy. It thus eliminates the need for continuously operating clocks that govern sleep/wake periods, thereby eliminating their power consumption.
If on the other hand one prefers to use more conventional mote transceivers that consume tens of milli-watts when on, then the sleep/wake strategy is paramount to save energy, and the clock sets the power bottleneck. A commercially available real time clock (RTC) typically consumes 1µW of power, which is considerably more than the tens of nano-watts more applicable for giant sensor networks. Fortunately, even when not used as a wireless transceiver, the all-mechanical circuit herein provides a solution to the clock problem, as well.
Specifically, the second part of this dissertation presents an RF powered wireless clock that feeds on a received FSK-modulated or Continuous-Wave input and generates a local clock output signal, while consuming only 13nW of battery power, which is more than 50 times lower than an off-the-shelf RTC. Calculations further predict a three-order-of-magnitude power reduction if an on-chip next stage inverter replaces the off-chip discrete component in the current demonstration. The input capacitance of the off-chip inverter is 1.5pF, which translates to nano-watts of power for kilohertz clock frequencies. Use of an on-chip inverter with a few femto-Farads of input capacitance would reduce the power consumption to pico-watts.
Resoswitches as receivers or as clock generators, while very compelling, face several challenges that must be further explored before they can be commercially viable. For instance, the frequency stability of a resoswitch directly determines the error rate of a receiver or the stability of a clock. This frequency stability depends on perturbations of mechanical resonance impact dynamics that create unstable switching and instantaneous frequency jitter, a phenomena referred to as squegging. The quality factor, the contact material property, and the external excitation voltage all affect the squegging dynamics, but controlling these factors is not an easy task. On the other hand, manipulating the electrode configuration can also alter the squegging behavior, and is much easier to design and implement. This dissertation models and experimentally verifies observed squegging phenomena. The methods and models presented here provide a path to improving the stability of resoswitches towards commercial viability for use in future ultra-low power wireless networks.