Transfer function for track average amplitude prediction

A method and apparatus produce a read signal using a read head. A variable gain value is set that is used by a variable gain amplifier to amplify the read signal. The variable gain value is used to determine whether the head is functioning properly.

FIELD OF THE INVENTION

The present invention relates generally to read heads, and more particularly but not by limitation to testing read heads used in devices such as data storage devices.

BACKGROUND OF THE INVENTION

Data retrieval devices retrieve bits of data that have been stored on a recordable medium by passing a read head over the medium. Typically, the read head detects a physical property of a portion of the media such as the direction of a magnetic domain or the optical reflectance of the media in order to detect the data stored on the media. The detection of this property results in a read signal.

In order for a retrieval device to operate properly, it is critical that the read head generate a read signal within an acceptable amplitude range. In order to determine if a head will produce an acceptable signal, it is common to measure the head's track average amplitude (TAA), which refers to the average peak-to-peak amplitude of the pulses produced by the read head from a pattern on the media. The optimal value of this measurement depends on the density of the pattern and the sensitivity of the read-channel. If the TAA is too high, the read head may saturate, which distorts the waveforms in the read signal. If the TAA is too low, the signal-to-noise ratio in the read signal will be too low, causing random bit errors.

In the prior art, in order to avoid placing low performing read heads into a device, it has been common to test the read heads before they are assembled into the device. This typically involves connecting test equipment to the output conductors of the heads and measuring the signals generated by the heads. Although such testing removes heads that fail the test, it has been recognized that some heads can pass this screening test yet still perform poorly when integrated into the retrieval device. To address this problem, the prior art has repeated the track average amplitude testing after the heads have been integrated into the retrieval device. This typically involves connecting probes of an external testing system to the output conductors of a common preamplifier in the retrieval device that is connected to each of the heads.

Such testing is undesirable because it requires external test equipment and because it requires a significant amount of time to connect the test equipment to the lines containing the read signal. Thus, a system is needed that allows for testing of the head without requiring external test equipment and without requiring that probes be connected to conductors exiting the device's preamplifier.

Embodiments of the present invention provide solutions to these and other problems, and offer other advantages over the prior art.

SUMMARY OF THE INVENTION

A method and apparatus produce a read signal using a read head. A variable gain value is set that is used by a variable gain amplifier to amplify the read signal. The variable gain value is used to determine whether the head is functioning properly.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

FIG. 1is an isometric view of a disc drive100in which embodiments of the present invention are useful. Disc drive100includes a housing with a base102and a top cover (not shown). Disc drive100further includes a disc pack106, which is mounted on a spindle motor (not shown) by a disc clamp108. Disc pack106includes a plurality of individual discs107, which are mounted for co-rotation about central axis109. Each disc surface has an associated disc head slider110, which is mounted to disc drive100for communication with the disc surface. In the example shown inFIG. 1, sliders110are supported by suspensions112, which are in turn attached to track accessing arms114of an actuator116. The actuator shown inFIG. 1is of the type known as a rotary moving coil actuator and includes a voice coil motor (VCM), shown generally at118. Voice coil motor118rotates pivot housing116with its attached heads110about a pivot shaft120to position heads110over a desired data track along an arcuate path122between a disc inner diameter124and a disc outer diameter126. Voice coil motor118is driven by servo electronics, also known as controller,128based on signals generated by heads110and a host computer (not shown).

FIG. 2illustrates a flow chart170of a method of testing a device having a read head to determine whether the read head has adequate amplitude. The method illustrated inFIG. 2can be completed rapidly without making actual measurements of track average amplitude (TAA) of a read signal with an oscilloscope or similar measuring device. The process illustrated inFIG. 2can be performed using firmware (or a combination of firmware and software) such as shown in the example illustrated inFIG. 3to determine whether the read head in the device is operating properly. In the embodiment ofFIG. 3, a test system300includes a test computer370and a storage device301that is being tested. A processor342in test computer370controls the testing and interacts with storage device301through a host interface390.

The method illustrated inFIG. 2begins at start172and proceeds to step174where a suitable track for testing is identified by processor342of test computer370. Step176includes determining whether the track can accept data by determining if the track contains a media defect. If the track is found to contain a media defect, the process returns to step174where processor342selects a new track.

If the track is found to be defect free, the method proceeds to step180where a test pattern is written to the track. The test pattern can have a frequency in the low, middle, or high frequency range or have a random frequency. However, the preferred range is a pattern in the mid-frequency range, such as a 2T pattern. To write the pattern, processor342sends 2T data through host interface390to interface controller348. Interface controller348passes the data to write channel345, which converts the data into a single analog signal that is passed through pre-amplifier312to the appropriate head310. Note that if write channel345includes a scrambling or encoding circuit, this circuit should be bypassed or disabled so that the 2T pattern is written correctly. Based on the analog signal from pre-amplifier312, head310writes the 2T pattern to the disc.

Under many embodiments, the pattern is written to tracks at different radial positions along the disc. In one particular embodiment, the pattern is written at an inner diameter of the disc near the spindle, at a middle diameter of the disc and at an outer diameter of the disc. Note that in other embodiments, the 2T pattern can be pre-written into the media before the media is installed in the storage device.

Once the pattern has been written to the disc, the process continues at step184, where a read signal is generated by passing head310over the test pattern in the selected tracks. The read signal that is generated by head310passes into pre-amplifier312where it is amplified. The amplified read signal then passes into read channel316.

Read channel316includes a variable gain amplifier (VGA)318, an equalizer320, and a detector322. The amplified read signal is amplified by variable gain amplifier318based on a gain value stored in a gain register336that is converted into an analog value by digital to analog converter328. The amplified signal is equalized by equalizer320so that the equalized signal approaches a desired channel characteristic. Note that equalizer320is optional but is generally used in partial response channels. The equalized signal is passed to detector322, which decodes the equalized signal to generate a set of binary values. These values are then passed to interface controller348, which checks the data for errors before passing it to host interface390.

In order for detector322to operate properly, the equalized signal provided to the detector should have a consistent maximum amplitude. To ensure a consistent amplitude, read channel316uses an automatic gain control (AGC) module consisting of VGA318, equalizer320, analog-to-digital converter330, reference register332, digital comparator334, gain register336and digital-to-analog converter328. In operation, analog-to-digital converter330periodically samples the output signal of equalizer320to generate a digital value that is indicative of the amplitude of the equalized signal. This digital value is provided to digital comparator334together with a target or reference amplitude stored in reference register332. If the amplitude of the equalized signal is below the reference amplitude, digital comparator334increases the gain value stored in gain register336. If the amplitude of the equalized signal is above the reference amplitude, digital comparator334reduces the gain value stored in gain register336.

As noted above, the gain value in gain register336is used to set the amplification level of variable gain amplifier318. Thus, increasing the gain value increase the amplification and thus increases the amplitude of the equalized signal while decreasing the gain value decreases the amplification and thus decreases the amplitude of the equalized signal. Note that the feedback loop of the AGC module is designed so that it corrects only slowly varying or low frequency changes and is not responsive to high frequency content.

During the read operation, the gain values in gain register336are periodically read by servo processor and control logic346as indicated at step186. Although steps184and186are shown as two separate steps, they occur at the same time. Servo processor and control logic346passes the gain values to a host interface microprocessor306, which forwards them through interface controller348and host interface390to processor342. Processor342stores the gain values in a memory304for later processing. In one embodiment, gain values are measured for each of the track locations where the pattern was written, such as the inner diameter track, the middle diameter track, and the outer diameter track.

At step188, processor342averages the gain values for each track to produce an average gain value for each track. At step190, processor342uses the average gain values for each track to determine whether the head is performing properly.

Under one embodiment, the determination of whether the head is operating properly is based on a correlation the present inventors discovered between track average amplitude and average gain values. In particular, the present inventors have discovered that there is an inverse relationship between the track average amplitude (TAA) and the average gain value. Experimentally, there was an especially good correlation between the mid-frequency 2T TAA and average gain value when the read head of an entire disc drive was tested. Using regression techniques, the present inventors identified the following equations as describing the relationship between the average gain value and the measured 2T TAA value:
TAA=298.632−1.94582·V+0.0035143·V2Eq. 1
TAA=257.845−2.02454·V+0.0045936·V2Eq. 2
TAA=224.105−1.62403·V+0.0033865·V2Eq. 3

Where equation 1 is for a pattern on an inner diameter, equation 2 is for a pattern on a middle diameter, equation 3 is for a pattern on an outer diameter, TAA is the track average amplitude and V is the average gain value.

Using equations 1-3, it is possible to generate estimates of the TAA values from the average gain values. To measure how well these equations describe the relationship between TAA and average gain value, the present inventors compared estimates of the TAA values generated using equations 1-3 with actual TAA values measured for the same heads. At the inner diameter of 0.7792″, the observed R-Square value between the 2T TAA measured value and the estimated TAA was 94.2%. For the middle diameter of 1.3700″, the R-Square value was 90.4%. Finally, for the outer diameter of 1.8231″, the R-Square value was 87.3%. Other frequencies such as 1T, 7T, and random patterns were tested, but the 2T pattern and the second order linear regression of equations 1-3 were found to provide the closest correlation between TAA and the average gain value.

To determine whether the read head is within specification at step190, the TAA can be approximated using equations 1, 2 and/or 3 and the average gain values. The approximate TAA is then compared with a specification TAA value or a range of TAA values. If the approximated TAA values are within a desired range for TAA values set by the specification TAA value, the head passes at step192. Otherwise the head fails at step194. Alternatively, equations 1-3 can be used to identify an average gain value specification based on a TAA specification. The average gain value specification is then compared with measured gain value to determine whether the read head is operating properly. An indication of whether the head is operating properly is then provided to a user through a user interface392that can include a display or a printer.

In one embodiment, the head must have an average gain value that provides a TAA within the desired specifications for each radial position (inner diameter, middle diameter, and outer diameter) where the head is tested in order to pass.

It will be recognized by those skilled in the art that the average gain value and TAA calculation can be performed by firmware placed in the electronics of disc drive301such as the host interface microprocessor306. In such cases, a separate test computer370is not needed.

A method of testing a read head310in a device301comprising producing a read signal using read head310; setting a variable gain value336that is used by a variable gain amplifier318to amplify the read signal; and using the variable gain value to determine if the read head310is operating properly.

It is to be understood that even though numerous characteristics and advantages of various embodiments of the invention have been set forth in the foregoing description, together with details of the structure and function of various embodiments of the invention, this disclosure is illustrative only, and changes may be made in detail, especially in matters of structure and arrangement of parts within the principles of the present invention to the full extent indicated by the broad general meaning of the terms in which the appended claims are expressed. For example, the particular elements may vary depending on the particular application for the method and apparatus of testing a device having a read head system while maintaining substantially the same functionality without departing from the scope and spirit of the present invention. In addition, although the preferred embodiment described herein is directed to a screening system for a device having a read head before shipping, it will be appreciated by those skilled in the art that the teachings of the present invention can be applied to a method of testing a read head or an operational retrieval device having a read head to predict baseline instability, without departing from the scope and spirit of the present invention.