Transition Jitter Research Articles

Bit errors due to channel modulation of media jitter

Recording media written-in transition jitter noise can limit magnetic recording systems. Read channel intersymbol interference (ISI) modulates media jitter in a head/disk/channel, attenuating jitter at low linear density due to classical ISI but amplifying at high density. This paper assesses whether jitter modulation is significant in typical disk drive peak detect (PD) and partial response (PR) channel design, where maximum linear bit density is sought at a specified bit error rate (BER). Experimental isolated pulse data, transition noise, and electronic noise measurements from a 65 mm head/disk recording design are used as inputs to a Matlab simulation of PD and PR Lorentzian channel low pass and boost filters, PD differentiator filter, PR digital filter, and PD/PR bit detectors. The goal of such models is optimizing the designable PD and PR channel parameters for realistic maximum linear density prediction. However their complexity obscures the jitter modulation contribution, which is demonstrated here by the shift in the EER versus density curves, when the jitter modulation is mathematically turned off. Jitter modulation is thereby shown to be significant but not dominant in (1,7) PD density limits. About an order of magnitude BER loss due to jitter modulation can occur at typical PD linear channel bit densities P/sub 50//T/spl ap/2. Jitter amplification in PD channels can occur at channel density 3.8 (above usable PD limits). PR channels only amplify media jitter, degrading EER by about an order of magnitude at typical (0, k) PR4 densities, and degrading timing channel accuracy twice as fast as channel density increases. Lorentzian isolated pulse (ISOP) modeling is compared to recording physics mathematical pulses, and simulation statistical validity checks are made.

Media noise and signal-to-noise ratio estimates for high areal density recording

Transition jitter, σjitter, and pulse-width fluctuations, σPW50, limit the attainable signal-to-noise ratio (SNR) in a magnetic recording system. We estimate the media noise power for systems with high areal density. The case of a Lorentzian-shaped voltage pulse is worked out in detail. Experiments show that the model reasonably reproduces data. The analysis is applied to areal densities of 10 Gbit/in.2 and beyond. If the ratio of PW50 to bit length B (the so-called channel density U) is kept constant, the ratio σjitter/B is found to depend only on SNR and U, and is about 3% for SNR=26 dB, at any density. Under those assumptions, σjitter≈0.75 nm at 1 Mfci. Otherwise, for constant PW50, σjitter/B decreases monotonically with density, and is about 1% for PW50=120 nm, 1/B=1 Mfci, SNR=27 dB.

Time interval analysis errors in measuring recording media transition jitter

Media transition position jitter noise can be a dominant signal-to-noise ratio (SNR) issue in magnetic recording systems with magnetoresistive (MR) heads and (0,k) PRML channels, since the high MR head gain emphasizes media jitter noise over electronic noise, and (0,k) codes are sensitive to dibit jitter. Time interval analyzer (TIA) instruments are useful for measuring transition jitter (write jitter), since they can separate write jitter from electronic noise (read jitter) with minimal synchronization and speed variation difficulties. This paper analyzes TIA errors due to read channel intersymbol interference (ISI), which occurs even though TIA measurements are made with periodic LF or HF bit patterns at equal bit intervals T, which should be unchanged by ISI. Theory and data show that TIA measurements underestimate jitter by as much as 40% at low channel bit density D/sub c//spl equiv/P/sub 50//T/spl ap/1.5-1.8, and overstate jitter at high density. Even though typical TIA jitter measurements are made between nonadjacent pattern pulses, these errors still occur due to ISI by unmeasured neighbor pulses. It is shown that similar errors occur with TIA electronic read jitter noise measurements. At channel densities above D/sub c//spl ap/2.6, channel amplification of transition jitter occurs, which may be a factor in PRML channel bit density limits.

Effect of Trackwidth and Linear Spacing on Stability and Noise in Longitudinal and Perpendicular Recording

Results from spin-stand and MFM experiments showing the effect of demagnetizing fields for both perpendicular and longitudinal media are presented. Spin-stand experiments involved measurement of the current required to erase square wave recordings to half amplitude. While longitudinal media require smaller currents to erase higher density recordings, perpendicular media require higher currents. MFM studies indicate differences in the onset of the magnetization reversal. The reversal initiates near the transition in longitudinal media but further from the transition in perpendicular media. Experimental results showing an increase in transition jitter with decreasing trackwidths are also reported for both longitudinal and perpendicular media.

Gauge testing of transition jitter measurements for longitudinal thin film magnetic recording

A gauge test for measuring transition jitter with acceptable accuracy is discussed and presented. The dependence of the transition jitter measurement on head design for a given longitudinal thin film magnetic recording medium is also presented. It is shown that the head type is an important parameter of the measurement of transition jitter in a write process, and the effect due to different head types must be taken into account.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Optimization of low-noise media for 100-Mb/in.2 recording with a magnetoresistive head

Single-layer and multilayer longitudinal media were evaluated for 100-Mb/in.2 recording at a 6-μin. flying height with a magnetoresistive head. It was found that the multilayer media produced transition jitter values that were at least 23% below comparable single-layer results. Timing margins, estimated from the measured jitter and asymmetry data, indicate that single-layer media may prove marginal for this application. Multilayer timing margins were projected to be 40%. Signal-to-noise ratio, resolution, and overwrite comparisons are also made for both types of media. The improvements demonstrated with the multilayer media are attributed to the reduced media noise associated with this type of structure. Increased Cr spacer layer thickness in the multilayer media was found to increase jitter and reduce the D70 density response.

Longitudinal media for 1 Gb/in/sup 2/ areal density

Media with low noise at high transition density that demonstrate satisfactory recording performance at an areal density, of 1 Gb/in/sup 2/ when combined with dual-element (magnetoresistive read/inductive write) heads have been fabricated. A media structure of C/CoPtCr/Cr was utilized over a range of magnetic parameters: coercivity approximately=1600-1800 Oe, remanence-thickness product approximately=0.7*10/sup -3/ emu/cm/sup 2/, and coercive squareness approximately=0.7-0.8. Media noise reduction was accomplished by optimizing the film-growth characteristics to reduce intergranular exchange coupling in the magnetic layer. The low-noise characteristics of the media are manifested in their low transition jitter values, 5 nm for 3- mu m track width, and the absence of supralinear increase in media noise power with linear density up to 3000-3500 fc/mm. The -6-dB rolloff densities are in the range 4000-5000 fc/mm. Overwrite values are typically better than 40 dB. Microstructural analysis indicates that the reduced transition noise of the present media is due to physical separation of the grains in the magnetic films, which reduces the exchange coupling between the magnetic grains. The reduced coercive squareness of the low-noise media degrades the overwrite performance and is also expected to decrease the linear density resolution of the media. >

Characteristics of overwrite-induced bit shift

Overwrite-induced bit shift was investigated on high-density helical scan tape recorders using ferrite heads and metal particle tape. A high-precision time interval (TI) counter was used to measure the transition jitter. By comparing the TI histogram of pseudorandom-data-sequence and single-tone-frequency recordings, it was found the overwrite-induced bit shift is mostly due to a low-frequency noise. Properly filtering the low-frequency content of the signal through either channel code selection or playback data detection techniques can greatly reduce the shift. Implementation of overwrite digital VTRs using DC-free Miller squared code is described.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Jitter vs. additive noise in magnetic recording: Effects on detection

Noise on thin-film magnetic recording media is often modeled as a transition jitter, whereas particulate media noise is taken to be additive. Although these two noise mechanisms can produce similar spectra their effects on detection can be markedly different. This paper analyzes these differences for several signaling/detection schemes and concludes that where transition jitter dominates, peak detection of

Transition position and amplitude fluctuation noise model for longitudinal thin film media

Integrated noise power in longitudinal thin film media increases with transition density. This phenomenon has been attributed to the medium noise being concentrated in the transition regions with the areas between transitions being relatively quiet. This paper presents a transition noise model for square wave recording on longitudinal thin film media considering two sources of noise: phase shift (or transition jitter) and amplitude. The transition jitter noise is due to random fluctuations in the transition positions, while the amplitude noise is due to random shape fluctuations in the transitions. It is assumed that random medium coercivity fluctuations cause transition position fluctuations. The random transition shape fluctuations are modeled by allowing the transition parameter a to be a random variable. These transition parameter variations are assumed to be caused by random medium magnetization fluctuations. Each written transition produces a demagnetizing field that affects the positions and shapes of subsequent transitions. The properties of both types of noise are discussed.

Timing recovery in optical receivers for NRZ signalling

The performance of a bit timing recovery scheme for NRZ signalling in an optical receiver employing an avalanche photodiode is investigated. The influence of pattern noise, bit transition jitter and receiver parameters on the bit timing jitter of the recovered clock is quantitatively assessed.