U.S. patent application number 11/189594 was filed with the patent office on 2007-02-01 for system, method, and apparatus for detecting signal-to-noise ratio decay in perpendicular magnetic recording.
Invention is credited to Shanlin Duan, Jizhong He, Zhupei Shi, Jane Jie Zhang.
Application Number | 20070024276 11/189594 |
Document ID | / |
Family ID | 37693618 |
Filed Date | 2007-02-01 |
United States Patent
Application |
20070024276 |
Kind Code |
A1 |
Duan; Shanlin ; et
al. |
February 1, 2007 |
SYSTEM, METHOD, AND APPARATUS FOR DETECTING SIGNAL-TO-NOISE RATIO
DECAY IN PERPENDICULAR MAGNETIC RECORDING
Abstract
A magnetic test module runs on a spin stand to detect amplitude
decay and noise evolution at the same time. Signal-to-noise ratio
(SNR) decay is directly measured. The recording performance is
correlated better with SNR instead of signal only. The thermal
stability of the system is evaluated more accurately with this SNR
decay method. A heater is placed under the media disk, and a remote
sensing thermometer and temperature controller form a subsystem to
set up desired environmental temperature. The heater creates a
heated band and the read/write head flies above the heated band.
The temperature control system may be removed when SNR decay
measurement is performed under room temperature.
Inventors: |
Duan; Shanlin; (Fremont,
CA) ; He; Jizhong; (San Jose, CA) ; Shi;
Zhupei; (San Jose, CA) ; Zhang; Jane Jie; (San
Jose, CA) |
Correspondence
Address: |
BRACEWELL & GIULIANI LLP
P.O. Box 61389
HOUSTON
TX
77208-1389
US
|
Family ID: |
37693618 |
Appl. No.: |
11/189594 |
Filed: |
July 26, 2005 |
Current U.S.
Class: |
324/212 ;
G9B/5.024; G9B/5.026; G9B/5.033 |
Current CPC
Class: |
G11B 5/4555 20130101;
G11B 5/012 20130101; G11B 2005/0021 20130101; G11B 2005/001
20130101; G11B 5/02 20130101; G01R 33/1207 20130101; G11B 5/09
20130101 |
Class at
Publication: |
324/212 |
International
Class: |
G01R 33/12 20060101
G01R033/12; G01V 3/00 20060101 G01V003/00 |
Claims
1. A method of magnetic testing of disk drive on a spin stand, the
method comprising: (a) providing magnetic storage media; (b)
mounting the magnetic storage media to a spin stand; (c)
establishing a desired environmental temperature for the magnetic
storage media; (d) operating the magnetic storage media; and (e)
detecting signal amplitude decay and noise evolution in the
magnetic storage media at the same time to directly measure
signal-to-noise ratio (SNR) decay in the disk drive over time.
2. A method according to claim 1, wherein the magnetic storage
media utilizes one of perpendicular magnetic recording (PMR) having
a soft magnetic underlayer (SUL) and longitudinal magnetic
recording (LMR).
3. A method according to claim 1, wherein step (c) comprises
positioning a heater, a remote sensing thermometer, and a
temperature controller adjacent the magnetic storage media to
establish the desired environmental temperature.
4. A method according to claim 3, wherein the heater heats a radial
swath of the magnetic storage media to form a heated band while a
read/write head flies above the heated band.
5. A method according to claim 1, wherein step (c) comprises
operating the magnetic storage media at room temperature.
6. A method according to claim 1, further comprising interleaving
an aged signal and a test signal on a same written track on the
magnetic storage media, and using write gate and read gate features
to control a sequence of aged magnetic bits and test magnetic
bits.
7. A method according to claim 6, wherein the aged magnetic bits
act as a reference signal to eliminate adverse effects of thermal
drift and sensitivity change.
8. A method according to claim 1, wherein the SNR decay module
performs multi-density measurement including KFCI and frequency
measurement at the same time to enhance test throughput and
characterization efficiency.
9. A method according to claim 1, further comprising implementing a
spectrum analyzer into the spin stand to measure noise in frequency
domain, and obtaining integrated noise when frequency sweep is
performed with a spectrum analyzer as a function of time.
10. A method according to claim 1, further comprising improving a
data-taking efficiency by properly selecting noise samples and
constructing noise sensitivity to define integrated noise as a
quantity for monitoring noise evolution.
11. A method of magnetic testing of disk drive on a spin stand, the
method comprising: (a) providing magnetic storage media; (b)
mounting the magnetic storage media to a spin stand; (c)
establishing a desired environmental temperature for the magnetic
storage media by positioning a heater, a remote sensing
thermometer, and a temperature controller adjacent the magnetic
storage media to establish the desired environmental temperature;
(d) operating the magnetic storage media; (e) detecting signal
amplitude decay and noise evolution in the magnetic storage media
at the same time to directly measure signal-to-noise ratio (SNR)
decay in the disk drive over time; and (f) interleaving an aged
signal and a test signal on a same written track on the magnetic
storage media, and using write gate and read gate features to
control a sequence of aged magnetic bits and test magnetic bits,
such that the aged magnetic bits act as a reference signal to
eliminate adverse effects of thermal drift and sensitivity
change.
12. A method according to claim 11, wherein the magnetic storage
media utilizes one of perpendicular magnetic recording (PMR) having
a soft magnetic underlayer (SUL) and longitudinal magnetic
recording (LMR).
13. A method according to claim 11, wherein the heater heats a
radial swath of the magnetic storage media to form a heated band
while a read/write head flies above the heated band.
14. A method according to claim 11, wherein step (c) comprises
operating the magnetic storage media at room temperature.
15. A method according to claim 11, wherein the SNR decay module
performs multi-density measurement including KFCI and frequency
measurement at the same time to enhance test throughput and
characterization efficiency.
16. A method according to claim 11, further comprising implementing
a spectrum analyzer into the spin stand to measure noise in
frequency domain, and obtaining integrated noise when frequency
sweep is performed with a spectrum analyzer as a function of
time.
17. A method according to claim 11, further comprising improving a
data-taking efficiency by properly selecting noise samples and
constructing noise sensitivity to define integrated noise as a
quantity for monitoring noise evolution.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Technical Field
[0002] The present invention relates in general to perpendicular
magnetic recording in disk drives and, in particular, to an
improved system, method, and apparatus for detecting
signal-to-noise ratio decay in perpendicular magnetic
recording.
[0003] 2. Description of the Related Art
[0004] Thermal decay due to the "superparamagnetic effect" in
longitudinal magnetic recording is becoming a significant concern
as the rate of areal density increases rapidly. In the past,
substantial effort was devoted to characterizing media thermal
decay and understanding the impact of media decay on the ultimate
recording performance. Precise and quantitative prediction of media
thermal decay lifetime was commonly believed to be an important
task. In the prior art, amplitude or magnetization versus time was
measured, then the decay rate was determined in percentage decrease
per decade of log(time).
[0005] Perpendicular magnetic recording (PMR) has been investigated
as a way to extend beyond the "superparamagnetic limit" of
conventional longitudinal recording. Hard disk drive (HDD)
companies are intensively engaged in PMR development. Generic
architectures of longitudinal recording 11 and perpendicular
recording 13 are depicted in FIG. 1. An important feature of PMR is
the "deep gap field" generated by the single pole writer and the
soft magnetic underlayer (SUL) in the recording media. However,
there are two concerns with PMR. The demagnetization field is
generally larger in the PMR, which may induce an increase in
background noise over time. Secondly, the SUL is part of the writer
during the writing process, but is embedded in the PMR media. SUL
is a soft magnetic material (e.g., NiFe permealloy) and easily
forms a variety of complex domain structures. The domain movement
over time and temperature in the SUL may generate adverse noise
evolution. The characterization of these noise dynamics is
important to successfully launch PMR HDD products.
[0006] The article, An Experimental Study of the Effect of Thermal
Decay on Noise and Nonlinear Distortions in Perpendicular Media, by
W. Zhu, H. Zhou, and J. Judy (IEEE Magn. 40, No. 4, p. 2610-2612
(2004)), provides details of writing a pseudo-random sequence on
perpendicular magnetic media and using an oscilloscope to monitor
nonlinear distortion and its related noise background for
addressing SNR decay. That method was employed in longitudinal
magnetic recording in the past and, thus, is merely an example of
applying the same method to perpendicular media without any
improvements. Unfortunately, that method cannot be implemented in
any PMR disk media manufacture testing or at an early stage of PMR
media development due to its complexity, the difficulty of
capturing very small signal distortion signals, and the difficulty
of accurately monitoring an evolution of small distortion as a
function of time.
[0007] U.S. Patent Application No. 2003/0016461, provides a method
of determining a time domain equalized, signal-to-total distortion
ratio and an equalized signal-to-noise ratio via writing a
pseudo-random 127-bit pattern on a magnetic media. That disclosure
determines thermal characterization in HDDs and is not applicable
to component level testing, such as magnetic disk media screening.
It also requires a channel IC chip for waveform equalization, and
needs several specific analog-to-digital conversion (ADC) and
digital-to-analog conversion (DAC) units for manipulation. It is
impossible to implement that design on conventional spin stand
magnetic testers. Thus, one skilled in the art would not employ
that prior art method for routine PMR disk media testing and
screening. Finally, U.S. Pat. No. 6,630,824, and U.S. Patent
Application No. 2002/0063559, provide performance evaluation
methods for signal decay, but not SNR decay.
[0008] Thus, although the prior art measures signal decay in
magnetic recording systems, it is unable to analyze the
noise-induced instability in PMR systems. It would be desirable to
develop a comprehensive method to resolve this issue. In the
present disclosure, a solution is presented for detecting signal
decay and noise evolution at the same time.
SUMMARY OF THE INVENTION
[0009] In one embodiment of the present invention, a magnetic test
module runs on a spin stand to detect amplitude decay and noise
evolution at the same time so that signal-to-noise ratio (SNR)
decay can be directly measured. The PMR recording performance, such
as BER (bit error rate), is correlated better with SNR instead of
signal only. The thermal stability of the PMR system is evaluated
more accurately with this SNR decay method. Thus, in one
embodiment, the present invention discloses a method of detecting
SNR decay, while the prior art only detects signal decay.
[0010] The SNR decay is integrated into a spin stand for magnetic
testing. One embodiment includes a heater located under the media
disk, a remote sensing thermometer, and a temperature controller
that work as a subsystem to establish a desired environmental
temperature. The heater creates a heated band in a disk while the
read/write head flies above the heated band. The temperature
control system may be removed when SNR decay measurement is
performed under room temperature.
[0011] Media that may exhibit a significant amount of signal decay
after several years of recording are not practically useful. The
HDD product media that would exhibit little signal decay after
several years of recording would likely exhibit almost negligible
signal decay for a reasonably short period after recording. It
poses a challenge to reliably measure extremely small changes of
the recorded signals because of thermal drift and sensitivity
changes of the transducer of the recording head.
[0012] In one embodiment, the aged signal and test signal are
interleaved on the same written track. Write gate and read gate
features are used to control the sequence of aged magnetic bits and
test magnetic bits. The aged magnetic bits also act as a reference
signal to eliminate the adverse effects of thermal drift and
sensitivity change mentioned above.
[0013] A SNR decay module was developed to operate decay
measurement on a spin stand. For example, SNR decay is integrated
into the operation system of one type of spin stand tester. In the
SNR decay module, a mechanism of separating aged signal and test
signal is illustrated in terms of data processing. As discussed
above, the combined result of thermal drift and sensitivity change
leads to an undesirable fluctuation in the measured aged signal and
test signal. Such fluctuation can be effectively eliminated by
taking the ratio of the test signal and aged signal. Taking aged
written bits as a reference works well to detect extremely small
changes in the recorded signal decay.
[0014] In order to measure SNR decay, the noise evolution is
measured at the same time of detecting signal decay. A spectrum
analyzer is implemented into the spin stand to measure the noise in
frequency domain. Integrated noise can be obtained when a frequency
sweep is performed. One can monitor this integrated noise as a
function of time. Frequency sweep and noise integration is a long
process and takes much more time than detecting the aged/test
signal. The data-taking efficiency can be improved by properly
selecting noise samples and constructing noise sensitivity to
define integrated noise as a quantity for monitoring noise
evolution.
[0015] Running the SNR decay module on a spin stand automatically
provides signal decay and noise evolution at the same time. The
decay rate can be evaluated by processing this collected data. The
signal decay, noise increase, and SNR decay are a function of
magnetic bit density. SNR decay is dominated by noise evolution at
lower magnetic bit density. This result reveals one aspect of PMR
that demagnetization at lower density creates significant noise
background. The developed SNR decay module is a significant
characterization tool as it impacts the PMR HDD product design,
development, and manufacture. The developed SNR decay module can
also be applied to current longitudinal magnetic recording. The
measured SNR decay for one type of HDD media is well correlated
with the HDD file data. This provides another example of the SNR
decay module to be used for manufacture yield analysis.
[0016] The foregoing and other objects and advantages of the
present invention will be apparent to those skilled in the art, in
view of the following detailed description of the present
invention, taken in conjunction with the appended claims and the
accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] So that the manner in which the features and advantages of
the invention, as well as others which will become apparent are
attained and can be understood in more detail, more particular
description of the invention briefly summarized above may be had by
reference to the embodiment thereof which is illustrated in the
appended drawings, which drawings form a part of this
specification. It is to be noted, however, that the drawings
illustrate only an embodiment of the invention and therefore are
not to be considered limiting of its scope as the invention may
admit to other equally effective embodiments.
[0018] FIG. 1 is a schematic drawing of longitudinal and
perpendicular recording architectures;
[0019] FIG. 2 is a schematic diagram of one embodiment of an SNR
decay test setup constructed in accordance with the present
invention;
[0020] FIG. 3 is a schematic diagram of one embodiment of
interleaved aged and test signals constructed in accordance with
the present invention;
[0021] FIG. 4 depicts one embodiment of a graphical user interface
showing the integration of SNR decay into one type of tester
operation system and is constructed in accordance with the present
invention;
[0022] FIG. 5 depicts one embodiment of a graphical user interface
showing the SNR decay setup for a tester operation system and is
constructed in accordance with the present invention;
[0023] FIG. 6 is a plot of aged and test signals and a ratio
thereof and is constructed in accordance with the present
invention;
[0024] FIG. 7 is a plot of signal decay and noise evolution and is
constructed in accordance with the present invention;
[0025] FIG. 8 is a plot of signal decay, noise increase, and SNR
decay as a function of magnetic bit density and is constructed in
accordance with the present invention; and
[0026] FIG. 9 is a plot of thermal decay correlation and is
constructed in accordance with the present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0027] Perpendicular magnetic recording (PMR) is a promising
technology for sustaining growth in data storage capacity. PMR
employs a soft magnetic underlayer (SUL) and is potentially
superior to longitudinal recording with respect to efficient
writability and thermal stability of the written bits. An effective
SUL is important for better writability and yet should not
contribute significantly to the media noise. Unfortunately, most
SUL materials are not only susceptible to stray fields causing
random spike noise, but also deteriorate the recording
performance.
[0028] The prior art may evaluate the signal decay in PMR systems,
but is unable to address the noise evolution in PMR systems. It is
desirable to have a magnetic test capability to detect signal decay
and noise increment at the same time. In the present invention, a
magnetic test module runs on a spin stand to detect amplitude decay
and noise evolution at the same time so that signal-to-noise ratio
(SNR) decay can be directly measured. The PMR recording
performance, such as BER (bit error rate), is correlated better
with SNR instead of signal only. The thermal stability of the PMR
system is evaluated more accurately with this SNR decay method.
Thus, in one embodiment, the present invention discloses a method
of detecting SNR decay, while the prior art only detects signal
decay.
[0029] The SNR decay is integrated into a spin stand for magnetic
testing. One embodiment is depicted in FIG. 2, and includes a
heater 21 located under the magnetic media disk 23, a remote
sensing thermometer 25, and a temperature controller 27 that work
as a subsystem to establish a desired environmental temperature for
the disk 23. The heater 21 creates a heated band 29 in the disk 23
while the read/write head 31 flies above the heated band 29. The
temperature control system may be removed when SNR decay
measurement is performed under room temperature.
[0030] Media that may exhibit a significant amount of signal decay
after several years of recording are not practically useful. The
HDD product media that would exhibit little signal decay after
several years of recording would likely exhibit almost negligible
signal decay for a reasonably short period after recording. It
poses a challenge to reliably measure extremely small changes of
the recorded signals because of thermal drift (i.e., the head moves
across the written track due to temperature changes in the
environment over time) and sensitivity changes of the transducer
(e.g., GMR or TMR sensor) of the recording head. To resolve this
challenge, the aged signal 33 and test signal 35 are interleaved on
the same written track 29 (FIG. 3). Write gate 37 and read gate 39
features are used to control the sequence of aged magnetic bits and
test magnetic bits. The aged magnetic bit also acts as a reference
signal to eliminate the adverse effects of thermal drift and
sensitivity change mentioned above.
[0031] An SNR decay module was developed to operate decay
measurement on a spin stand. For example, FIG. 4 shows a control
panel for "SNR decay" that is integrated into the operation system
41 of one type of spin stand tester. The SNR decay module may be
developed from computer code (i.e., source code) such as
programming languages Visual Basic, C, C++, etc., and perform
operations on a spin stand. The servo feature of the spin stand
makes SNR decay measurement more accurate. Other functional buttons
are listed for comparison or reference.
[0032] FIG. 5 is an example of an SNR decay setup screen 51, which
reveals test conditions and parameters. The critical test
parameters and test conditions may be integrated into this control
panel so that an operator may perform it as a routine operation
test for disk media.
[0033] In the device sector, one may specify spindle rotational
speed, the location (e.g., track number and test radius) on the
disk media to perform SNR decay measurement, skew angle of the
read/write head, write current for the written transitions (e.g.,
aged and test bits), and the number of sectors for writing and
servo control.
[0034] In the head disk information sector, one may input head
identification for the specified SNR decay test. There is a
selection to choose SNR decay measurement at room temperature or
other specified temperatures to mimic HDD environments. Also, the
specified disk identification can be logged.
[0035] In the spectrum and noise decay sector, one can specify the
frequency range (e.g., data transfer rate) that is converted into a
KFCI range reported in the summary area on the computer display.
Other parameters, such as start time and end time, and the number
of repeats (Nr of Repeat) are used to estimate the total test
duration (e.g., in seconds) for SNR decay measurement. The SNR
decay setup control panel provides a dynamic interface for an
operator to easily perform the SNR decay test, which fits the disk
media test in manufacturing.
[0036] In addition, it shows that SNR decay module can perform
multi-density (e.g., KFCI or frequency) measurement at the same
time, which largely enhances the test throughput and
characterization efficiency. In the SNR decay module, a mechanism
of separating aged signal and test signal is illustrated in terms
of data processing.
[0037] The measured aged signal 61 and test signal 63 fluctuate as
a function of time, as shown in FIG. 6. This results from the
combination of thermal drift and sensitivity change as discussed
above. However, the ratio 65 of test signal 63 and aged signal 61
eliminates the fluctuation due to thermal drift and sensitivity
change. Taking aged written bits as a reference works well to
detect extremely small change of the recorded signal decay.
[0038] In order to measure SNR decay, the noise evolution is
measured at the same time as detecting signal decay. A spectrum
analyzer is implemented into the spin stand to measure the noise in
frequency domain. Integrated noise can be obtained when frequency
sweep may be performed with any industry standard spectrum
analyzer. One can monitor this integrated noise as a function of
time. Frequency sweep and noise integration is a long process and
takes much more time than detecting the aged/test signal. The
data-taking efficiency can be improved by properly selecting noise
samples and constructing noise sensitivity to define integrated
noise as a quantity for monitoring noise evolution.
[0039] Running the SNR decay module on a spin stand automatically
provides signal decay 71 and noise evolution 73 at the same time,
as plotted in FIG. 7. The decay rate (e.g., %/decade) can be
evaluated by processing this collected data. The signal decay 81,
noise increase 83, and SNR decay 85 are plotted in FIG. 8 as a
function of magnetic bit density. SNR decay is dominated by noise
evolution at lower magnetic bit density. This result reveals one
aspect of PMR that demagnetization at lower density creates
significant noise background. The developed SNR decay module is a
significant characterization tool as it impacts the PMR HDD product
design, development, and manufacture.
[0040] The developed SNR decay module can also be applied to
current longitudinal magnetic recording. The measured SNR decay 91
for one type of HDD media is well correlated with the HDD file
data, as shown in FIG. 9. This provides another example of the SNR
decay module to be used for manufacture yield analysis.
[0041] The present invention also comprises a method of magnetic
testing of disk drive on a spin stand. In one embodiment, the
method comprises providing magnetic storage media; mounting the
magnetic storage media to a spin stand; establishing a desired
environmental temperature for the magnetic storage media; operating
the magnetic storage media; and then detecting signal amplitude
decay and noise evolution in the magnetic storage media at the same
time to directly measure signal-to-noise ratio (SNR) decay in the
disk drive over time. The magnetic storage media may utilize one of
perpendicular magnetic recording (PMR) having a soft magnetic
underlayer (SUL) and longitudinal magnetic recording (LMR).
[0042] The method may comprise positioning a heater, a remote
sensing thermometer, and a temperature controller adjacent the
magnetic storage media to establish the desired environmental
temperature. The heater may be used to heat a radial swath of the
magnetic storage media to form a heated band while a read/write
head flies above the heated band, or the magnetic storage media may
be operated at room temperature.
[0043] The method may further comprise interleaving an aged signal
and a test signal on a same written track on the magnetic storage
media, and using write gate and read gate features to control a
sequence of aged magnetic bits and test magnetic bits, wherein the
aged magnetic bits act as a reference signal to eliminate adverse
effects of thermal drift and sensitivity change, and wherein the
aged signal and the test signal fluctuate as a function of time.
The SNR decay module may perform multi-density measurement
including KFCI and frequency measurement at the same time to
enhance test throughput and characterization efficiency.
[0044] In addition, the method may further comprise implementing a
spectrum analyzer into the spin stand to measure noise in frequency
domain, and obtaining integrated noise when frequency sweep is
performed with a spectrum analyzer as a function of time, and/or
improving a data-taking efficiency by properly selecting noise
samples and constructing noise sensitivity to define integrated
noise as a quantity for monitoring noise evolution.
[0045] The present invention has several advantages, including the
ability to detect the SNR (signal-to-noise) decay while the prior
art only can measure the signal decay. This solution includes a new
measurement algorithm and magnetic test methodology that detects
SNR decay in PMR systems. The present invention includes unique
features and technical merits in terms of measurement algorithm,
easy implementation, wide adoptability by spin stand test equipment
companies and the HDD industry in general. It directly monitors
signal decay and noise evolution in a proper time domain, which
detects SNR decay in PMR systems.
[0046] While the invention has been shown or described in only some
of its forms, it should be apparent to those skilled in the art
that it is not so limited, but is susceptible to various changes
without departing from the scope of the invention.
* * * * *