數位磁性記錄系統之時序回復與等化技術研究

Abstract

在數位磁性記錄系統中，通常使用信號處理技術來增加儲存資料之密度。 然而，在高記錄密度的磁性記錄系統中，所讀出之信號常常含有非線性碼 際干擾。因此，本論文使用一種非線性之判別回授等化器用以消除非線性 碼際干擾之影響，此一非線性等化器通常將傳統的線性等化器之線性回授 部分以一非線性濾波器取代之，例如一般所使用的福特拉濾波器，稱之為 福特拉判別回授等化器。從理論分析中，我們可以得知福特拉判別回授等 化器之最小均方差為取樣相位之函數，此一函數之關係直接影響到等化器 之輸出信號雜訊比，而結合通道估測器之包率取樣時序回復系統以求得最 佳之取樣相位。由電腦模擬之結果，我們可以得知福特拉判別回授等化器 無論是在輸出信號雜訊比或是在錯誤率上之效益都比傳統之線性判別回授 等化器好。在部份響應通道上，我們提出一種擁有快速“搜尋”和避免“ 拖延”現像此兩種優點之修正型門檻判別時序回復系統用以找尋最佳之取 樣相位。而對於一般之磁性記錄通道，部份響應等化器之作用是將磁性記 錄通道根據目標信號等化成所需之部份響應通道，而由於此一系統之效能 與所取樣之相位有密切之關係，並且其最大輸出信號雜訊比是在通道暫態 響應之最高點的相位上，因此我們在時序回復之架構中提出將時序函數定 義成通道暫態響應的線性組合，並依此提出一時序誤差估計演算法以求得 最佳取樣相位。 In high density digital magnetic recording systems, the nonlinear ISI may appears in the readback waveform. In this dissertation, the Volterra-DFE is proposed to equalize the nonlinear ISI. By theoretical analysis, the minimum MSE of the Volterra-DFE is found to be a function of sampling phase. This will affect the output SNR of the playback system. A baud-rate timing recovery system in conjunction with channel estimator is used to operate the sampling clock on the optimum phase. By computer simulation, the Volterra-DFE is found to outperform the conventional DFE in both output SNR and BER in a high density magnetic recording system. In partial response channel, the modified threshold decision can achieve fast acquisition and avoid hangup effects in the timing recovery. The recording channel can be equalized by the partial response equalization to a partial response channel. The equalizer has maximum output SNR at the peak position of transition response. By defining the timing function with respect to the transition response and using a timing error estimation algorithm, the timing recovery scheme will converge to the optimum sampling phase.