Complexity issues in RAM-DFE design for magnetic disk drives

Design of a high-speed adaptive decision feedback equalizer (DFE) for detection of magnetic disk data is described. A RAM is used in the feedback path instead of the usual transversal filter, and the equalizer is referred to as the RAM-DFE. The RAM compensates for trailing nonlinear intersymbol interference in the disk data. The structure and operation of the RAM-DFE is described, and it is shown that implementation of the RAM-DFT requires circuitry for timing recovery, gain adjustment, synchronization, and training. The overhead of these subsystems coupled with the high-speed requirements of disk drives makes implementational complexity a key design issue. The effects of finite precision, latency, implementation of the feedback path, and training on the performance and complexity of the system are discussed. The RAM-DFE has been designed on a single VLSI chip using BiCMOS technology. Simulations predict clock speeds in excess of 54 MHz, corresponding to data rates in excess of 54 Mbit/s