Mohammad Abu Khater

Vibration mitigation for evanescent-mode cavity filters

This paper presents the first vibration mitigation technique for evanescent-mode cavity filters. The effect of vibration is substantially reduced by over 20 dB by injecting a voltage signal on the bias of the tuning piezoelectric device. The vibration is measured by an accelerometer and the frequency and amplitude of this signal is chosen based on the analysis of the measured acceleration. The proposed technique has the potential to be fully implemented in an electronic monitoring system. The vibration mitigation approach is tested on a 2-pole Bandstop Filter (BSF) tuned to 1.3 GHz. The sidebands caused by vibration are reduced by 24-26 dB.

Real-Time Feedback Control System for Tuning Evanescent-Mode Cavity Filters

This paper presents a real-time feedback control system for evanescent-mode cavity filters. The control loop monitors the frequency of each cavity in the filter and adjusts the bias voltage on the tuning piezoelectric actuator based on the monitoring data, such that each resonator of the filter is at the desired frequency. The control system can tune the filter regardless of any effects from hysteresis or creep. While applicable to a variety of filter structures, the presented control system has been validated on a second-order bandstop filter built using a standard PCB process. The control system can successfully tune the filter from 0.9 to 1.45 GHz (550-MHz tuning range or 61%) without affecting the RF performance. The control system is fully electronic with a digital interface for easier integration. It also exhibits a measured frequency resolution of 33-6 MHz (or 3.5%-0.4%).

Spectrum-aware jammer suppression using evanescent-mode cavity filters

This work presents the first spectrum-aware closed-loop tunable jammer suppression system, utilizing evanescent-mode cavity filters. The presented system automatically sweeps a bandstop filter (BSF) across its tuning range and detects the location of the jammer in the spectrum by reading the output power of the filter. Once the frequency of the jammer is detected, the BSF is automatically tuned to suppress it. The system operates in real-time as it does not require a special hardware to directly sense the filters resonators. The system is implemented and tested under different frequency conditions. The tuning range is 1.8-2.16 GHz, and it can suppress jammers 20 MHz away from the signal by 41-45 dB using a three-pole BSF. This suppression is sufficient to bring the symbol recovery rate up to at least 99.9%, from less than 17% when no jammer suppression is employed. Error vector magnitudes are also experimentally studied for the 16 QAM modulation scheme that is used in the measurements.

Tunable Cavity-Based Diplexer With Spectrum-Aware Automatic Tuning

This paper presents, for the first time, an automatically tunable diplexer using evanescent-mode cavity filters, with spectrum-aware and interference mitigation capabilities. The diplexer consists of two second-order bandpass filters and a bandstop filter (BSF) at the receiving path. The BSF is utilized in a spectrum sensing methodology to detect potential jammers. The automatic tuning system monitors each resonator in the diplexer and biases them to accurately reach the targeted frequency accordingly. The presented diplexer presents a measured tuning range 0.75–1 GHz, with an insertion loss (IL) of less than 3.7 dB in the receive path and 2.2 dB in the transmit path and a tuning resolution of ~0.05%. The closed-feedback loop tuning time, which accounts for all mechanical and electrical time constants, is around 250 ms. The spectrum sensing capability has been experimentally demonstrated to detect a jammer. Automatic diplexer tuning results in a total jammer suppression of 30 dB with 33-MHz transmit–receive separation. Further rejection of up to ~50 dB can be achieved for higher separations. The automatic tuning control system is also analyzed for stability.

Temperature-compensated lumped element tunable bandpass filter

This paper presents and compares two techniques for compensating temperature effects in varactor diode-based lumped element tunable bandpass filters (BPFs). Temperature compensation relies on first measuring the temperature and subsequently re-biasing the varactors of the filter based on pre-calculated lookup table. The lookup table is calibrated using two different methods: a) constant varactor capacitance, and b) constant filter response. Both approaches are evaluated when the filter is subjected to temperatures ranging from -40 to 100°C. In the first approach the capacitance of each of the filters varactors is held constant at the value found to yield the desired filter response at room temperature. On the other hand, the second approach utilizes a global optimization scheme that re-biases all varactors until the filter yields the desired transfer function. The results from both methods are measured and compared for a third-order BPF tunable from 226 MHz to 333 MHz. While the first method is simpler to implement and may appear intuitive, it actually causes a worse temperature-induced drift because it ignores substrate and inductor effects. Specifically, the resulting temperature drift of the center frequency with the first method is 4.3% as compared to 3.2% when no compensation is applied. Optimal results are achieved with the second approach where the filters center frequency remains constant to within 0.1% over the -40 to 100°C temperature range.

An Electronically Tunable High-Power Impedance Tuner With Integrated Closed-Loop Control

In this letter, a novel electronically tunable, high-power and highly linear impedance tuner with integrated closed-loop control is experimentally demonstrated. The structure of the proposed tuner is based on two individually controlled substrate-integrated cavities, implemented as evanescent-mode magnetically coupled resonators. A prototype tuner at 3.3 GHz is designed, fabricated, and successfully measured with up to 90-W input power. The reported experimental results show more than 90% coverage of Smith chart at the center frequency and about 37% frequency bandwidth for covering at least half of the Smith chart. Also, the measured IIP3 and power loss are +64.3 dBm and 0.77 dB, respectively. The proposed tuner is suitable for high-power applications including radars and base stations.

Real-time temperature compensation control system for tunable cavity-based high-Q filters

In this paper, a real-time temperature compensation control system for a tunable high-Q cavity-based filter is designed, implemented and experimentally validated. A high-Q (Q = 400) bandstop resonator that can be tuned by a piezoelectric actuator from 1.27 GHz to 1.79 GHz is monitored in real time by an integrated monitoring resonator that shares the same piezoelectric actuator. Consequently, the RF resonance has a one-to-one correspondence to the monitoring resonance. A VCO is coupled with the monitoring resonator resulting in an oscillatory output signal at a frequency that depends on the RF resonator frequency. The monitoring resonance is controlled by comparing it to a user-provided resonance through a closed-loop in real-time. The system resolution varies from 2 MHz to 13 MHz depending on the resonators center frequency with a 1.6 μs sensing period and averaging set at 512. The system is capable of compensating for frequency drifts by adjusting bias voltage to the piezoelectric actuator for a wide range of temperatures between -40°C and 40°C. The frequency control error at 1.5 GHz is measured at 4 MHz (0.25%), which is much smaller than the open-loop change of 195 MHz (12.97%) for the same temperature change.

Control methodology for on-chip switching power supplies for biomedical implants

Recent developments in biomedical implants have increased the demand for on-chip power supplies. Switching power supplies provide high efficiency solution, which also serves in reducing the size of energy harvesting or energy storage devices. Hence, on-chip switching power supplies are ideal for implantable applications. However, they have the problem of higher ripple at the output than the conventional linear regulator due to the impractical values of on-chip components. In this paper, we present a new control methodology for on-chip switching power supply optimized for inductively powered implantable devices without any off-chip filtering components. The control methodology smoothens the transition between switching states based on pulse density modulation (PDM) feedback, in addition to an output voltage slope-limiting circuit. Simulated results of output ripple and efficiency based on 130nm RF-CMOS are shown. The control methodology reduce the ripple with minimal effect on power efficiency and without the need for any external filtering components.

CMOS-based monitoring of contact events up to 4 MHz in ohmic RF MEMS switches

This paper presents an ultra-low power, fully electronic methodology for real-time monitoring of the contact events of ohmic RF MEMS switches. The measurement is based on a resistive readout circuit composed of 67 transistors with a 105 µm × 105 µm footprint. This is coupled with a novel implementation of a single-crystal silicon switch capable of operating from DC-40 GHz. The CMOS readout electronics tap the RF circuitry through two 1.6 kΩ resistors that add negligible insertion loss to the switch. Experimental and theoretical results demonstrate that timing information for the switch contact behavior is accurately measured for all consecutive bounce events that occur during the time it takes for the switch to come to a fully closed state (25–30 µs). A simple one-dimensional contact model agrees to within 10–20% with the measured landing times. In addition, a finite contact duration of 2–3 µs for each landing is accurately captured experimentally. This demonstrates the potential of this technique to real-time on-chip dynamic monitoring of contacts for packaged RF MEMS switches through their entire lifetime and after their integration in the final system.

Real-time temperature compensation for tunable cavity-based BPFs and BSFs

In this study, a real-time temperature compensation control system for tunable high-Q cavity-based filters are designed, implemented, and experimentally validated. Both bandpass (BPFs) (700–1000 MHz) and bandstop filters (BSFs) (1300–1600 MHz) with high-Q ( Q ≃ 400 ) resonators are monitored in real time to compensate for any temperature variations. The monitoring scheme includes additional resonators that share the same tuning piezoelectric actuators with the resonators of the radio frequency (RF) filters. An oscillator is coupled with each monitoring resonator resulting in an output signal at a frequency directly linked to the RF resonance. Each monitoring resonator is controlled by a user-provided input through a closed-loop in real time. The presented system is capable of compensating for temperature variations in the − 40 and 80 ∘ C range. The average system resolution varies from 0.23 to 9 MHz, depending on temperature, with a 1 ms sensing period. The closed-loop frequency shift is 6.5 MHz (0.93%) and 8.75 MHz (0.65%) for the BPFs and BSFs, respectively, in the − 40 to 80 ∘ C temperature range. This is to be compared with the open-loop change of 256 MHz (36%) and 590 MHz (44%) for the same temperature change. The monitoring oscillator power leakage to the RF cavities is optimised and measured to −101 dBm.