U.S. patent application number 16/139364 was filed with the patent office on 2020-03-26 for mitigation of noise generated by random excitation of asymmetric oscillation modes.
The applicant listed for this patent is Seagate Technology LLC. Invention is credited to Andrzej Adam Stankiewicz, Scott Wilson Stokes, Krishnan Subramanian.
Application Number | 20200099456 16/139364 |
Document ID | / |
Family ID | 69883721 |
Filed Date | 2020-03-26 |
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United States Patent
Application |
20200099456 |
Kind Code |
A1 |
Stankiewicz; Andrzej Adam ;
et al. |
March 26, 2020 |
MITIGATION OF NOISE GENERATED BY RANDOM EXCITATION OF ASYMMETRIC
OSCILLATION MODES
Abstract
An apparatus and method for mitigating noise in a waveform from
a signal-producing device. The method includes receiving the
waveform from the signal-producing device, and decomposing the
waveform into low frequency (LF) and high frequency (HF)
components. The method also includes determining an instant
amplitude of the HF component, and employing the instant amplitude
of the HF component and possibly the LF component to obtain a noise
correction value for the LF component. The method further includes
adding the obtained noise correction value to the LF component to
obtain an output signal.
Inventors: |
Stankiewicz; Andrzej Adam;
(Edina, MN) ; Stokes; Scott Wilson; (Brooklyn
Park, MN) ; Subramanian; Krishnan; (Shakopee,
MN) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Seagate Technology LLC |
Cupertino |
CA |
US |
|
|
Family ID: |
69883721 |
Appl. No.: |
16/139364 |
Filed: |
September 24, 2018 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H04B 17/345 20150115;
H04B 17/14 20150115; H04B 15/02 20130101; H04B 15/005 20130101 |
International
Class: |
H04B 15/02 20060101
H04B015/02; H04B 15/00 20060101 H04B015/00; H04B 17/14 20060101
H04B017/14; H04B 17/345 20060101 H04B017/345 |
Claims
1. A method comprising: performing noise detection operations and
noise correction calibration operations for an apparatus comprising
a signal-producing device, the noise detection and noise correction
calibration operations comprising: receiving a first sample
waveform from the signal-producing device; decomposing the first
sample waveform into first low frequency (LF) and first high
frequency (HF) components; generating an instantaneous oscillation
amplitude waveform using the first HF component, the instantaneous
oscillation amplitude waveform being calculated as a difference
between an upper envelope and a lower envelope of the first HF
component; detecting correlations between the instantaneous
oscillation amplitude waveform and portions of the first LF
component; calculating noise correction values for the detected
correlated portions of the first LF component; and storing the
noise correction values in a lookup table on a non-transitory data
storage medium of the apparatus.
2. The method of claim 1, further comprising: receiving a second
sample waveform from the signal-producing device; decomposing the
second sample waveform into second low frequency (LF) and second
high frequency (HF) components; determining an instant amplitude of
the second HF component; employing the instant amplitude of the
second HF component to select one of the noise correction values
from the lookup table; and adding the selected noise correction
value to the second LF component to obtain an output signal.
3. The method of claim 1, wherein detecting the correlations
between the instantaneous oscillation amplitude waveform and
portions of the first LF component comprises finding a regression
line of the first LF component and the instantaneous oscillation
amplitude waveform.
4. The method of claim 3, further comprising: repeating the
receiving, decomposing and finding steps for each of a plurality of
noise-contributing conditions during calibration.
5. The method of claim 4, further comprising: receiving a second
sample waveform from the signal-producing device; decomposing the
second sample waveform into second low frequency (LF) and second
high frequency (HF) components; employing the second LF component
to select one of the noise correction values from the lookup table
for undeterminable noise-contributing conditions during normal
operation of the apparatus; and adding the selected noise
correction value to the second LF component to obtain an output
signal.
6. The method of claim 4, wherein the plurality of
noise-contribution conditions comprises an external magnetic field
proximate to the signal-producing device and a temperature of the
signal-producing device.
7. The method of claim 1, wherein the signal-producing device
comprises a micro-electromechanical device or a
nano-electromechanical device, and wherein receiving the first
sample waveform from the signal-producing device comprises
receiving the first sample waveform from the
micro-electromechanical device or the nano-electromechanical
device.
8. The method of claim 1, wherein the apparatus comprises a disc
drive and the signal-producing device comprises a read head, and
wherein receiving the first sample waveform from the
signal-producing device comprises receiving the first sample
waveform from the read head.
9. The method of claim 1, wherein decomposing the first sample
waveform into the first LF and the first HF components comprises:
utilizing a predetermined threshold frequency to split the first
sample waveform into the first LF and the first HF components, with
the first LF component comprising frequencies below the
predetermined threshold frequency and the first HF component
comprising frequencies above the predetermined threshold frequency;
or utilizing band pass filtering with differing bands to obtain the
first LF and the first HF components.
10. The method of claim 2, wherein the apparatus comprises a disc
drive and the signal-producing device comprises a read head, and
wherein the output signal is provided to a component of a
preamplifier of the disc drive.
11. An apparatus comprising: a signal-producing device; and a
noise-mitigation circuit configured to: receive a waveform from the
signal-producing device; decompose the waveform into low frequency
(LF) and high frequency (HF) components; determine an instantaneous
oscillation amplitude waveform of the HF component, the
instantaneous oscillation amplitude waveform being calculated as a
difference between an upper envelope and a lower envelope of the
first HF component; and employ at least the instantaneous
oscillation amplitude waveform of the HF component to enable noise
correction for the LF component.
12. The apparatus of claim 11, wherein the noise-mitigation circuit
is configured to obtain a noise correction value for the LF
component by employing the instant amplitude of the HF component to
select a noise correction value from a lookup table stored on a
non-transitory data storage medium of the apparatus.
13. The apparatus of claim 12, wherein the noise-mitigation circuit
is configured to obtain a noise correction value for the LF
component by also employing the LF component to select the noise
correction value from a lookup table stored on a non-transitory
data storage medium of the apparatus.
14. The apparatus of claim 11, wherein the noise-mitigation circuit
is configured to decompose the waveform into the LF and the HF
components by: utilizing a predetermined threshold frequency to
split the waveform into the LF and the HF components, with the LF
component comprising frequencies below the predetermined threshold
frequency and the HF component comprising frequencies above the
predetermined threshold frequency; or utilizing band pass filter
circuits with differing bands to obtain the first LF and the first
HF components.
15. The apparatus of claim 11, wherein: the signal-producing device
comprises a micro-electromechanical device or a
nano-electromechanical device; or the apparatus comprise a disc
drive and wherein the signal-producing device comprises a read
head.
16. The apparatus of claim 15, wherein the output signal is
provided to a component of a preamplifier of the disc drive.
17. A method comprising: performing noise detection operations for
an apparatus comprising a signal-producing device, the noise
detection operations comprising: receiving a first waveform from
the signal-producing device; decomposing the first waveform into
first low frequency (LF) and first high frequency (HF) components;
generating an instantaneous oscillation amplitude waveform using
the first HF components, the instantaneous oscillation amplitude
being calculated as a difference between an upper envelope and a
lower envelope of the first HF component; detecting correlations
between the instantaneous amplitude waveform and portions of the
first LF component; and performing noise correction calibration
operations based on the detected correlations between the
instantaneous amplitude waveform and portions of the first LF
component.
18. The method of claim 17, wherein performing the noise correction
calibration operations comprises tuning an adjustable non-linear
circuit to produce a correcting waveform based on the detected
correlations between the instantaneous amplitude waveform and
portions of the first LF component.
19. The method of claim 17, wherein performing the noise correction
calibration operations comprises: calculating noise correction
values for the detected correlated portions of the first LF
component; and storing the noise correction values in a lookup
table on a non-transitory data storage medium of the apparatus.
20. The method of claim 19, further comprising: receiving a second
waveform from the signal-producing device; decomposing the second
waveform into second low frequency (LF) and second high frequency
(HF) components; determining an instant amplitude of the second HF
component; employing at least the instant amplitude of the second
HF component to select one of the noise correction values from the
lookup table; and adding the selected noise correction value to the
second LF component to obtain an output signal.
Description
SUMMARY
[0001] In one embodiment, a method is provided. The method includes
performing noise detection operations and noise correction
calibration operations for an apparatus comprising a
signal-producing device. The noise detection and noise correction
calibration operations include receiving a first sample waveform
from the signal-producing device, and decomposing the first sample
waveform into first low frequency (LF) and first high frequency
(HF) components. An instantaneous amplitude waveform is generated
using the first HF component. Correlations between the
instantaneous amplitude waveform and portions of the first LF
component are then detected. Noise correction values for the
detected correlated portions of the first LF component are
calculated. The noise correction values are stored in a lookup
table on a non-transitory data storage medium of the apparatus.
[0002] In another embodiment, an apparatus is provided. The
apparatus includes a signal-producing device and a noise-mitigation
circuit. The noise-mitigation circuit receives a waveform from the
signal-producing device and decomposes the waveform into low
frequency (LF) and high frequency (HF) components. The
noise-mitigation circuit also determines an instant amplitude of
the HF component. Additionally, the noise-mitigation circuit
employs at least the instant amplitude of the HF component to
enable noise correction for the LF component.
[0003] In yet another embodiment, a method is provided. The method
includes performing noise detection operations for an apparatus
comprising a signal-producing device. The noise detection
operations include receiving a first waveform from the
signal-producing device, and decomposing the first waveform into
first low frequency (LF) and first high frequency (HF) components.
The noise detection operations also include generating an
instantaneous amplitude waveform using the first HF components, and
detecting correlations between the instantaneous amplitude waveform
and portions of the first LF component. The method further includes
performing noise correction calibration operations based on the
detected correlations between the instantaneous amplitude waveform
and portions of the first LF component.
[0004] Other features and benefits that characterize embodiments of
the disclosure will be apparent upon reading the following detailed
description and review of the associated drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0005] FIG. 1 is a graph that depicts simulated oscillation spectra
for a bi-harmonic oscillator potential.
[0006] FIG. 2A is a graph showing time-domain low frequency and
high frequency signal decomposition for a bi-harmonic
oscillator.
[0007] FIG. 2B is a two-dimensional histogram of low frequency (LF)
noise and high frequency (HF) instantaneous amplitude.
[0008] FIGS. 3A and 3B are graphs showing asymmetric energy minima
and a corresponding restoring force, respectively, based on skew
normal distribution.
[0009] FIG. 4 is a graph that shows a power spectrum density
plot.
[0010] FIG. 5A is a graph showing a portion of a reader noise
waveform.
[0011] FIG. 5B is a plot that represents a corresponding power
spectral density distribution for the reader noise waveform shown
in FIG. 5A.
[0012] FIG. 6 is a two-dimensional histogram of LF noise and HF
instantaneous amplitude.
[0013] FIG. 7A is a block diagram of a noise mitigation system in
accordance with one embodiment.
[0014] FIGS. 7B and 7C are flowcharts of a method embodiment.
[0015] FIG. 8A is a block diagram of a noise mitigation system in
accordance with another embodiment.
[0016] FIG. 8B is a block diagram of a disc drive in accordance
with one embodiment.
[0017] FIGS. 9A and 9B show a two-dimensional histogram and a
spectrum, respectively, after correction for a noisy reader.
DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS
[0018] Embodiments of the disclosure relate to mitigation of noise
generated by random excitation of asymmetric oscillation modes.
Prior to providing a description of the different embodiments, a
description of such oscillation modes is provided below.
[0019] It should be noted that like reference numerals are used in
different figures for same or similar elements. It should also be
understood that the terminology used herein is for the purpose of
describing embodiments, and the terminology is not intended to be
limiting. Unless indicated otherwise, ordinal numbers (e.g., first,
second, third, etc.) are used to distinguish or identify different
elements or steps in a group of elements or steps, and do not
supply a serial or numerical limitation on the elements or steps of
the embodiments thereof. For example, "first," "second," and
"third" elements or steps need not necessarily appear in that
order, and the embodiments thereof need not necessarily be limited
to three elements or steps. It should also be understood that,
unless indicated otherwise, any labels such as "left," "right,"
"front," "back," "top," "bottom," "forward," "reverse,"
"clockwise," "counter clockwise," "up," "down," or other similar
terms such as "upper," "lower," "aft," "fore," "vertical,"
"horizontal," "proximal," "distal," "intermediate" and the like are
used for convenience and are not intended to imply, for example,
any particular fixed location, orientation, or direction. Instead,
such labels are used to reflect, for example, relative location,
orientation, or directions. It should also be understood that the
singular forms of "a," "an," and "the" include plural references
unless the context clearly dictates otherwise.
[0020] Many physical systems and devices operate in a quasi-static
regime in contact with a thermal bath, which leads to fluctuating
oscillations around an energy minimum. For linear systems, the
spectrum of these oscillations replicates a resonance curve of a
damped oscillator. In particular, power spectral density grows
monotonically with frequency in a low frequency (LF) range, until
reaching a maximum at a resonant frequency. Small sizes of
contemporary magneto-resistive readers and other
micro-electromechanical systems (MEMS)/nano-electromechanical
systems (NEMS) devices push oscillations to non-linear regimes,
which may lead to resonance line broadening, bifurcations, etc.
However, non-linearity does not change a monotonicity of thermal
fluctuations in the LF range, until the energy profile is symmetric
around the energy minimum.
[0021] Asymmetry of the energy profile around the stable point
leads to LF thermal noise with a 1/f-like spectral profile. This
phenomenon results in M-shaped LF noise dependence versus magnetic
field in magnetic tunneling readers.
[0022] In magnetoresistive readers, it has been shown that magnetic
precession can be approximated in terms of a one-dimensional damped
oscillator. It serves as a primary basis for this investigation. 5
gigahertz (GHz) asymmetric oscillators (which may be a resonant
frequency for a reader free layer) in contact with a thermal bath
were analyzed using the Langevin equation. Potential energy
profiles were generated using bi-harmonic and skew normal
approximations, providing the ability to generate symmetric and
asymmetric potentials using a single adjustable parameter. FIG. 1
is a graph 100 that depicts simulated oscillation spectra for a
bi-harmonic oscillator potential. In graph 100, a horizontal axis
102 represents frequency in GHz and a vertical axis 104 represents
power spectral density (PSD) in arbitrary units (a.u.). The
bi-harmonic plot of FIG. 1 shows 3 curves 106, 108 and 110, with
curve 106 representing symmetric (harmonic) oscillations and curves
108 and 110 representing asymmetric oscillations. Curves 106, 108
and 110 have a main resonance peak that is essentially the same at
5 GHz. Differences are outlined in insert graph 112, which has a
horizontal axis 114 that represents displacement, a first vertical
axis 116 that represents energy and a second vertical axis 118 that
represents force. Plots 120, 122 and 124 are energy plots that
correspond to curves 106, 108 and 110, respectively. Similarly,
plots 126, 128 and 130 are force plots that correspond to curves
106, 108 and 110, respectively. FIG. 1 shows that symmetric
oscillations result in low and flat LF noise. Also, FIG. 1 shows an
increasing power spectral density at low frequencies as asymmetry
is introduced. Asymmetric oscillations introduce a direct current
(DC) shift, and the presence of random fluctuations in this
asymmetric potential produces a broadening of the DC frequency
spike leading to a pronounced 1/f-like low-frequency shape to the
spectral density.
[0023] In order to further characterize the nature of this 1/f-like
low frequency noise profile, the time domain signal for the
asymmetric oscillator was decomposed into low and high frequency
components as shown in graph 200 of FIG. 2A. An optimal
decomposition technique depends on particular device properties.
Empirical Mode Decomposition was used here, and similar results can
be obtained using low- and high/band-pass filters. Band-pass
filtering may be particularly useful when more than one asymmetric
resonance contributes to LF noise, allowing the calculating of
corrections due to each of them.
[0024] In FIG. 2A, a horizontal axis 202 represents frequency in
GHz and a vertical axis 204 represents PSD in decibels (dB)/Hz.
Plot 206 is a low frequency component plot and plot 208 is a high
frequency component plot. Then upper and lower envelopes of the
high frequency component were extracted, and instantaneous
oscillation amplitude was calculated as a difference between them.
FIG. 2B shows a two dimensional histogram 210 of LF noise
(represented by a vertical axis 214) and high frequency
instantaneous amplitude (represented by a horizontal axis 212 for
free layer (FL) instantaneous amplitude). The strong correlation
seen in FIG. 2B confirms that the 1/f-like low frequency noise
originates from fluctuations of asymmetric oscillations. This model
demonstrates a new mechanism of 1/f noise generation.
[0025] Another instructive model function is based on a skew normal
distribution (SND), shown in FIG. 3A and FIG. 3B. FIG. 3A is a
graph 300 showing asymmetric energy minima and FIG. 3B is a graph
302 showing a corresponding restoring force. In FIG. 3A, a
horizontal axis 304 represents displacement and a vertical axis 306
represents energy. FIG. 3B employs the same horizontal axis 304 as
FIG. 3A and includes a vertical axis 308 that represents force. The
SND-based model shown in FIGS. 3A and 3B adds nonlinearity for both
energy branches, and asymmetry is controlled by a single parameter
Lambda (e.g., a slant parameter). Other model parameters were
adjusted to ensure constant 5 GHz frequency of very small
oscillations for any Lambda. The different curves 310-320 of FIG.
3A are for Lambda values 0-5, respectively. Similarly, curves
322-332 of FIG. 3B represent Lambda values 0-5, respectively.
Nonlinearity contributes to resonance peak shift and broadening,
but clearly the LF noise is still a result of asymmetry (as can be
seen in FIG. 4). FIG. 4 shows a power spectrum density plot 400 of
5 GHz oscillation modelled by the Langevin equation in potential
energy depicted in FIG. 3A. In FIG. 4, a horizontal axis 402
represents frequency in GHz. Plots 404-414 represent Lambda values
of 0, 2, 4, 6, 8 and 10, respectively. In particular, the plot 404
(corresponding to symmetric oscillations) shows low and flat LF
noise, while for asymmetric cases 1/f noise dependence is preserved
down to very low frequencies.
[0026] Typical magnetoresistive (tunneling) readers are comprised
of ferromagnetic free, reference and pinned layers (FL, RL, PL,
respectively), which are stabilized with interlayer interactions
and bias magnetic fields. External fields (e.g. due to magnetic
media) cause magnetization rotations in the FL, resulting in
resistance changes. At finite temperature, the magnetization
fluctuates around the energy minimum, resulting in output noise. In
nanostructures, these oscillations may be quite large (e.g., 10-20
degrees), and go beyond the range of the harmonic approximation.
However, the resonant frequency is large (e.g., about 5 GHz), and
the resulting noise in the operational band (e.g., <2 GHz)
remains small, until the excitations become asymmetric around the
equilibrium point, resulting in 1/f-like noise.
[0027] In an experiment, the quasi-static reader output signal was
probed in the time domain using an Integral Solutions International
(ISI) ferromagnetic resonance spectroscopy (FMR) tester and an
external oscilloscope. The system analog bandwidth was 18 GHz, and
waveforms were collected at a sampling rate of 80 giga-samples
(GSa)/s. FIG. 5A shows a small section of a reader noise waveform
500. In FIG. 5A, a horizontal axis 502 represents time in
nanoseconds (ns) and a vertical axis 504 represents an output
signal in a.u. Oscillation asymmetry is evident in FIG. 5A. FIG. 5B
is a plot 506 that represents a corresponding PSD distribution. In
FIG. 5B, a horizontal axis 508 represents frequency in GHz and a
vertical axis represents PSD in a.u. PSD in FIG. 5B reveals a
prominent FL resonance peak (at -5 GHz), along with other resonance
lines at high frequencies (>10 GHz), and strong 1/f-like noise
below 2 GHz (b) (e.g., in the LF range).
[0028] In order to prove that LF noise is mostly a result of the
observed oscillation asymmetry, the signal is decomposed into LF
(<2 GHz) and HF (>2 GHz) components. Such operations may be
carried out by either empirical mode decomposition (EMD), or by
zero-phase-shift numerical filters. The HF portion is then used to
produce the instantaneous amplitude waveform, using a simple
envelope algorithm (the Hilbert transform may be used as well). A
two-dimensional histogram 600 of the LF noise (represented by a
vertical axis 604) and HF instantaneous amplitude (represented by a
horizontal axis 602) is shown in FIG. 6. A strong correlation
(e.g., within ellipse 606) and a shift of the equilibrium signal
(corresponding to null instantaneous HF amplitude) confirm
oscillation asymmetry to be a source of LF noise, while regression
nonlinearity (e.g., line 608) reveals oscillation nonlinearity.
[0029] While the above description is substantially focused on the
presence of low frequency noise in asymmetric magnetic readers, it
is fully expected to be manifest in many other systems, possibly
explaining the widespread presence of 1/f noise.
[0030] The ability to retrieve noise dependence from broadband
waveforms (FIGS. 5A and 5B) provides an opportunity to compensate
LF noise by producing and adding a correction waveform. An example
LF noise compensation algorithm/method includes: [0031] Collecting
a sample noise waveform, decomposing the collected sample noise
waveform into LF and HF components, and creating a correcting
function by finding a regression line of LF signal versus HF
instantaneous amplitude. [0032] If asymmetry depends on particular
conditions (e.g. external field, temperature, etc.), repeating the
previous steps for all applicable conditions. [0033] During device
operation, detecting HF amplitude, producing a correcting signal,
and adding the correcting signal to the output signal in the real
time. LF noise of an obtained resulting signal is substantially
reduced. [0034] If some of the external conditions cannot be
determined during device operation (e.g. external field in a
magnetoresistive reader), the LF component may be substituted as an
approximated reference.
[0035] FIG. 7A illustrates a noise mitigation system/circuit 700 in
accordance with one embodiment. The noise mitigation system 700 of
FIG. 7A employs a frequency splitter circuit 704, an instant
amplitude detection circuit 706, a correction
determination/calculation circuit 708 and a summing node 710 to
carry out the above-described steps. In some embodiments,
system/circuit 700 may include a condition detector 711 that is
capable of detecting an external field, a temperature, etc. As
indicated above, noise detection and noise calibration-related
operations are first performed. The operations are shown in FIG.
7B, which is a flow diagram of a method 750. Referring to FIGS. 7A
and 7B, the method initially involves, at block 752 of FIG. 7B,
receiving a first sample waveform (e.g., a test sample waveform)
from a signal-producing device (e.g., device under investigation
(DUI) 702 of FIG. 7A). At block 754 of FIG. 7B, the first sample
waveform is decomposed into first LF and first HF components. This
may be carried out by frequency splitter 704 of FIG. 7A that
utilizes a predetermined threshold frequency (f.sub.tresh) to
obtain LF components that have frequency values below f.sub.tresh
and HF components that have frequency values above f.sub.tresh. In
some cases, band-pass filtering may be more suitable optimal. At
block 756 of FIG. 7B, the HF components are fed to instant
amplitude detector 706 of FIG. 7A that generates an instantaneous
amplitude waveform using the first HF component. At block 758 of
FIG. 7B, correlations between the instantaneous amplitude waveform
and portions of the first LF component are detected. At block 760
of FIG. 7B, noise correction values are calculated for the detected
correlated portions of the first LF component. At block 762 of FIG.
7B, the noise correction values are stored in a one- or
two-dimensional lookup table on a non-transitory data storage
medium. The one-dimensional lookup table includes different HF
amplitude values and noise correction values corresponding to the
different HF amplitude values. The two-dimension lookup tables
include different HF amplitude values along with detected condition
values and/or LF signal values, and corresponding noise correction
values. The stored noise correction values may be utilized for LF
noise compensation during normal operation of the signal producing
device 702 in a manner described below in connection FIG. 7C, which
is a flow diagram of a method 770.
[0036] Referring now to FIGS. 7A and 7C, at block 772 of FIG. 7C, a
second sample waveform (e.g., a normal-operation sample waveform)
is received from the signal-producing device 702 of FIG. 7A. At
block 774 of FIG. 7C, the second sample waveform is decomposed into
second LF and second high HF components by frequency splitter 704
of FIG. 7A. At block 776 of FIG. 7C, an instant amplitude of the
second HF component is determined by instant amplitude detector 706
of FIG. 7A. At block 778, the second the instant amplitude of the
second HF component and LF component (if applicable) are employed
by correction calculator 708 of FIG. 7A to select one of the noise
correction values from the one- or two-dimensional lookup table. In
some embodiments, the correction calculator 708 includes processing
circuitry that carries out a lookup table search based on
instantaneous HF amplitude for a one-dimensional lookup table, and
additionally based on an external condition reference for a
two-dimensional lookup table. If the external condition(s) cannot
be detected in real time, the LF signal may be used as an
approximated reference. The different detected/captured values of
HF amplitude, external conditions and LF signals may be different
at different time instants. Thus, in such embodiments, the lookup
table search carried out by the processing circuitry involves
matching a value of HF amplitude captured at a particular time
instant with a stored HF amplitude value in the lookup table for
the one-dimensional lookup table case. For the two-dimensional
lookup table case, the processing circuitry may additionally match
an external condition value (e.g., an external magnetic field
value) captured at the particular time instant with an external
conditional value stored in the lookup table and, if the additional
condition cannot be detected, matches the LF signal value obtained
at the particular time instant with a LF signal value stored in the
lookup table. Once a suitable match is obtained, the noise
correction value in that table record is selected as the
compensation value. At block 780 of FIG. 7C, the selected noise
correction value is added to the second LF component by summing
node 710 of FIG. 7A to obtain an output signal 712 of FIG. 7A.
[0037] It should be noted that elements 704, 706, 708, etc., are
employed for LF noise compensation in the above-described
embodiments because simply employing a filter for LF noise
filtering will result in a removal of useful HF components of the
signal from the signal-providing device 702. Employing the elements
704, 706, 708, etc., in the connection configuration shown in FIG.
7A preserves the useful HF signal components and substantially
reduces LF noise. It should also be noted that analog or digital
implementations of the noise mitigation system may be utilized in
different embodiments. In an analog implementation, the noise
correction calibration operations comprises tuning an adjustable
non-linear circuit to produce a correcting waveform based on the
detected correlations between the instantaneous amplitude waveform
and portions of the first LF component. An example of a digital
implementation is briefly described below in connection with FIG.
8A.
[0038] FIG. 8A illustrates a noise mitigation system 800, which may
be employed in magnetic recording, in accordance with another
embodiment. As in the case of noise mitigation system 700 of FIG.
7A, the noise mitigation system 800 of FIG. 8A employs a frequency
splitter 804, an instant amplitude detector 806, a correlation
calculator 808 and a summing node 810. Additionally, system 800
includes a first digitizer 812 and a second digitizer 814.
Therefore, correction calculator 808 receives digital values as
inputs. As can be seen in FIG. 8A, noise mitigation system 800 is
positioned between a reader (e.g., a tunnel magnetoresistive (TMR)
reader) 802 and a channel 816 of a data storage device (e.g., a
disc drive of the type described below in connection with FIG. 8B).
The configuration shown in FIG. 8A assumes that an interconnect
between the reader 802 and channel 816 cuts out the HF signal, and
therefore system 800 obtains the HF signal before it is cut out. In
an alternate embodiment, if the interconnect is able to carry the
HF signal to a preamplifier board, all electronic components of
noise mitigation system 800 may be placed on the preamplifier
board. In the embodiment of FIG. 8A, a threshold frequency of about
2 GHz may be used to split the signal received from reader 802 into
LF and HF components. Alternatively, the LF components obtained may
simply be within the preamplifier band, and the HF components may
be a narrower band than all frequencies above 2 GHz. Noise
mitigation system 800 of FIG. 8A operates in a manner substantially
similar to noise mitigation system 700 of FIG. 7A and therefore a
description of the operation of the noise mitigation system is not
repeated in connection with FIG. 8A. A description of a disc drive
in which noise mitigation system 800 may be employed is provided
below in connection with FIG. 8B.
[0039] Referring now to FIG. 8B, an example embodiment of a disc
drive data storage system 820 in which in which noise mitigation
may be carried out using system 800 of FIG. 8A is shown. Disc
storage system 820 includes a printed circuit board assembly (PCBA)
822 and a head-disc assembly (HDA) 824. PCBA 822 includes circuitry
and processors, which provide a target interface controller (or
drive controller) for communicating between a host system 826 and
HDA 824. Host system 826 can include a microprocessor-based data
processing system such as a personal computer or other system
capable of performing a sequence of logical operations. Data is
transmitted between host system 826 and PCBA 822 via a host bus
connector 828. HDA 824 includes an actuator assembly 830, a
preamplifier 832, and a disc assembly 834. Disc assembly 834
includes one or more media discs 835, stacked on a spindle assembly
838. Spindle assembly 838 is mechanically coupled to a spindle
motor 840 for rotating the disc(s) at a high rate of speed.
[0040] Actuator assembly 830 includes a voice coil motor, and
multiple actuator arms. Located at the end of each actuator arm are
one or more sliders/transducer heads such as 842, which are
associated with a respective disc surface. Transducer heads 842
communicate with disc controller circuit board 822 via a cable
assembly 844 connected to preamplifier 832 for reading and writing
data to the transducer head's associated disc surface. Preamplifier
832 provides an amplified signal to a read/write channel 846 of
PCBA 822. Read/write channel 846 performs encoding and decoding of
data written to and read from the disc.
[0041] A servo processor 866 provides intelligent control of
actuator assembly 830 and spindle motor 840 through a servo
controller 868. By commands issued to servo controller 868 by servo
processor 866, VCM driver 870 is coupled to move actuator assembly
230 and spindle motor driver 872 is coupled to maintain a
substantially constant spin rate of spindle motor 840.
[0042] PCBA 802 includes a host interface disc controller (HIDC)
application-specific integrated circuit (ASIC) 848. ASIC 848
includes a host interface 850, a buffer controller 852, and a disc
controller 854. Host interface 850 communicates with host system
826 via host bus connector 828 by receiving commands and data from
and transmitting status and data back to host system 206. A command
queuing engine (CQE) 258 is incorporated in host interface 230.
[0043] Buffer controller 852 controls a non-volatile buffer memory
856. Disc controller 854 tracks the timing of data sectors passing
under a currently selected transducer head and accordingly sends
data to and receives data from read/write channel 846. Disc
controller 854 also provides for error correction and error
detection on data transmitted to and read from discs 824.
[0044] An interface processor 858 manages a queue of commands
received from host 826 with the assistance of the CQE 878 embedded
in host interface 850. Interface processor 858 interfaces with
functional elements of PCBA 822 over a bus 860, for transfer of
commands, data, and status.
[0045] Disc system operational programs may be stored in
non-volatile program storage memory 874, such as read-only memory
(ROM) or flash memory, and are loaded into random access memory
(RAM) or program loading memory 876 for execution by interface
processor 858. Suitably, servo processor 866 may have integrated or
separate memory 880 for storage of servo programs.
[0046] As noted above, disc storage system 820 includes noise
mitigation system 800. In the embodiment of FIG. 8B, noise
mitigation system is included between heads 842, which may each
include a reader, and preamplifier 212. In the interest of
simplification, only one noise mitigation system 800 is shown in
FIG. 8B. However, each head 222 may be connected to a different
noise mitigation system 800 and therefore disc storage system 800
may include eight different noise mitigation systems. Also, as
indicated earlier, noise mitigation system 800 may be within preamp
212 in some embodiments. Noise mitigation system 800 operates in a
manner described earlier in connection with FIGS. 7A, 7B and
7C.
[0047] For validation, the method described above in connection
with FIGS. 7A and 7B, was implemented in MATLAB (matrix
laboratory). FIGS. 9A and 9B show a two-dimensional histogram 600
and a spectrum 906, respectively, after correction for the noisy
reader presented in FIGS. 5A, 5B and 6. In FIG. 9A, a horizontal
axis 902 represents FL instantaneous amplitude and a vertical axis
904 represents corrected noise. In FIG. 9B, a horizontal axis 908
represents frequency in GHz and a vertical axis 910 represents PSD
in a.u. In FIG. 9B, curve 912 represents a spectrum for a reader
waveform to which no noise mitigation is applied, and curve 914
represents a spectrum for the reader waveform after noise
mitigation is applied. From FIGS. 9A and 9B, it is seen that the
correlation between LF noise and oscillation amplitude is gone, and
noise PSD is reduced by an order of magnitude for very low
frequencies. This demonstration was performed in constant external
field, so one-dimensional look-up-table was sufficient, and the LF
signal was not used for correction calculation.
[0048] The illustrations of the embodiments described herein are
intended to provide a general understanding of the structure of the
various embodiments. The illustrations are not intended to serve as
a complete description of all of the elements and features of
apparatus and systems that utilize the structures or methods
described herein. Many other embodiments may be apparent to those
of skill in the art upon reviewing the disclosure. Other
embodiments may be utilized and derived from the disclosure, such
that structural and logical substitutions and changes may be made
without departing from the scope of the disclosure. Additionally,
the illustrations are merely representational and may not be drawn
to scale. Certain proportions within the illustrations may be
exaggerated, while other proportions may be reduced. Accordingly,
the disclosure and the figures are to be regarded as illustrative
rather than restrictive.
[0049] One or more embodiments of the disclosure may be referred to
herein, individually and/or collectively, by the term "invention"
merely for convenience and without intending to limit the scope of
this application to any particular invention or inventive concept.
Moreover, although specific embodiments have been illustrated and
described herein, it should be appreciated that any subsequent
arrangement designed to achieve the same or similar purpose may be
substituted for the specific embodiments shown. This disclosure is
intended to cover any and all subsequent adaptations or variations
of various embodiments. Combinations of the above embodiments, and
other embodiments not specifically described herein, will be
apparent to those of skill in the art upon reviewing the
description.
[0050] The Abstract of the Disclosure is provided to comply with 37
C.F.R. .sctn. 1.72(b) and is submitted with the understanding that
it will not be used to interpret or limit the scope or meaning of
the claims. In addition, in the foregoing Detailed Description,
various features may be grouped together or described in a single
embodiment for the purpose of streamlining the disclosure. This
disclosure is not to be interpreted as reflecting an intention that
the claimed embodiments employ more features than are expressly
recited in each claim. Rather, as the following claims reflect,
inventive subject matter may be directed to less than all of the
features of any of the disclosed embodiments.
[0051] The above-disclosed subject matter is to be considered
illustrative, and not restrictive, and the appended claims are
intended to cover all such modifications, enhancements, and other
embodiments, which fall within the true spirit and scope of the
present disclosure. Thus, to the maximum extent allowed by law, the
scope of the present disclosure is to be determined by the broadest
permissible interpretation of the following claims and their
equivalents, and shall not be restricted or limited by the
foregoing detailed description.
* * * * *