We have constructed one such ring that resonates at 1.46GHz and has a Q of 15 248, the current world record for an on-chip resonator operating above 1 GHz at room temperature. In a cellphone, that would translate into a filter with a pass band about 100 kilohertz wide—much more selective than the 35-MHz filters now found in cellphones. Indeed, it’s narrow enough to remove all interfering signals and pass just a single communications channel. Because the processing circuitry that follows wouldn’t have to deal with large-amplitude interfering signals, it could operate at lower power levels. And this basic design should work for much higher frequencies as well.
Ring resonators have other advantages too. Unlike what happens with a disk, it’s easy for the designer to specify the electrical impedance of the device without changing its resonant frequency. This then allows the impedance to be matched to the circuits attached to the resonator, which is important for the same reason that it’s important between, say, a stereo amplifier and its speakers: Without a good impedance match, power isn’t transferred efficiently.
As nice as rings are, it might, in fact, be advantageous to gang several disks together in a mechanical circuit. A set of such disks will accomplish the necessary impedance matching and be physically smaller than an equivalent ring resonator. So a disk array may be a better choice when space is at a premium.
Some researchers, including Albert P. Pisano, my colleague at Berkeley, and Gianluca Piazza, of the University of Pennsylvania, are looking at another way to achieve the desired impedance and to handle high power levels. They are using piezoelectric MEMS resonators similar to FBARs, but with frequencies that can be controlled by adjusting certain lateral dimensions. The devices they’ve built should be sufficient even for the kinds of power levels found in transmitter circuitry, which are always much higher than what’s encountered on the receiving end. The problem with piezoelectric resonators is that their Q values have so far been limited to about 3000.
In addition to working with disks and rings, I and other researchers around the world have experimented with MEMS resonators of other types: beams, squares, and combinations of these shapes. Lots of geometries are possible, of course, and no one will be surprised if something entirely new eventually proves even more capable than anything that’s been built so far.
Some component vendors—for example, SiTime Corp. of Sunnyvale, Calif., and Discera, a company I founded in 2001, based in San Jose, Calif.—are currently marketing MEMS resonators for use in precision oscillators. These add to the growing number of applications where MEMS devices are turning up: principally in accelerometers, pressure sensors, gyroscopes, ultrasonic transducers, and microphones. The FBAR filters found in today’s cellphones are also MEMS devices, albeit ones with lower Q values. I expect to see companies gearing up to apply more advanced MEMS technology to the construction of high-Q filters.
Suffice it to say that oscillators built with these mechanical resonators are far superior to their electrical counterparts. But these are not the only virtues of this technology. The best thing about these mechanical marvels is that they can do much more than just oscillate. If you’re clever, you can transform a MEMS resonator into a complete radio receiver stage—one that can take an incoming RF signal and amplify it, down-convert its frequency, and filter the result—all with just one minuscule, passive mechanical device. This may seem like magic, so let me explain in more detail how this micromechanical prestidigitation works.
Whether built as a disk, a ring, or something else entirely, the vibrating mechanical part of these new MEMS resonators isn’t placed in physical contact with its input or output terminals. Rather, it’s coupled to the input and output signals by means of an electric charge placed on it. Because of the force between electric charges, the moving piece begins vibrating when an oscillating electrical signal is applied to the nearby input electrode. And similarly, the vibration of the electrically charged resonator induces an oscillatory signal on the adjacent output electrode.
To generate the required electric charge, you simply apply a bias voltage using a third terminal attached to the oscillating part of the resonator. Setting that bias voltage to zero turns the resonator off, effectively opening a switch between the input and output terminals. (That’s better than putting a transistorized switch in the signal path, which is what you’d have to do with an electrical filter, because such switches degrade the signal.)
The bias voltage can, in fact, do more than just turn the resonator on or off: It can also amplify the incoming signal, down-convert its frequency, and filter it, as I mentioned. The trick is to apply an oscillating bias instead of a DC voltage.