Although passive filters, such as surface acoustic wave (SAW) filters, film bulk acoustic resonator filters, and the evanescent-mode tunable filters mentioned above, offer excellent filtering performances in terms of rejection, passband flatness, and linearity, their relatively large physical size prevents them from being integrated with the active circuits. In recent years, N-path filters have regained interest because of its potential in providing very high filter shape factors within integrated circuit processes. The most unique characteristics for N-path filters is that their center frequency is determined by the clock frequency, therefore allowing for extremely wide tuning range often approaching 10:1.
Using an N-path resonator as a building block, we have been investigating advanced tunable on-chip RF filter designs. In several recent works, we have proposed and demonstrated the capability of creating very notch frequencies very close to the filter passband. This is achieved by the parallel combination of one N-path bandpass filter and one N-path bandstop filter. The interferometric cancellation of the signals traveling through the two paths results in sharp rejection bands. In the actual design implemented in a 65-nm CMOS process, the parasitic capacitances lead to less attenuation in the notch bands, which can be compensated by introducing tuning elements (switches) in the baseband capacitors and by adding an additional signal cancellation path.
Tunable filters can significantly reduce the complexity of future software-defined frequency- and bandwidth-agile wireless systems. Achieving wide frequency tuning range and a high unloaded quality factor (Qu) at the same time has long been a challenge for tunable filter design and implementations. Highly loaded evanescent-mode cavity filters offer a great balance between the two requirements.
As a graduate student, Dr. Liu pioneered the use of electrostatic micro-electromechanical actuators for the implementation of high-Q tunable evenescent-mode filters. An electronic tunable 2-pole filter was demonstrated for 3-4.7 GHz with 0.7% fractional bandwidth, insertion loss of 3.6-2.4 dB, and extracted Qu of 300-600. Further works demonstrated tunable bandpass and bandstop filters with similarly high Qu and tuning range for frequencies up to 20 GHz.
At UC Davis, we have continued the work exploring various aspects of the design and fabrication of novel tunable filters. We have demonstrated a design strategy for integrating lumped tuning elements with cavity filters. We have also introduced a frequency and bandwidth tunable filter design method based on dispersive coupling structures, showing bandwidth tunability of 0-10%. At the 0% bandwidth state, this coupling structure effectively shuts off the filter and can work as an RF switch without relying on an actual switch. More recently, we have also demonstrated a W-band (75-110 GHz) waveguide tunable filter with recording-breaking tunability and insertion loss.
Due to its low insertion loss, high isolation, high linearity, wide bandwidth, and near-zero dc power consumption, radio frequency micro-electromechanical (RF-MEMS) switches have been an emerging technology with significant potential in many high frequency circuits and systems, especially those requiring reconfiguration. Compared to other high frequency switching technologies, metal contact RF-MEMS switches have the highest figure of merit (FoM, defined as 1/RonCoff) because of its ultimately low contact resistance and off-state capacitance. In addition, metal contact RF-MEMS switches have extremely large bandwidth that can extend from dc to beyond 100 GHz.
However, the reliability issues associated with RF-MEMS contact switches have been a major barrier for the wider adoption of the technology. Significant efforts have been devoted to improving the lifetime (primarily in terms of cycling time) of RF-MEMS contact switches. For example, the Radant MEMS switch can be cycled up to 100 billion times under cold-switching conditions. Under hot-switching conditions, however, the reliability of these switches degrades quickly with a sharp increase in contact resistance and insertion loss after a few tens of thousands of cycles. For applications where hot switching is needed, improving the reliability of RF-MEMS switches has been a significant challenge.
Our group has recently demonstrated an RF-MEMS switch design that can significantly extend the hot-switching life-time of RF-MEMS contact switches. The design concept is shown in the figure above. To prevent the contact degradation during in hot-switching events, a pair of series protection contacts are added in parallel with the "real" contacts. When the switch closes, the protection contacts close first and creates a low-voltage, near cold-switching condition for the "real" contacts. Although protection switches have been proposed in the past, this work combines the protection and "real" contact actuation into a single mechanical structure, and demonstrated unequivocally the significant improvement in hot-switching life-time. For unpackaged devices using Au-Au as the "real" contact material and Pt-Au as the protection contact material, we have demonstrated 150-million actuation cycles at 1-W hot-switching power and 50-million at 2 W. This is much better than anything we have seen!
This design concept can in fact be extended to a few more switch design variations which we are currently investigating. We have also been working on improving the contact materials and packaging for further switch life-time enhancement.
In contrast to a low-pass sampling receiver, the bandpass sampling receiver samples at a much lower frequency with respect to the center frequency of the signal. The analog-to-digital converter (ADC) in the band-pass sampling radio therefore operates at a much lower bandwidth, resulting in a significant reduction in power consumption. The I/Q separation and baseband processing (channel filtering, base-band AGC, etc) can be carried out entirely in the digital domain. This will improve flexibility in terms of adapting to different waveforms and wireless standards. Compared with existing solutions, the tunable bandpass sampling architecture pushes digitization as close to the antenna as possible without having to sacrifice the dynamic range and has the potential to significantly increase the utilization of the ever more crowded radio frequency spectrum.
The DART lab has been working with industry partners to apply our expertise in high frequency electronics to medical applications. One example is the use of ultra-low-power radar sensors for the detection of heart health conditions. Contrary to many existing work on radar-based stand-off detection of vital signs, our solution relies on contact-based measurement which significanlty reduces the power consumption, increases the accuracy, and improves noise/clutter rejection.