6GHz縮小化FMCW雷達系統

Abstract

本篇論文是一套6GHz 縮小化FMCW雷達。從天線到射頻模組以及FMCW演算法的改進。天線部份其一是超寬頻天線架構。有別於傳統超寬頻天線大多是用一大片金屬來當輻射體或是多個不同頻段之天線來達到寬頻的要求，此天線架構是利用一段靠近地的間隙來使天線的輸入阻抗在極大的頻寬內沒有很大的變化。此天線架構類似常見的倒F型天線(Inverted-F antenna)，但縮小天線與地的間距，即可得到一相當寬頻的天線。另一個是天線陣列，使用槽孔天線當天線元件做成的2x1的天線陣列。來滿足所需之天線指向性。 FMCW雷達演算法改進是基於學長之前所做的24GHz FMCW雷達系統再加以改良。本來是用一連串的鋸齒波，並對回波訊號做2D的FFT來達到偵測目標物距離與相對速度的功能。但舊有的演算法須要較大的運算量以及較高的硬體要求，所以我用了不同的調變方法。有別於一連串的鋸齒波調變，我使用一個較慢的三角調變波，可避免相度速度的影響，且大幅減少運算量以及硬體的要求。此外，為了在有限頻寬內提高距離解析度，所以加入了補零的動作，並根據舊有的訊號特性改進了原來的中頻放大器。在距離解析度上至少提高了一倍的準確度，改進後的中頻放大器也讓雷達系統的靈敏度提高10dB以上。 6GHz FMCW縮小化射頻模組，跟24GHz FMCW雷達不一樣。為了縮小化所以使用同一根天線當接收與發射使用。且系統中只使用了一個6GHz 壓控震盪器以及一個平衡混頻器。使用單天線會受到天線特性而對系統靈敏度有很大的影響，不過基於24GHz FMCW雷達的經驗，能壓抑天線特性的影響進而完成模組的整合。

A 6GHz compact size radar system is proposed in this article. There are several parts; antenna, Rf module and DSP improvement. This article proposed two different kinds of antenna. One is a novel wideband antenna. A small gap between radiator and ground smoothes the input impedance variation. The antenna structure proposed quite like a Inverted-F antenna. Just lower the gap size, this common antenna type comes a wideband antenna. Another proposed antenna is a 2x1 aperture couple antenna array. Aperture couple antenna has good directivity naturally. So a array with two antenna element can satisfied the requirement. Digital signal processing is base on the previous approach and adds some improvement. 2D FFT is used in previous approach. Using different control wave can save system resource and 1D Chirp Transform is enough. Zero padding technology is also used to improve distance and relative velocity resolution. And a modified IF amplifier is also proposed. This amplifier upgrades system sensitivity to 10dB better. At the last is a compact size 6GHz FMCW radar module. In order to have smaller dimension this RF module has a VCO, a hybrid mixer and a filter only. Not like 24GHz radar system, this radar system uses the same antenna for Tx/Rx. Using single antenna can save half antenna area but the characteristic of antenna affects a lot. Base on the experience in 24GHz radar system there are some approaches to suppress the effect of antenna and implement this compact size radar system successfully.