阻抗感測器與1.95GHz鏡像抑制混頻器設計

Abstract

隨著通訊世代的演進，手機中射頻前端模組的複雜性呈現指數級的成長，如何提高天線效率在智慧型手機的整體射頻性能是最關鍵的挑戰。其中集成天線的應用較容易受到外部條件改變所影響，進而導致天線與射頻前端間的阻抗不匹配。阻抗不匹配時會降低射頻前端和天線間的功率傳輸並增加通信單元對雜訊的敏感性。因此本論文設計實現之阻抗感測器 (Impedance Sensor)，用來感測環境中是否存在外部干擾，並將阻抗調諧網路設置至最佳的狀態，以便在不同的使用條件下都能夠使裝置運作在較高的效能範圍內。第一顆電路為阻抗感測器，透過定向耦合器 (Directional Coupler) 以及雙端天線結構來提升感測器的解析度。透過雙端天線的二次反射來增加阻抗檢測訊號的變化量，並提出兩種架構來提高對於阻抗感測的敏感度。第二顆電路為鏡像抑制混頻器，採用I/Q 訊號調變的方式消除鏡像訊號，並在IF端加上緩衝放大器來提升整體轉換增益。當LO驅動功率為3 dBm時，在1.5 GHz至2 GHz的鏡像拒斥比皆小於 - 35 dBc，轉換增益為3.863 dB，功耗約為10.8 mW，晶片佈局面積為1.2 mm × 1.14 mm。

With the evolution of communication generations, the complexity of RF front-end modules in mobile phones has grown exponentially. How to improve the antenna efficiency is the most critical challenge in the overall RF performance of smart phones. Among them, the application of the integrated antenna is more easily affected by the change of external conditions, which leads to the impedance mismatch between the antenna and the RF front-end. Impedance mismatches reduce power transfer between the RF front end and the antenna and increase the susceptibility of the communication unit to noise. Therefore, the Impedance Sensor designed and implemented in this paper is used to sense whether there is external interference in the environment, and set the impedance tuning network to the best state, so that the device can operate in a higher performance range under different usage conditions.The first circuit is an Impedance Sensor, which uses a Directional Coupler and a double-ended antenna structure to improve the resolution of the sensor. The variation of the impedance detection signal is increased through the secondary reflection of the double-ended antenna, and two structures are proposed to improve the sensitivity of the impedance detection.The second circuit is a 1.95 GHz Image Rejection Mixer, which uses I/Q signal modulation to eliminate the image signal, and adds a buffer amplifier at the IF port to increase the overall conversion gain. When the LO power is 3 dBm, the image rejection from 1.5 GHz to 2 GHz is less than -35 dBc, the conversion gain is 3.863 dB, the power consumption is about 10.8 mW, and the chip size is 1.2 mm × 1.14 mm.