Dynamic magnetic information storage or retrieval – Checking record characteristics or modifying recording...
Reexamination Certificate
1998-10-01
2001-05-01
Kim, W. Chris (Department: 2651)
Dynamic magnetic information storage or retrieval
Checking record characteristics or modifying recording...
C360S067000
Reexamination Certificate
active
06226136
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to improving the performance of magnetic storage media reading devices. More specifically, the invention relates to gain compensation for thermal asperity correction. In particular, the invention relates to a method of adding a gain compensation factor to a variable gain amplifier that compensates for a lowered input resistance at the input of the variable gain amplifier where the decreased input resistance to the variable gain amplifier was introduced for the purpose of correcting for a thermal asperity event.
2. Relationship to the Art
In the modem disk drives that employ magnetoresistive (MR) recording heads, one common problem that leads to errors or unreadable data is so called “thermal asperity” (thermal asperity) effects. A magnetoresistive head is a device which has a variable resistance in the presence of a variable magnetic field. Thermal asperity effects are caused by media defects or particulate impacting the MR head. Such impacts generate heat which can cause the MR head resistance to change. Changes in the MR head resistance caused by thermal events caused by impacts can either wash out a read signal or can appear to be an intended variation in a read signal. Thus, thermal asperity effects can impair the signal read from the disk and can cause errors during read back.
Because the effect of a thermal asperity event is similar to a low frequency baseline DC shift superimposed on the original signal, one method of compensating for such events is to attempt to remove the DC shift when a thermal asperity event is detected. This may be accomplished by lowering the input resistance of an amplifier that amplifies the signal from the MR head as is described below.
FIG. 1
is a block diagram illustrating a typical read channel for a magnetic disk reader that uses a magnetoresistive head. A disk
100
is read using a magnetoresistive head
102
. By passing a constant current through the variable resistance and measuring the voltage across the variable resistance, it is possible to determine the state of the magnetic field in the region of the disk being read. The voltage output from the magnetoresistive head
102
is input into a preamplifier
104
. Preamplifier
104
outputs a differential signal that is coupled to a read channel amplifier
106
using a pair of coupling capacitors
108
. The interface between the preamplifier and the read channel amplifier is shown in greater detail in FIG.
2
.
FIG. 2
is a block diagram illustrating the circuit which couples a preamplifier
210
to a read channel amplifier circuit
220
. Typically, read channel amplifier circuit
220
includes a variable gain amplifier. The variable gain amplifier adjusts the incoming signal magnitude to realize the best signal quality. Typically, that is done in the digital domain. Vs represents the preamplifier signal that is obtained by reading the MR head, “Rs” is the output resistance of the preamplifier, which is represented by a pair of internal output resistors
212
. A pair of coupling capacitors
214
couple the signal from the preamplifier to the read channel amplifier circuit
220
on a line
222
and a line
224
. The coupling capacitance is chosen to be large enough so that the lowest frequency of interest may be passed without much attenuation.
The input resistance of the read channel amplifier “Rin” is represented by internal input resistor
240
. The signal that is input to the read channel amplifier is roughly proportional to Rin/(Rs+Rin). Typically the Rin is much larger than Rs, so that most of the signal from the preamplifier is delivered to the read channel amplifier with very little loss.
Because the read channel amplifier circuit
220
is coupled to the preamplifier chip through capacitors
214
, the DC difference between the two chips is stored on the coupling capacitors. When a thermal asperity event happens, the coupling capacitors are charged up due to the DC baseline shift. To restore the DC quickly, differential input resistance
240
on the read channel amplifier side may be lowered to reduce the input time constant and quickly discharge the coupling capacitors to the correct DC levels. One problem associated with the lowering the input resistance in this manner during a thermal asperity event is that lowering the input resistance to the read channel amplifier causes the signal to be attenuated at the same time, which causes the overall gain of the read channel to change. Thus, even though the DC can be quickly restored, the signal is distorted due to lowered input resistance.
The input resistance of variable gain amplifier
260
may be lowered by simply adding a shunt resistor
242
between line
222
and
224
. As noted above, this reduces the charging and discharging time of Cin and also reduces the input resistance of variable gain amplifier
260
, which reduces the signal input to the read channel amplifier. It should be noted that this effect generally is not compensated for digitally by the variable gain amplifier because the VGA gain is actually frozen during a thermal asperity event. The VGA is frozen because the incoming signal is being disturbed too much at the time for gain loop updates to be performed.
What is needed is a way of compensating for the decreased signal gain in the read channel that is caused by lowering the input resistance of the read channel amplifier.
SUMMARY OF THE INVENTION
Accordingly, the present invention provides a gain compensation scheme that restores the gain of the read channel chip during a thermal asperity event. The thermal asperity event is detected and the input resistance of the read channel amplifier is reduced to quickly discharge the coupling capacitors to the correct DC levels. At the same time, the gain of the read channel amplifier is increased by an amount that compensates for the attenuation caused by the lowered read channel amplifier input resistance. As a result, the signal distortion due to lowered input resistance is reduced and the chip performance is enhanced.
It should be appreciated that the present invention can be implemented in numerous ways, including as a process, an apparatus, a system, a device, a method, or a computer readable medium. Several inventive embodiments of the present invention are described below.
In one embodiment, A method of reading data from a magnetic disk is disclosed. The method includes generating a preamplified data signal by reading the magnetic state of the disk using a magnetoresistive head. The preamplified data signal is capacitively coupled to a variable gain read channel amplifier. The variable gain read channel amplifier has an input, an output, and a programmable gain. The input of the variable gain read channel amplifier has a variable gain read channel amplifier input resistance. The occurrence of a thermal asperity event is detected and an adjustment is made to the variable gain read channel amplifier input resistance to compensate for the thermal asperity event. An adjustment to the programmable gain of the variable gain read channel amplifier is made to compensate for the adjustment to the variable gain read channel amplifier input resistance.
In another embodiment, a method of adjusting the gain of a variable gain read channel amplifier having an input, an output and a programmable gain upon the occurrence of a thermal asperity event is disclosed. The method includes detecting a thermal asperity event and changing the resistance across the inputs of the variable gain read channel amplifier to allow energy generated by the thermal asperity event to dissipate. The change in the resistance across the inputs of the variable gain read channel amplifier causes an attenuation in the output of the variable gain read channel amplifier. A thermal asperity gain adjustment signal is generated for the variable gain read channel amplifier. The gain adjustment signal corresponds to a change in gain of the variable gain read channel amplifier that substantially cancels out the attenuati
Kim W. Chris
LSI Logic Corporation
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