All high performance data storage devices utilizing magnetic disc technology require some type of servo mechanism to accurately and repeatedly position the write/read heads at the locations where data is to stored or retrieved. Timing information giving the head position relative to the disc rotation is also required. One method of implementing the servomechanism and generating the required timing involves permanently recording on one surface of one disc in the disc drive, a pattern of magnetic transitions or magnetic zones commonly called dedicated servo code. No data is ever recorded on this surface and the surface is "dedicated" servo and timing functions.
The pattern of magnetic transitions or magnetic zones, utilized for the servo code conventionally comprises a two-phase code. Two recording signals are used. These consist of pulses at the same repetition rate (frequency) but displaced in time relative to each other by one half of the period (180 degrees in phase), producing a discrete magnetic zone in each of two adjacent tracks in circumferentially spaced positions.
Position servoing is achieved by traversing recorded magnetic zones and reading the amplitude of generated pulses separated by 180 electrical degrees and using this information to control the radial position of the magnetic head so that the amplitudes are equal. The center of the magnetic head will then be half way between the locations where the two tracks of magnetic servo code were written.
Decoding the servo code to generate a position error signal requires knowledge of the timing relationship between the phases of the servo code. The position signal is basically the difference in amplitude between the two generated pulses with are 180 degrees apart. The timing is usually obtained from a phase locked loop which is locked to a specific phase of the servo code. Since the servo code produces more than one phase of generated signals the phase locked loop is not guaranteed to lock to the correct phase. Once locked, however, a properly designed phase locked loop will remain locked to whichever phase it has acquired.
A technique commonly used to force the phase locked loop to lock to the proper phase, employs an area of single phase code adjacent to the normal servo code. This area of single phase code is called a guard band and may be at either the inside diameter or the outside diameter, or both, of the servo code on the dedicated servo disc. The magnetic servo head can then be positioned over the single phase guard band and the phase locked loop forced to lock to the proper phase. The phase lock will be maintained when the head is positioned over the servo code and the position error signal can be properly decoded.
A magnetic servo head flying very close to the surface of a magnetic disc is affected by magnetic transitions located some distance on either side of the flux sensing element of the magnetic servo head. The sensitive distance can be as much as one half of the width of the body or slider of the magnetic servo head, which is equivalent to a significant number of tracks in a high density disc drive. When a servo head is positioned near a single phase guard band, with one side of the body or slider in the guard band, the affect of the guard band on the read signal is to slightly add to the servo code pulse which is in phase with the guard band pulse, but there will be no affect on the servo code pulse which is 180 degrees out of phase with the guard band. This introduces an offset or error in the position signal decoded from the servo code near the guard band. The magnitude of the error will depend on the magnetic head design and the proximity of the body or slider of the magnetic head to the guard band. In high performance disc drives, this error is sufficient to require correction. This present difficult servo design problems.