Patent Publication Number: US-8120867-B2

Title: Apparatus and methods of generating a test pattern of data, analysing a test pattern of data, and testing a data storage disk medium and/or a read/write head

Description:
The present invention relates to apparatus and methods of generating a test pattern of data, analysing a test pattern of data, and testing a data storage disk medium and/or a read/write head. 
     In embodiments, the present invention relates generally to head media test apparatus such as are commonly known as “spinstands” or “dynamic electrical test machines” in the art. Spinstands were first developed in the art as a tool for use during research and development to allow the performance of the various components of disk drives, for example the heads, disks and channels, to be evaluated and optimised. It is now common to also use spinstands in the field of disk drive manufacturing to test each manufactured read/write head or disk before it is assembled into a disk drive unit. 
     A typical spinstand comprises a motor-driven spindle on which a disk to be tested can be mounted and spun, and a head load mechanism for holding and positioning the read/write head to be tested. The spinstand also comprises a spinstand controller under the control of which the head is “flown” over the surface of the disk when spun, so that test data can be written to and read from the disk with the head. When conducting a test with a spinstand, the head is first positioned over a track on the disk and test data is written to the track. The test data is subsequently read back by the head, measured, and analysed and the results then displayed to the user. Various parameters under which the data is written and/or read back can be controlled and varied by the spinstand, allowing the performance and characteristics of the head or disk to be investigated under various conditions. Usually, a computer or similar processing apparatus is provided to carry out the various tests performed on the spinstand, and to analyse and display the measurements made with the spinstand. Additionally, dedicated parametric measurement electronics, a spectrum analyser or an oscilloscope may be provided for analysing and displaying the measurements made with the spinstand. In this way a series of tests may be conducted, including for example so-called bit error rate (BER) bathtubs, track squeeze, track centre, read/write offset, overwrite, etc. 
     One possible source of error in testing is due to fluctuations in the speed of rotation of the disk. A highly accurate motor is usually used to rotate the spindle and thereby the disk attached to the spindle. Nevertheless, the motor speed can only be controlled to within a certain tolerance, giving rise to undesired fluctuations in the rotational speed of the disk. Such fluctuations cause the test data pattern to be distorted both when being written to the disk as well as when being read back from the disk. These distortions can cause significant problems in performing tests where high precision is required. 
     According to a first aspect of the present invention, there is provided a method of generating a test pattern of data to be written to a data storage disk medium for testing, the method comprising: rotating the disk; detecting fluctuations in the speed of rotation of the disk; producing a reference clock signal in accordance with said fluctuations so as to be synchronised with the rotation of the disk; and, generating a test pattern of data using said reference clock signal as a timing reference. 
     By producing a reference clock signal that is synchronised with the rotation of the disk and using this to generate a test pattern of data, it is possible to generate a test pattern that accounts or allows for fluctuations in the rotation of the disk such that when the test pattern is written to the disk, it is spatially correct with respect to the surface of the disk. This means that the data can be read back and the signal obtained will be a truer representation of the original test pattern of data. 
     For example, where a test frequency is written to a track on the disk, in prior art arrangements the test frequency actually present on the disk will vary to some degree along the track, meaning that the recovered frequency will also be spread, leading to loss of accuracy in measuring the signal. In contrast, this aspect of the present invention allows the frequency to be modulated to accommodate the rotational error of the disk, thereby making the written frequency pattern more spatially correct on the disk so that the signal read back from the disk will be less spread. This means that detecting the frequency in the read back signal will be simpler as the frequency power will be more sharply concentrated about the correct frequency and less spread. 
     In a preferred embodiment, the step of detecting the fluctuations comprises: detecting a plurality of marks that rotate synchronously with the disk; and, producing a clock signal corresponding to the movement of the detected marks, said clock signal being used in the production of the reference clock signal. This arrangement provides a preferred method of measuring the fluctuations in the rotation of the disk to allow the reference clock signal to be produced. The marks may be optical marks that are read using laser interferometry. The optical marks may be attached to the disk itself or to the spindle or to any other part that rotates with the disk. Alternatively, the marks may be magnetic marks on the disk that are detected magnetically. It is generally preferred to have many marks, to allow a higher resolution in determining the fluctuations in the rotation of the disk. 
     In a preferred embodiment, the method comprises providing the clock signal as an input to a phase locked loop, the phase locked loop being arranged to produce said reference clock signal. This provides a simple way of producing the reference clock signal. Using a phase locked loop allows the production of the reference signal to be tailored to suit the application according to the well known principles governing the operation of phase locked loops. 
     In an embodiment, the method comprises producing an oscillator clock signal from an oscillator; the disk being rotated with a motor that is locked to the oscillator clock signal; and, the reference clock signal being generated using the oscillator clock signal, the oscillator clock signal being modulated in accordance with the fluctuations in the speed of rotation of the disk in the generation of the reference clock signal. In effect, in this embodiment one oscillator is used to produce the reference clock and to clock the motor. If separate clocks were used for each, then thermal drift could cause the clocks to drift with respect to each other, which would have the effect of introducing phase error in the pattern written to the disk. Having one clock for both has the advantage of substantially eliminating this error. 
     The method may comprise rotating the disk; detecting fluctuations in the speed of rotation of the disk; producing a reference clock signal in accordance with said fluctuations so as to be synchronised with the rotation of the disk; reading a test pattern of data from the disk to provide a test data signal; and, analysing said test data signal using the reference clock signal as a timing reference. This arrangement provides a preferred phase locked loop architecture. The divide-by-n counter means that the reference clock signal can be at a different frequency from the clock signal. 
     According to a second aspect of the present invention, there is provided a method of analysing a test pattern of data read from a data storage disk medium, the method comprising: rotating the disk; detecting fluctuations in the speed of rotation of the disk; producing a reference clock signal in accordance with said fluctuations so as to be synchronised with the rotation of the disk; reading a test pattern of data from the disk to provide a test data signal; and, analysing said test data signal using the reference clock signal as a timing reference. 
     By producing a reference clock signal that is synchronised with the rotation of the disk and using this to analyse the test data signal, it is possible to account or allow for fluctuations in the rotation of the disk when reading back the data. This makes analysing the data more simple and improves the accuracy of the results. 
     For example, where a test frequency is written to a track on the disk, in prior art arrangements the recovered test data signal read from the disk will be distorted due to the fluctuations in rotation of the disk, meaning that the recovered frequency will be frequency spread, leading to difficulties in measuring the signal. In contrast, this aspect allows the recovered data test signal to be demodulated with the rotational error of the disk, rejecting fluctuations in the speed of rotation of the disk, so that the analysis of the signal read back from the disk will be less affected by frequency spreading. This means that detecting the frequency in the read back signal will be simpler as the frequency power will be more sharply concentrated about the correct frequency and less spread. 
     The step of analysing may comprise spectrum analysing said test data signal. As discussed below, the present arrangement has particular advantages when used in narrow band measurement tests. 
     The step of spectrum analysing may comprise converting said test data signal to an intermediate frequency by mixing said test data signal with a sinusoidal signal generated using said reference clock as a timing reference. This arrangement allows a more accurate super-heterodyning operation to be carried out on the data signal as part of a spectrum analysing test and allows the spectrum analyser to become normalised to disk speed. 
     The step of spectrum analysing may comprise sampling and digitising the test data signal using an analogue-to-digital converter that is clocked by the reference clock. This allows the spectrum analyser to become normalised to disk speed. The present arrangement can also be used in a spectrum analyser that performs some or all of the spectrum analysis digitally. 
     According to a third aspect of the present invention, there is provided a method of testing at least one of a data storage disk medium and a read/write head, the method comprising: rotating the disk; detecting fluctuations in the speed of rotation of the disk; producing a reference clock signal in accordance with said fluctuations so as to be synchronised with the rotation of the disk; generating a test pattern of data using said reference clock signal as a timing reference; writing the test pattern of data to the disk with the head; reading the test pattern of data from the disk with the head to provide a test data signal; and, analysing said test data signal using the reference clock signal as a timing reference. 
     This provides a more accurate method of testing by utilising both methods described above. The reference clock signal is used both when generating the pattern of data to be written to the disk to compensate for fluctuations in the disk rotation and also when analysing the data signal read back from the disk to compensate for fluctuations in the disk rotation. 
     In an embodiment, the method comprises: 1) the disk being rotated by a motor that is locked to an oscillator clock signal; and, 2) the reference clock signal being provided from an oscillator clock signal, the oscillator clock signal being modulated in accordance with the fluctuations in the speed of rotation of the disk in the production of the reference clock signal, wherein the same oscillator clock signal is used for steps 1) and 2). In this arrangement, a single oscillator clock is used as reference for the entire testing apparatus rather than having separate oscillator clocks in the apparatus. This avoids the effects of thermal drift which typically arise over time between two separate clocks, and therefore helps avoid a source of error entering the system, both when writing data to the disk and when reading back and analysing data from the disk. 
     According to a fourth aspect of the present invention, there is provided apparatus for generating a test pattern of data to be written to a data storage disk medium for testing, the apparatus comprising: a spindle for rotating a said disk; a detector arranged to detect fluctuations in the speed of rotation of the disk; a processor arranged to produce a reference clock signal in accordance with said fluctuations so as to be synchronised with the rotation of the disk; and, a pattern generator arranged to generate a test pattern of data using said reference clock signal as a timing reference. 
     According to a fifth aspect of the present invention, there is provided apparatus for analysing a test pattern of data read from a data storage disk medium, the apparatus comprising: a spindle for rotating a said disk; a detector arranged to detect fluctuations in the speed of rotation of the disk; a processor arranged to produce a reference clock signal in accordance with said fluctuations so as to be synchronised with the rotation of the disk; a controller arranged to read a test pattern of data from the disk to provide a test data signal; and, an analyser arranged to analyse the test data signal using the reference clock signal as a timing reference. 
     According to a sixth aspect of the present invention, there is provided apparatus for testing at least one of a data storage disk medium and a read/write head, the apparatus comprising: a spindle for rotating a said disk; a detector arranged to detect fluctuations in the speed of rotation of the disk; a processor arranged to produce a reference clock signal in accordance with said fluctuations so as to be synchronised with the rotation of the disk; a pattern generator arranged to generate a test pattern of data using said reference clock signal as a timing reference; a controller arranged to write the test pattern of data to the disk with a said head, and to subsequently read a test data from the disk with the head to provide a test data signal; and, an analyser arranged to analyse the test data signal using the reference clock signal as a timing reference. 
    
    
     
       Embodiments of the present invention will now be described by way of example with reference to the accompanying drawings, in which: 
         FIG. 1  shows schematically an example of a spinstand according to an embodiment of the present invention; and, 
         FIG. 2  shows schematically an example of a spectrum analyser to be used in conjunction with the spinstand of  FIG. 1 . 
     
    
    
     Referring to  FIG. 1 , a spinstand  1  comprises an air-bearing spindle  2  driven by a motor  3  to which a disk  4  is mounted. The spinstand  1  also has a head load mechanism (not shown) for holding a head gimbal assembly  5  and positioning the read/write head  6  of the head gimbal assembly  5  over the surface of the disk  4  such that data can be written to and read from a track on the surface of the disk  4 . As described thus far, the spinstand  1  is of a known type. 
     The spinstand  1  has a laser encoder system  7  associated with its spindle  2  to allow the position of the spindle  2  to be accurately determined. The principles of a laser encoder system  7  are generally well known in the art and therefore will not be described in detail herein. In particular, suitable laser encoder systems  7  have been developed for use with servo track writing machines, where they are used to allow successively written servo tracks to be kept in the correct phase with each other with the necessary high degree of accuracy. Nevertheless in brief, the spindle has an encoder scale  7   a  attached thereto at its end opposite the disk  4 . The scale  7   a  comprises a diffraction grating, i.e. a series of transitions from black to white, arranged in a ring positioned to be concentric with the axis of the spindle  2 . There are preferably at least thousands of such transitions around the ring, and possibly millions of transitions. The transitions preferably occur with an equiangular spacing around the ring. 
     A grating interferometer  7   b  is mounted to the spinstand  1  for reading the scale  7   a  on the spindle  2 . The interferometer  7   b  has a laser (not specifically shown) which is directed onto the scale  7   a  to allow the transitions in the scale  7   a  to be detected by a detector (not specifically shown) in the interferometer  7   b  as they move past the laser. In this way the laser encoder system  7  produces a raw encoder signal  51  measuring the detected scale  7   a.    
     The raw encoder signal  51  is passed to optical timing clock electronics  8  which are arranged to process the raw encoder signal  51  to produce a processed encoder signal in the form of a clock signal  52  whose edges correspond to the detected transitions of the scale  7   a.    
     It should be noted that other arrangements are possible for measuring fluctuations in the rotation of the disk  4 . For example, the scale  7   a  can be attached to the disk  4  or any other part that rotates with the disk  4  instead of being attached to the spindle  2 . It is not necessary to use optical marks. Magnetic marks can be used. The magnetic marks can be formed on the surface of the disk  4 . 
     The clock signal  52  is fed to a phase locked loop (PLL)  10 , which produces as its output a reference clock signal  53 . As will be appreciated by one skilled in the relevant art, there are many ways of implementing a PLL. Only one way will be described in detail in the following. 
     The PLL  10  generally extracts phase information from the clock signal  52  and modulates an oscillator clock signal  54  from a crystal oscillator  11  with the phase information to produce the reference clock signal  53 . This is done by feeding back the reference clock signal  53  in a negative feedback loop. In this example, the reference clock signal  53  is first divided by a divide-by-n counter  12  to provide a divided reference signal  55 . This division lowers the frequency to somewhere near the frequency of the clock signal  52 . The clock signal  52  and the divided reference signal  55  are fed to a phase detector  13 , which determines the phase difference between the two. The phase difference is expressed as a DC level at the detector output  56 . This may be optionally filtered by a filter  14  to improve the performance of the PLL  10 , as is known in the art. This filtered signal  57  is provided to a vector modulator  15  as a control signal. A vector modulator  15  has the filtered signal  57  as a first input. The crystal oscillator  11  produces the oscillator clock signal  54  which is also fed to the vector modulator  15 . The vector modulator  15  is arranged to modulate the phase of the incoming oscillator signal  54  so that the phase difference between the divided reference clock signal  55  and the clock signal  52  derived from the motor  3  is reduced to or reduced towards zero. 
     Thus a reference clock signal  53  is generated that is modulated to take account of the fluctuations in the rotation of the disk  4  so as to be synchronised with the rotation of the disk  4 . 
     Now, a number of standard tests made by a spinstand require using narrow band power measurements. One such test is the so-called “overwrite” test. This test involves writing a pattern of test data having a first frequency to a track on the disk  4  with the read/write head  6 , and then subsequently overwriting this first pattern with a second pattern of test data having a different frequency. The track is then read back with the head  6 , and the signal obtained is analysed to measure the residual signal power of the overwritten pattern at the first frequency. This is accomplished by a narrow band measurement system focussed on the first frequency. It is generally desirable that the narrow band measurement system is operable over a wide frequency range with good resolution so as to be flexible enough to work with different frequencies of the data patterns written to the disk in testing. In practice, a tuned spectrum analyser or apparatus similar thereto is commonly used to perform the signal analysis. 
     Because the residual signal power of the first frequency pattern is generally relatively low, it is necessary to use a measurement system with a high dynamic range in detecting and measuring the signal. In practice, this means that the spectrum analyser needs to have a low resolution bandwidth (RBW) for this purpose. As an example, next generation spinstands are specifying a RBW of down to 10 kHz. However, due to fluctuations in the speed of the motor both when writing the test data and when reading back the test data, the frequency content of the data signal ultimately read by the head can be distorted and spread. For example, for a typical motor speed error of ±0.001%, in the worst case there would be an error of ±0.002% in the frequency of the measurement signal (i.e., ±0.001% error when the pattern is written, and ±0.001% error when the pattern is read back). When this error is applied to a typical written frequency signal of 500 MHz, the read back frequency will vary by more than ±10 kHz in the worse case. Thus, it is becoming increasingly problematic to implement a RBW that is narrow enough to pick up the low level signal whilst coping with possible spreading of the frequency. Furthermore, this is a growing problem as the trend in the art is for the signal-to-noise ratio of modern heads  6  to become ever lower, meaning that it is necessary to further lower the width of the narrow band filter in the spectrum analyser. Also, next generation spinstand equipment is expected to operate with a written frequency of up to 2.5 GHz. Thus standard techniques are becoming increasingly inadequate. 
     Similar or equivalent problems caused by fluctuation in the spindle speed arise in other types of spinstand test. 
     In one embodiment, the reference clock signal  53  is fed to a pattern generator  16  and used as a timing reference for generating a test pattern of data  58 . The test pattern of data  58  is then written to the disk  4  by the head  6 . The spatial spreading of the test pattern of data  58  written to the disk  4  due to fluctuations in the rotation of the disk  4  is therefore reduced. Thus, when for example conducting an overwrite test as described above, this technique of generating a test pattern of data  58  may be used in writing the first frequency pattern of test data and/or the second frequency pattern of test data to the disk  4  in order to mitigate the problems of distortion and spreading of the written frequency. 
     The fluctuations in the rotation of the disk  4  will also affect the fidelity of the test data signal  59  (also known as the head signal  59 ) that is read back by the head  6 . To mitigate the effects of this, in one embodiment, the analysis is performed on the test data signal  59  using the reference clock signal  53  as a timing reference so that the test data signal  59  is demodulated with the reference clock signal  53 . Thus, again when conducting an overwrite test, this technique of analysing the test data signal  59  can be used to perform spectrum analysis on test data signal  59  to detect the residual signal power of the overwritten first frequency pattern of test data so as to further mitigate the problems of distortion and spreading of the written frequency when it is read back from the disk  4 . 
       FIG. 2  shows an example of apparatus  20  for performing spectral analysis on the test data signal  59 . The spectrum analyser  20  has an input stage that works on the “super-heterodyning” principle. In this, the test data signal  59  measured by the head  6  is mixed with a selected frequency sine wave  60  in order to frequency shift the frequency of interest to an intermediate frequency (IF). In the present example, a tunable synthesiser  21  is used to generate the desired frequency  60 . The test data signal  59  and the generated frequency  60  are fed to a mixer  22 , where the signals are mixed in order to shift the frequency of interest to the IF. Alternatively, the tunable synthesiser can be linked to a sweep generator so as to perform a spectrum analysis across a frequency range of interest. 
     The shifted test data signal  61  is filtered by an IF filter  23  to remove unwanted heterodyning tones. The filtered signal  62  is then sampled and digitised by an analogue-to-digital converter (ADC)  24 . The clock  63  for the ADC  24  is derived from the reference clock signal  53 . The reference clock signal  53  is fed to a ADC clock synthesiser  25  which generates a clock signal  63  for the ADC  24  based upon the reference clock signal  53  as a timing reference. Thus the ADC  24  is clocked by the reference clock signal  53  and so the sampled signal  64  produced by the ADC  24  rejects frequency spreading due to fluctuations in the rotation of the disk  4 . 
     The sampled signal  64  is then passed to a digital signal processing (DSP) system  30 . A first-in-first-out buffer  31  (FIFO) is used as the input stage to the DSP system  30  and receives the sampled signal  64 . The FIFO  31  preferably has protection for crossing asynchronous clock domains so that the DSP system  30  performing the processing can be clocked at a much higher rate than the sampling frequency of the ADC  24  to improve the latency of the analyser  20 . The DSP system  30  also comprises a digital down-converter  32  which receives the sampled signal  64  from the FIFO  31  and then down-converts the sampled signal  64  to a baseband signal  65 . This baseband signal  65  is passed through a low pass filter  33 , which acts as a RBW filter. Finally, the filtered signal  66  is passed to a power detector  34 , which measures the power in the signal  66 , so that this can be recorded or displayed to the user. 
     Since the ADC sampling clock  63  is derived from the reference clock signal  53  modulated with the speed of rotation of the disk  4 , the sampled signal  64  produced by the ADC  24  rejects frequency spreading due to fluctuations in the rotation of the disk  4 . This means that the spectrum analyser  20  becomes normalised to disk speed. 
     Nonetheless, there are other ways in which the spectrum analyser  20  can be made normalised to disk speed. For example, instead of deriving the ADC sampling clock  63  from the reference clock signal  53 , the super heterodyning frequency  60  can be derived from the reference clock signal  53 . This also has the effect that the sampled signal  64  produced by the ADC  24  rejects frequency spreading due to fluctuations in the rotation of the disk  4 , and so normalises the spectrum analyser  20  to disk speed. 
     In addition, referring to  FIG. 1 , the motor controller  40 , which is used to control the rotation of the disk  4 , is clocked by, or otherwise locked to, the same oscillator  11  that clocks the PLL  10 . In this way, a single oscillator clock  54  is used as a reference for the entire testing apparatus  1 , 20  rather than having separate oscillator clocks in the apparatus. This avoids the effects of thermal drift between two separate clocks and therefore helps avoid a source of error entering the system, both when writing data to the disk  4  and when reading back and analysing data from the disk  4 . 
     These are only some examples of how the reference clock signal  53  can be used as a timing reference for analysing the test data signal  59 . Other forms are possible depending on the application. 
     Embodiments of the present invention have been described with particular reference to the examples illustrated. However, it will be appreciated that variations and modifications may be made to the examples described within the scope of the present invention.