GPS接收機窄頻干擾抑制與頻率轉移之設計

Abstract

摘 要 全球定位系統(GPS)可在各種導航、時頻同步等工程應用領域提供精確的位置與時間資訊。但在某些場合，GPS接收機受到外界射頻訊號(RF)的干擾，發生GPS定位精確度降低的現象，更甚者使接收機同步功能失鎖而無法接收衛星訊號。有鑑於此，GPS接收機的干擾抑制研究，已成為一重要且實際的課題。

在本論文中，運用重疊快速傅立葉轉換(OFFT)來呈現對GPS窄頻干擾的抑制效果，首先利用快速傅立葉轉換(FFT)將訊號轉換至頻域，找出頻譜上之干擾成分並予以抑制或去除，再將去除干擾後之頻譜施以快速反傅立葉轉換(IFFT)而得時間訊號。除了干擾抑制處理外，為了使信號規格與GP2015和GP2021晶片匹配，加入頻率轉移將訊號中心頻率1.16MHz轉換至1.405MHz，此設計採用移相形成法，以完成單邊帶調變技術。

最後，將本設計應用在Altera Stratix EP1S25F780C模擬板上，並利用Altera QuartusⅡ發展軟體作電路的合成、電路配置與繞線以及時序模擬，並透過Matlab軟體來驗證窄頻干擾抑制與頻率轉移電路之可行性。經過模擬及驗證後可知，窄頻干擾抑制電路針對單頻及多頻干擾有較佳的抑制效果，但所需的硬體成本較高，且運算時間較長。

Abstract GPS is an accurate navigating system and generally used in civilian or military applications. But GPS receiver performance is usually degraded by intentional or unintentional RF interference. The interference may cause the navigation error and even saturate the receiver. In this thesis, we use overlapped fast Fourier transform (OFFT) to simulate the performance of narrowband interference suppression. First, fast Fourier transform (FFT) can translate time sequence after analog to digital converter (ADC) to frequency domain. Then excision processing suppresses interference power in the spectrum. Inverse fast Fourier transform (IFFT) reconstruct the time sequence after interference excision. Besides interference suppression processing, in order to match specifications of the GP2015 and GP2021 chips, we design the frequency translation to transfer the central frequency of the GPS signal from 1.16MHz to 1.405MHz. The utilization of the phase-shifted method completes single sideband modulation. Finally, according to the fixed-point simulation, we choose Altera Stratix EP1S25F780C5 chip to design the hardware circuits. Altera Quartus Ⅱ software is utilized for logic synthesis, circuit place and route and timing simulation. The Matlab software is used to verify the design of the narrowband suppression circuit and the frequency translation circuit. The simulation and the verification can show that overlapped fast Fourier transform is a powerful approach to suppress the single tone and multi-tone narrowband interference. But it costs much latency and needs much hardware cost.