Joshua Small

Theory and Design of Octave Tunable Filters With Lumped Tuning Elements

This paper presents octave-tunable resonators and filters with surface mounted lumped tuning elements. Detailed theoretical analysis and modeling in terms of tuning range and unloaded quality factor (Qu) are presented in agreement with simulated and measured results. Based on the models, a systematic design method to maximize tuning ratio and optimize Qu of the resonator is suggested. A resonator tuning from 0.5 to 1.1 GHz with Qu ranging from 90 to 214 is demonstrated using solid-state varactors. A two-pole filter with a tuning range of 0.5-1.1 GHz with a constant 3-dB fractional bandwidth (FBW) of 4±0.1% and insertion loss of 1.67 dB at 1.1 GHz is demonstrated along with a three-pole filter with a tuning range of 0.58-1.22 GHz with a constant 3-dB FBW of 4±0.2% and insertion loss of 2.05 dB at 1.22 GHz. The measured input third-order intermodulation is better than 17 dBm over the frequency range for the two-pole filter.

A novel high-Q u octave-tunable resonator with lumped tuning elements

This paper presents a novel design of high-quality factor (Qu) continuously-tunable resonator compatible with standard PCB fabrication technology. The proposed resonator is tuned by lumped element varactors placed on the top surface of a substrate-integrated evanescent-mode cavity, thus significantly reducing the complexity of integration without sacrificing Qu. The proposed design approach is compatible with a variety of tuning elements, including solid-state, ferroelectric, and RF-MEMS varactors. A tunable resonator with solid-state varactors is demonstrated to validate this design approach. The resonator surpasses the state-of-the-art with a frequency tuning range of 0.5-1.2 GHz (tuning ratio of 2.4 : 1) and Qu of 82-197. An RF-MEMS varactor enabled tunable resonator based on the same design further shows Qu of 240 at 6.6 GHz.

Impact of Mechanical Vibration on the Performance of RF MEMS Evanescent-Mode Tunable Resonators

This letter presents the first experimental investigation on the impact of mechanical vibration on the performance of MEMS evanescent-mode tunable resonators. It is shown both conceptually and experimentally that mechanical vibration can introduce distortions to the RF signal. Signal distortions are found to be very small (<; -40 dBc of sideband or <; 0.5% change in-error vector magnitude) for a diaphragm based MEMS tunable resonator with a diaphragm size of 7 × 7 mm2 and mechanical vibration amplitude of 1g. A novel MEMS tunable evanescent-mode resonator based on two arrays of cantilever beams that replace the diaphragm is also presented to achieve even lower distortion in the presence of mechanical vibration. A 15-25 dB reduction in the vibration-induced sideband is observed.

A fast high-Q X-band RF-MEMS reconfigurable evanescent-mode cavity resonator

This paper presents the design, fabrication, and measurement of an X-band evanescent-mode reconfigurable RF-MEMS cavity-based resonator with a fast response (84–112 µs) and a high quality factor of 593–1,077 between 10.7–13 GHz. An array of MEMS fixed-fixed beams biased against their own substrate are used as the tuning mechanism. By designing the DC bias to be completely external to the cavity, the coupling between the DC bias-line and the RF signal is virtually eliminated. This is a key property in measuring a tunable resonator quality factor that exceeds 1,000 at 13 GHz. This paper also quantifies the quality factor reduction solely due to the MEMS tuning array spatial distribution. Specifically, compared to an ideal static 15.5 GHz evanescent-mode cavity resonator with a simulated Q = 1,750, the MEMS array diminishes the resonator quality factor by only 7.4% (simulated Q = 1,620; the measured Q value is 1,400). The measured switching times are 84 µs and 112 µs for the up-to-down and down-to-up operations, respectively. To the best of our knowledge, this is the fastest response for a tunable X-band cavity-resonator with this quality factor.

Impact of sacrificial layer type on thin-film metal residual stress

In this paper we study the impact of two sacrificial layers on the final residual stress of thin gold films. In particular, we comapre a typical photoresist layer (Shipley SC1827) to single-crystalline silicon. We fabricate and measure cantilever beams on both sacrificial layers and study their residual stresses by analyzing the final displacement profile of the released beams. All samples were fabricated at the same time and under identical conditions. The study clearly shows that the induced stress on thin films is dependent on the sacrificial layer. The gold film deposited over the single-crystalline silicon shows nearly zero gradient stress after release. On the other hand, gradient stress dominates the gold film deposited during the same run but over a photoresist layer. Such results are very useful in designing and fabricating a wide variety of low-stress actuators and sensors.

Electrostatically tunable analog single crystal silicon fringing-field MEMS varactors

This paper reports on the design of a new analog MEMS varactor that uses electrostatic fringing-field actuation and is based on a single-crystal silicon movable structure coated with a thin metallic layer. Electrostatic fringing-field actuation allows for an analog displacement with no pull-in instability that yields a much larger tuning ratio compared to conventional electrostatic designs. In addition, total lack of dielectric layers and the use of single crystal silicon for the moving membrane significantly enhances the robustness of our proposed varactor by making it devoid of dielectric charging and stiction, insensitive to process variations, amenable to high yield manufacturing and less susceptible to hysteresis and creep. Based on this idea, we present example designs and the associated fabrication processes for varactors that exhibit a tuning ratio of 4.5∶1 with capacitance values in the range of 43–200 fF achieved with DC voltages of 0–55 V. Such varactors are key elements in MEMS matching networks, tunable filters and reconfigurable antennas in the K/Ka/W-bands.

Electrostatic fringing-field actuation for pull-in free RF-MEMS analogue tunable resonators

This paper presents the design, fabrication and measurement of the first pull-in free tunable evanescent-mode microwave resonator based on arrays of electrostatically actuated fringing-field RF-MEMS tuners. Electrostatic fringing-field actuation (EFFA) is the key on achieving a wide tunable frequency range that is not limited by the conventional pull-in instability. Furthermore, total lack of dielectric layers and no overlap between the pull-down electrode and movable beams significantly enhance the robustness of our proposed tuning mechanism by making it devoid of dielectric charging and stiction and amenable to high-yield manufacturing. The proposed electrostatic fringing-field tuners are demonstrated in a highly loaded evanescent-mode cavity-based resonator. The measured unloaded quality factor is 280?515 from 12.5 to 15.5?GHz. In addition, a 10? improvement in switching time is demonstrated for the first time for EFFA tuners in a tunable microwave component by employing dc-dynamic biasing waveforms. With dynamic biasing, the measured up-to-down and down-to-up switching times of the resonator are 190 and 148 ?s, respectively. On the other hand, conventional step biasing results in switching times of 5.2 and 8?ms for up-to-down and down-to-up states, respectively.

Bandwidth-optimal single shunt-capacitor matching networks for parallel RC loads of Q ≫ 1

In this paper we demonstrate an optimized matching network that maximizes the matching bandwidth for parallel RC loads with quality factors greater than unity. The matching network includes a transmission line and a single shunt capacitor. We systematically investigate the dependance of the networks bandwidth on the impedance and electrical length of the transmission line. Results show that high impedance lines always yield maximum bandwidth for parallel RC loads with Q ≫ 1. A 4.3∶1 MEMS varactor-based tunable matching network is measured as a vehicle to demonstrate these results. In particular, we find that up to a two-fold increase in the 10-dB reflection bandwidth is possible by utilizing high impedance lines. Wih a single 4.3∶1 MEMS varactor a measured bandwidth of over 136% is obtained.

Post-Release Displacement Uncertainty of Micro-Cantilevers Due to Anchor Over/Under Etching

In this paper we follow an analytical and finite element modeling approach to study the effect of anchor over/under-etch on the post-release tip displacement of MEMS cantilever beams. We show that the last release step is particularly critical in controlling the beam’s post-release displacement, which is of primary importance in a variety of applications. In this paper we isolate the effects of mean stress and imperfect anchor. We assume negligible gradient stress since its effect has been studied in detail in the literature. Beams with perfect anchors and zero under-etch are also presented for comparison. The results emphasize that through careful control of the fabrication parameters and anchor structure, the initial displacement profile and thus performance of micro-cantilevers can be accurately controlled.Copyright

Real-Time DC-dynamic Biasing Method for Switching Time Improvement in Severely Underdamped Fringing-field Electrostatic MEMS Actuators

Mechanically underdamped electrostatic fringing-field MEMS actuators are well known for their fast switching operation in response to a unit step input bias voltage. However, the tradeoff for the improved switching performance is a relatively long settling time to reach each gap height in response to various applied voltages. Transient applied bias waveforms are employed to facilitate reduced switching times for electrostatic fringing-field MEMS actuators with high mechanical quality factors. Removing the underlying substrate of the fringing-field actuator creates the low mechanical damping environment necessary to effectively test the concept. The removal of the underlying substrate also a has substantial improvement on the reliability performance of the device in regards to failure due to stiction. Although DC-dynamic biasing is useful in improving settling time, the required slew rates for typical MEMS devices may place aggressive requirements on the charge pumps for fully-integrated on-chip designs. Additionally, there may be challenges integrating the substrate removal step into the back-end-of-line commercial CMOS processing steps. Experimental validation of fabricated actuators demonstrates an improvement of 50x in switching time when compared to conventional step biasing results. Compared to theoretical calculations, the experimental results are in good agreement.