Frequency Servo Research Articles

Evaluating Track-Following Servo Performance of High-Density Hard Disk Drives Using Patterned Media

In recent years, track densities of magnetic hard disks have continued to grow. A promising approach to continuing the trend to ultrahigh density is using bit-patterned media (BPM). However, the implementation of BPM in hard disk drives (HDDs) to achieve high recording density is challenging and requires various new techniques, such as new servo pattern designs and position error signal (PES) decoding schemes. In applying BPM in HDDs, it is important to select a servo pattern providing sufficient PES quality for head positioning. In this paper, we discuss evaluation of PES quality and servo pattern performance from a closed-loop (servo) point of view in order to evaluate the quality of several servo patterns. We consider three servo patterns (the amplitude pattern, chevron pattern, and differential frequency pattern) as case studies. We developed a PES simulation tool to provide a realistic HDD track-following simulation. Because of PES nonlinearity in the amplitude servo pattern, we considered time-based servo patterns as alternatives. For time-based servo patterns, we found that readback signal sampling and transition jitter greatly affect PES quality. Therefore, we conclude that the differential frequency servo pattern is superior to other patterns, since it is less sensitive to transition jitter and readback signal sampling.

Measurement of position offset in hard disk drive using dual frequency servo bursts

This paper presents a new scheme for encoding position information in hard disk drives (HDDs). Modern HDDs use embedded servo patterns where the servo sectors containing information related to the location of the tracks are placed interleaved with the data blocks. Reduced length of the servo sector, therefore, means availability of more space for storage of data. The scheme of dual-frequency servo burst used in this paper enables more than 50% reduction in the length of the servo bursts that encode the position error sensing signal (PES). Different methods suitable for decoding PES from the samples of these servo bursts are also discussed, and a detection technique based on the concepts of discrete Fourier transform (DFT) is proposed. High sensitivity and flexibility of the digital PES allows the tracks to be placed closer which, is required to meet the demands of future generation HDDs. The dual frequency servo bursts are written on a 2.5-in disk to create tracks at a spacing of 10 /spl mu/-in, and the PES is decoded from the samples of the burst waveforms using DFT-based algorithms. The linearity of the decoded position offset proves the effectiveness of the proposed encoding method and the decoding algorithm.

Slave resonator to master oscillator frequency servo loop in CW injection phase locked lasers

This paper describes an undithered servo loop technique well suited to hold the slave oscillator in resonance with the master oscillator emitted light in a CW injection phase locking experiment. Application to waveguide CO 2 lasers is reported.