Active RF Front-End Circuits Based on High-Q Cavity Resonators

Yu-Chen Wu, Purdue University

Abstract

High-quality cavity resonators are particularly attractive for the next-generation wireless communication systems since they enable frequency-selective functions at reasonably low loss and high selectivity. By utilizing these resonators, the RF front-end circuits can potentially achieve wideband and high-performance. In this work, several RF front-end blocks and frequency control systems based on high-Q cavity resonators are presented. In the first part, a real-time temperature compensation control system for tunable high-Q cavity-based filters is designed, implemented, and experimentally validated. Both bandpass (700–1000 MHz) and bandstop (1300–1600 MHz) filters 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 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°C and 80°C range. The average system resolution varies from 0.23 MHz 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 bandpass and bandstop filters, respectively, in the –40°C to 80°C temperature range. This is to be compared to the open-loop change of 256 MHz (36%) and 590 MHz (44%) for the same temperature change. Then, an L-band low phase noise evanescent-mode (EVA) cavity-based frequency synthesizer is developed, implemented, and experimentally validated. The oscillator is integrated on the substrate of a high-Q (Q u ≥ 430) EVA resonator board to reduce parasitics in the circuit. A piezoelectric actuator is used as a tuner. A phase-locked loop (PLL) is used to control the output frequency. The frequency tuning range is from 1000 MHz to 1700 MHz (1.7:1). The circuit achieves a phase noise as low as –94 dBc/Hz at 10 kHz offset, –122 dBc/Hz at 100 kHz offset, and –146 dBc/Hz at 1 MHz offset. The RMS jitter of the system is 267 fs at 1000 MHz. The frequency synthesizer consumes 64.3 mW without the active loop filter, which consumes 200 mW. Lastly, two high-efficiency power amplifiers (PAs) are proposed. The first PA is proposed by using a non-resistive high-Q bandwidth-extension technique. Specifically, the problem of proper termination of the second harmonic in power amplifiers with over 2:1 bandwidth is addressed. A tunable narrowband high-Q band-stop filter is included at the output of the power amplifier to properly terminate the transistor at its lower-band frequencies whose second harmonics fall within the bandwidth of the amplifier. The presented design is experimentally validated by manufacturing and measuring a 0.7–2.2 GHz GaN wideband high-efficiency power amplifier terminated by a high- Q (measured Q of 900) narrow-band (10 dB bandwidth of 2 MHz) band-stop filter tunable from 1.4–2.2 GHz. An over 3:1 bandwidth is achieved with measured output power of 40 dBm, average gain of 10 dB, and efficiency of ¿ 64%. The proposed termination results in an average improvement of over 10% throughout the entire bandwidth. The second PA, co-designed with a two-pole EVA cavity-base impedance tuner, is load-reconfigurable. A high-Q impedance tuner is used as the output matching network of the power amplifier to properly terminate the transistor various load impedances. The presented design is experimentally validated using GaN transistor and measured at 2.5 GHz. The quality factor of the impedance tuner is extracted from measurements and found to be approximately 300. The PA with the impedance tuner reaches 76% efficiency at VSWR = 1, 6375% at VSWR = 2, and 5062% at VSWR = 4. The maximum output power of the PA is 35 dBm (3.16 W). This work presents several high-performance RF front-end circuit designs, by utilizing high-Q cavity resonators, along with a control system. It also shows the potential and the possibility of wideband and high-quality RF front-end circuits and systems based on cavity resonators for future demanding wireless communication systems.

Degree

Ph.D.

Advisors

Peroulis, Purdue University.

Subject Area

Electrical engineering

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