低中頻數據機架構中的混頻器之設計

Abstract

無線通訊為求達到體積小，重量輕，低耗電率，以及價格便宜等四項 趨勢，未來可能只有用積體電路設計才能同時滿足以上的需求。一個完整 的無線通訊系統主要可分為三部份:一.射頻模組(RF module)。二.基頻信 號處理(BasebandSignal Processor)。三.通信規約處理(Protocol Processor)。其中僅有射頻模組尚無法積體化，本文將分析其積體化的困 難點，並參考各項資料提出較為適合積體化的三種接收器架構:一.直接轉 換架構(Direct Conversion)。二.近似直接轉換架構(Quasi-direct Conversion)。三.低中頻架構(Low-IF)。然後對這三種架構的優缺點作分 析比較。 為了評估其可行性，我們實驗室正利用低中頻架構來設 計2.4GHz的跳頻式展頻系統，然而低中頻架構最大的困難是由於選擇較低 的中頻頻率，使得在降頻前無法將接收信號的影像頻率干擾信號用濾波器 將它濾掉或是壓低，甚至有些時後影像頻率是落在我們的工作頻段中，根 本就不可以使用濾波器來壓制它，然而若沒有事先將影像頻率壓低到一定 的範圍下，一但我們的接收器在工作的同時有其他的信號出現在接收頻率 的影像頻率，我們接收器的混頻器會同時將我們所要的信號與存在於影像 頻率的信號轉換成我們的中頻，此時出現在影像頻率的信號就直接成為我 們接收信號的干擾源。因此在低中頻架構中一定需要一個能將影像頻率消 除或是壓低的混頻器。 本文將針對我們的2.4GHz跳頻式展頻系統設計 一個影像頻率消除混頻器(Image Rejection Mixer)，為了能夠快速地確 認它的功能，我們將使用電晶體､電阻､電容､電感以及微帶線在印刷電 路板上設計並製成測試模具(TestFixtures)測量其功能，文中同時會對影 像頻率消除混頻器其中各部份的理論與模擬作深入的探討。 In order to achieve the future tendency of wireless communication, including small size, low weight, low current consumption, and low cost, we must use the integrated circuit technology to implement the RF module. A complete wireless communication system includes a RF module,a baseband signal processor, and a protocol processor. Except the RFmodule, both baseband signal processor and protocol processor are alreadyimplemented by integrated circuit technology. We simultaneously introducethree receiver architectures which could be used to implement RF module easily. The first is Direct Conversion Architecture. The second is Quasi-direct Conversion Architecture. The third is Low-IF Architecture. In our experiment, we use the Low-IF architecture to design the 2.4GHz Frequency Hopping Spread Spectrum System. It is difficult to suppressthe power of image frequency by a fixed filter, because the intermediate frequency is very close to our operating frequency. Sometimes the image frequency is also the frequency which is used by the other subscriber ofthe same system. It is impossible to suppress the power of image frequencyby a fixed filter. If we do'nt suppress the power of image frequency, itwill interfere our signal. Therefore, it is necessary that we use a imagerejection mixer as the down-converter of the Low-IF architecture. We will design a image rejection mixer which will be used in our 2.4GHz Frequency Hopping Spread Spectrum system. In order to estimate the performance of the image rejection mixer quickly, we will use the hybridtechnology to implement the image rejection mixer. Simultaneously we willintroduce the theory and the simulation program for the image rejectionmixer. Finally, we will finish the test fixtures and discuss the differencesbetween simulations and measurements of the image rejection mixer.