# US 7,411,754 B2 · System and method for measuring readback signal amplitude asymmetry in a perpendicular magnetic recording disk drive

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- Number: US 7,411,754 B2
- Document ID: US7411754B2
- Status: Granted
- Kind: B2
- Application number: 11/463,307
- Filed: 2006-08-08
- Published: 2008-02-14
- Granted: 2008-08-12
- Earliest claimed: 2006-08-08
- Assignees: Hitachi Global Storage Technologies Netherlands B.V.
- Inventors: Mike X. Wang; Jing Zhang
- Web page: https://patentgrep.com/patent/US7411754B2

## CPC

- G11B 19/048: {Testing of disk drives, e.g. to detect defects or prevent sudden failure}
- G11B 5/455: Arrangements for functional testing of heads; Measuring arrangements for heads
- G11B 5/4555: {by using a spin-stand, i.e. a spinning disc or simulator}
- G11B: INFORMATION STORAGE BASED ON RELATIVE MOVEMENT BETWEEN RECORD CARRIER AND TRANSDUCER (recording measured values in a way that does not require playback through a transducer G01D9/00; recording or playback apparatus using mechanically marked tape, e.g. punched paper tape, or using unit records, e.g. punched or magnetically marked cards G06K; transferring data from one type of record carrier to another G06K1/18; circuits for coupling output of reproducer to radio receiver H04B1/20; gramophone pick-ups or like acoustic electromechanical transducers or circuits therefor H04R)

## Abstract

A method for operating a tester for testing heads and disks of a magnetic recording disk drive during manufacturing calculates the readback signal amplitude asymmetry in the frequency domain without the need for measurement in the time domain with a peak detection channel. The tester first signals the write head to write a first pattern on the disk to generate a readback signal with positive pulses. The read head then detects this first recorded pattern and sends the readback signal to a spectrum analyzer connected to the tester. The tester then signals the write head to write a second pattern on the disk to generate a readback signal with negative pulses. The read head then detects this second recorded pattern and sends the readback signal to the spectrum analyzer. The spectrum analyzer measures the amplitudes of the first and second readback signals in the frequency domain using a bandpass filter. A controller in the tester calculates readback signal amplitude asymmetry from the measured amplitudes of the first and second readback signals.

## Claims

### Claim 1

A method for measuring the amplitude asymmetry of a signal from a read head detecting magnetizations in a magnetic recording layer of a magnetic recording disk, the method comprising:
writing a first series of magnetizations in the magnetic recording layer to generate readback pulses having a positive amplitude;
writing a second series of consecutive magnetizations in the magnetic recording layer to generate readback pulses having a negative amplitude;
detecting with the read head the first series of magnetizations to generate a first readback signal;
detecting with the read head the second series of magnetizations to generate a second readback signal;
measuring in the frequency domain the amplitude of the pulses in said first readback signal;
measuring in the frequency domain the amplitude of the pulses in said second readback signal; and
calculating from said measured amplitudes the asymmetry between the first readback signal amplitude and the second readback signal amplitude.

### Claim 2 (depends on 1)

The method of claim 1 wherein detecting the first series of magnetizations and measuring in the frequency domain the amplitude of the pulses in said first readback signal are performed before writing the second series of magnetizations.

### Claim 3 (depends on 1)

The method of claim 1 wherein measuring in the frequency domain the amplitude of the pulses in each of said first and second readback signals comprises measuring with a spectrum analyzer having an oscillator, a mixer and a bandpass filter, the spectrum analyzer measuring comprising mixing the readback signal with the oscillator signal, passing the mixed signal through the bandpass filter, and measuring a voltage of the signal output from the bandpass filter.

### Claim 4 (depends on 1)

The method of claim 1 wherein calculating the amplitude asymmetry comprises calculating the amplitude asymmetry percentage (AASY) according to the equation
AASY=[(POS−NEG)/(POS+NEG)]*100,
wherein POS represents the measured amplitude of the pulses in one of said first and second readback signals, and NEG represents the measured amplitude of the pulses in the other of said first and second readback signals.

### Claim 5 (depends on 1)

The method of claim 1 wherein the magnetic recording layer is a perpendicular magnetic recording layer, wherein writing said first series of magnetizations comprises writing isolated regions magnetized in a first generally perpendicular direction, and wherein writing said second series of magnetizations comprises writing isolated regions magnetized in a second direction generally opposite said first direction.

### Claim 6 (depends on 5)

The method of claim 5 wherein writing said first series of magnetizations further comprises writing alternating first-direction and second-direction magnetizations between said isolated first-direction regions to form baseline regions, and wherein writing said second series of magnetizations further comprises writing alternating first-direction and second-direction magnetizations between said isolated second-direction regions to form baseline regions.

### Claim 7

A method of operating a head-disk tester having a spectrum analyzer, a processor, and memory accessible by the processor, the tester being capable of supporting a rotatable perpendicular magnetic recording disk and a read/write head for writing magnetizations generally perpendicularly on the disk and reading written magnetizations from the disk, the processor-implemented method comprising:
generating a signal to the head to cause the head to write a first pattern on the disk, said first pattern comprising a series of isolated regions magnetized in a first direction and separated by baseline regions of alternating first-direction and second-direction magnetizations, said second direction being generally opposite to said first direction;
generating a signal to the head to cause the head to write a second pattern on the disk, said second pattern comprising a series of isolated regions magnetized in said second direction and separated by baseline regions of alternating first-direction and second-direction magnetizations;
directing the head to read said first pattern to generate a first readback signal;
directing the head to read said second pattern to generate a second readback signal;
measuring with the spectrum analyzer the amplitude of the pulses in said first readback signal;
measuring with the spectrum analyzer the amplitude of the pulses in said second readback signal; and
calculating from said measured amplitudes the asymmetry between the first readback signal amplitude and the second readback signal amplitude.

### Claim 8 (depends on 7)

The method of claim 7 wherein directing the head to read said first pattern and measuring with the spectrum analyzer the amplitude of the pulses in said first readback signal are performed before generating a signal to the head to cause the head to write the second pattern.

### Claim 9 (depends on 7)

The method of claim 7 wherein the spectrum analyzer includes an oscillator, a mixer, a bandpass filter, and a voltage detector, and wherein measuring the amplitude of the pulses in the readback signal with the spectrum analyzer comprises mixing the readback signal with the oscillator signal, passing the mixed signal through the bandpass filter, and detecting a voltage of the signal output from the bandpass filter.

### Claim 10 (depends on 9)

The method of claim 9 wherein calculating the amplitude asymmetry comprises calculating the asymmetry from a first voltage output from the first readback signal and a second voltage output from the second readback signal.

### Claim 11 (depends on 7)

The method of claim 7 wherein calculating the amplitude asymmetry comprises calculating the amplitude asymmetry percentage (ASY %) according to the equation
AASY=[(POS−NEG)/(POS+NEG)]*100,
wherein POS represents the measured amplitude of the pulses in one of said first and second readback signals, and NEG represents the measured amplitude of the pulses in the other of said first and second readback signals.

### Claim 12

A computer program product for operating a head-disk tester capable of supporting a magnetic recording disk and adapted for connection to a write head for writing magnetizations on the disk and to a read head for detecting magnetizations from the disk, the computer program product comprising:
a computer-readable medium; and
computer-executable program code embodied in the computer-readable medium for
signaling the write head to write a series of isolated regions magnetized in a first direction;
signaling the write head to write a series of isolated regions magnetized in a second direction generally opposite said first direction;
detecting a first readback signal from the read head representing the regions magnetized in said first direction;
detecting a second readback signal from the read head representing the regions magnetized in said second direction;
measuring in the frequency domain the amplitude of the first readback signal;
measuring in the frequency domain the amplitude of the second readback signal; and
calculating from said measured amplitudes the asymmetry between the first readback signal amplitude and the second readback signal amplitude.

### Claim 13 (depends on 12)

The computer program product of claim 12 wherein the computer-executable program code further comprises:
signaling the write head to write alternating first-direction and second-direction magnetizations between said isolated first-direction regions to form baseline regions; and
signaling the write head to write alternating first-direction and second-direction magnetizations between said isolated second-direction regions to form baseline regions.