Dynamic magnetic information storage or retrieval – Checking record characteristics or modifying recording...
Reexamination Certificate
1999-02-01
2001-10-16
Faber, Alan T. (Department: 2651)
Dynamic magnetic information storage or retrieval
Checking record characteristics or modifying recording...
C360S046000, C360S067000, C360S053000
Reexamination Certificate
active
06304396
ABSTRACT:
The invention relates to an arrangement for reading information from a record carrier, the arrangement comprising:
a read head having at least one magneto-resistive element (R
mr1
),
a first transistor (Tr
1
) having a control terminal and first and second main terminals,
a second transistor (Tr
2
) having a control terminal and first and second main terminals, the control terminal of the first transistor being coupled to the first main electrode of the second transistor via a first capacitive element (
8
), the control terminal of the second transistor being coupled to the first main electrode of the first transistor via a second capacitive element (
6
), the first main electrode of the first transistor being coupled to a terminal of the said magneto-resistive element (R
mr1
), the first main electrode of the second transistor being coupled to a terminal of a second magneto-resistive element (R
mr2
), the second main electrodes of the first and the second transistor being coupled to a first point of constant potential (+) via first and second impedance means (R
1
), respectively and to a hard disk drive provided with the arrangement. Such arrangement is known from U.S. Pat. No. 5,559,646 and is used for reading information from eg. a hard disk.
Amplifiers for magneto-resistive (MR) heads sense very small variations in the MR resistance (typically 1%) and amplify these signals. MR elements are biased by either a voltage or a current. Variations in the resistance of the MR heads, due to a magnetic field, result in small signal voltages and/or currents which can be detected by the sense amplifiers. The biasing of the MR heads causes a DC voltage across the head. MR sense amplifiers must be AC coupled to the heads. To avoid filtering at the low end of the data frequency spectrum, which results in data distortion, the AC coupling has a long time constant, typically at 1 &mgr;s. Large transients, such as those caused during thermal asperities, pass the AC coupling and cause the amplifier to overload, resulting in loss of data during long periods.
The invention aims at providing an improved arrangement for reading information from the record carrier. In accordance with the invention, the arrangement is characterized in that
non-linear transconductance means (
14
) are provided having first and second inputs coupled to the second main electrodes of the first and second transistor, respectively, and first and second outputs coupled to the control terminal of the first and second transistor, respectively, the non-linear transconductance means being adapted to supply a control current at the first output in response to a voltage present across its inputs, such that for first and second input voltages across said inputs, resulting in first and second control currents, respectively, at said first output, the first input voltage being larger than the second input voltage, the amplification factor of the transconductance means being larger for generating the first control current than for generating the second control current.
The invention is based on the following recognition.
The MR heads fly above the disk at very low heights (typically 10 nm). When a collision with a particle on the disk takes place, kinetic energy is transferred to heat. The resistance of the MR elements increases (maximally some 10-20%). As the bias current remains constant, the baseline voltage (that is: the low frequency component of the output signal) increases rapidly. Further, the signal amplitude of the wanted signal changes by not more than the same 10-20%. After the thermal asperity, the MR element cools down slowly. These large unwanted baseline shifts saturate the read amplifier during long periods.
In accordance with the invention, the non-linear transconductance means result in a non-linear operation of the bias control loop of the amplifier (which determine the time constant of the AC coupling). This leads to a shift of the lower bandwidth of the read amplifier towards higher frequencies, in dependence of the amplitude of the thermal asperity (or any other transient). In this way, thermal asperities are compressed. This prevents overflow in the read amplifier and allows the channel IC to recover the data without making incorrectably many bit errors.
Further, in accordance with the invention, an additional DC correction can be carried out in order to further suppress the thermal asperities. This is the subject of the claims
8
,
9
and
10
. In addition, it may be required to add a detection circuit to detect thermal asperities, e.g. so as to freeze the circuits in the read out circuit, in the event that the thermal asperities would otherwise severely distort the read out circuit. This thermal asperity detection is the subject of claim
11
.
REFERENCES:
patent: 5331478 (1994-07-01), Aranovsky
patent: 5559646 (1996-09-01), Voorman et al.
Bergmans Johannes W.M.
Leclerc Patrick
Lugthart Marcel L
Ramaekers Jozef A. M.
Ramalho Joao N. V. L.
Faber Alan T.
Goodman Edward W.
U.S. Philips Corporation
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