基於滑動相關器原理之向量網路分析儀架構

Abstract

向量網路分析儀(Vector Network Analyzer-VNA)是量測及分析 高頻或微波元件特性的重要儀器。使用向量網路分析儀是為了讓我們能得 到高頻或微波元件的傳輸(Transmission)及反射(Reflection)特性，反映 在振幅(Magnitude)及相位(Phase)上的量值。目前一般商用向量網路分析 儀所使用的網路量測原理大多採用同調超外差式接收機(Coherent Super- heterodyne Receiver)的架構。這種架構是在頻域上的量測方法：信號源 一點一點連續地掃瞄一段欲量測的頻段，而接收機使用兩個中頻將信號解 調而找出相對應頻率點的特性，再將這些頻率點組合起來而獲得欲量測頻 段的頻域特性。在某些校準實驗室中，則可能使用六埠網路(Six-port Network)向量網路分析儀，其量測結果較同調超外差式接收機架構的向量 網路分析儀精準，但硬體的複雜度及價格則相對提高。另一種網路量測的 方法為高速脈波時域法，此法多在某些特殊實驗室中使用，尚屬於研究階 段。 本文提出一種設計向量網路分析儀的新架構。在展頻通訊系 統中，有一種用於量測無線通訊頻道響應的滑動相關器技術(Sliding Correlator Technique)，在此被應用於向量網路分析儀的設計。這種技 術的基本原理是藉由信號源和接收機所產生虛擬亂數碼序列 (Pseudo Noise Code Sequence) 間位元速率 (Chip Rate) 的微小差異，造成信號 源的虛擬亂數碼序列與接收機的虛擬亂數碼序列形成互相滑動的效果；同 時利用一個虛擬亂數碼序列具有週期性脈衝之自相關性(Auto- correlation)的特性，來得到在時域上待測網路的脈衝響應，再藉由快速 傅立葉轉換 (Fast Fourier Transform) 以獲得在頻域上待測網路的頻率 響應。 本文的目的，即在詳述如何應用滑動相關器技術來設計向 量網路分析儀，以及分析使用滑動相關器原理製作向量網路分析儀的優劣 點。 A vector network analyzer is an important instrument for measuring and characterizing RF or microwave devices and components. We can obtain the transmission and reflection characteristics of RF of microwave devices by using a vector network analyzer. Most network analyzers currently sold on the markets adopt a super-heterodyne receiver architecture for its measurements. This architecture is a measurement method in frequency domain. The frequency of a signal source change continuously to trigger the device under test (DUT). Then the frequency response of our desired band can be detected by the receiver. Some calibration laboratory might use a six-port network analyzer in order to achieve more accurate measurement results, nevertheless at the expanse of much higher system complexity and cost. Another way to do network analysis is to use very high speed pulses and conduct measurements in the time domain. This method is still under development in some research laboratories. In this article, we propose a new vector network analyzer architecture. We use the sliding correlator techniques commonly employed in a spread spectrum communication system to design a vector network analyzer. A sliding correlator technique is based on the time domain sliding effect caused by a small clock frequency offset in the pseudo random code generators of a transmitter and a receiver. Utilizing the periodic impulse train like autocorrelation function property of the pseudonoise (PN) code sequence, the receiver is able to measure the characteristics of the device under test (DUT). A time scaled version of the impulse response of the device under test (DUT) can be obtained from the time domain slided pseudo random code signals by using a correlator. A fast Fourier transform of the measured impulse response is the frequency response of the device under test. This paper describes how to design a vector network analyzer based on the sliding correlator technique and analyzes the advantage of using this method.