Patent Publication Number: US-6222375-B1

Title: Synchronous calibration test circuit for use in recording systems

Description:
FIELD OF INVENTION 
     Invention relates to calibration test circuits for recording system devices, and more particularly to calibration test circuits for detecting drive fly height that may indicate impending drive failure. 
     BACKGROUND OF INVENTION 
     Within disk drives, characterization circuits in the channel chip (i.e., PRML) are typically provided to measure head and media parametrics, thereby eliminating a need for external equipment such as an oscilloscope or a spectrum analyzer. Currently, these measurement circuits, such as the harmonic sensor circuit described in U.S. Pat. No. 5,784,296, obtain head/media measurements with all transfer functions of the calibration circuit locked in fixed, non-adaptive states. A particular application of the harmonic sensor is to measure changes in fly height of the playback head. Changes in fly height are important to disk drives (or tape drives) since these critical changes are indications of potential drive failure from such as unwanted contamination and sensitivity to atmospheric pressure, or sensitivity to mechanical parameters in the drive such as tape tension in a tape drive. However, a limitation of the harmonic sensor is that changes in the transfer function characteristics of the channel chip (such as, i.e., a PRML chip) due to environmental factors can be mistaken for changes in a head or media parameter, thereby corrupting the measurements sensed by the harmonic sensor. Specifically, environmental variations such as temperature, or supply voltage can change the gain of the transfer functions in the front end of a channel chip. These changes in circuit characteristics are not distinguishable from changes in the head/media interface in current measurements by the harmonic sensor. 
     An additional drawback of the harmonic sensor technology described in U.S. Pat. No. 5,784,296 is that the by holding the transfer function of the drive in a static mode (i.e. not allowing the chip to adapt to its environment), the detector&#39;s performance can be degraded by changes in the signal. Thus, for example, if the user wishes to perform a test such as measuring changes in fly height versus atmospheric pressure, the equalization of the incoming signal should be identical to that used when the test is started. Because the playback signal changes with atmospheric pressure, the equalization should correspondingly change to compensate for the new signal. However, the transfer functions are locked in non-adaptive state during head/media parametric measurements, thus resulting in misequalization and increased error rate. The increased error rate can limit the range over which the harmonic sensor can be used. 
     There is therefore a need to measure variations in head/media parameters, and distinguish those measurements from environmentally sensitive transfer function characteristics so to more accurately acquire measurements necessary for measuring fly height, conducive for detecting drive failure before data lost. 
     SUMMARY OF INVENTION 
     A synchronous calibration test circuit is provided in accordance with the principles of this invention that uses a bandgap reference voltage of a PRML chip to generate as output a programmable frequency reference signal whose amplitude is invariant with environmental conditions. A bandgap referenced signal is injected at the input of a signal path of the PRML chip and measured after the PRML&#39;s A/D converter using the synchronous test calibration circuit of this invention. Since the bandgap referenced signal is insensitive to voltage supply fluctuations as well as temperature and process variations this reference signal can be used to measure and characterize changes in the transfer function of the analog signal path in the PRML chip, unhampered by the environmental sensitivities of the circuits that comprise transfer function of the analog portion of the chip. Any variation measured in the referenced signal at the output of the PRML filter or gain stages is attributable to the changes in the transfer characteristics of the filter or the gain stage. 
     Thus, the calibration circuit of this invention allows calibration of analog and digital signal processing blocks so that absolute, rather than relative head/media measurements can be made. In one example, the calibration circuit enables the harmonic sensor to measure absolute changes in fly height over the life of a drive. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     FIG. 1 illustrates a circuit schematic of the synchronous calibration test circuit provided in accordance with the principles of this invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENT(S) 
     FIG. 1 illustrates a synchronous calibration test circuit  10  provided in accordance with the principles of this invention. In the preferred embodiment, synchronous calibration test circuit  10  is integrated in a PRML channel chip (not shown) of the disk drive, with a write synthesizer  12  of the channel chip used to generate a digital signal  13  at a channel chip clock rate. Preferably, as will be described in this embodiment, digital signal  13  comprises a square wave, although triangular wave, or sine wave signal are also contemplated as possible alternatives. Square wave signal  13  is then passed through an amplifier  18  that reproduces the input square wave signal, but with an amplitude referenced to the bandgap voltage of the channel chip. Square wave signal  13  is gated on and off using a read gate  14 . A frequency divider  16  comprising a divide by k circuit coupled between read gate  14  and amplifier  18  is provided to reduce the frequency of signal  13 . Thus, if k=2, 4, 8, or 16, for example, a simple set of flip flops can be used to perform the division. At the output of amplifier  18 , a resulting bandgap referenced signal  19  is thereby provided having an amplitude that is referenced to the chip&#39;s bandgap voltage, a fixed, environmentally insensitive voltage. 
     The conversion from a digital level signal to a square wave with a precise amplitude is a critical step and must be done with a circuit design that is independent of the supply voltage. Ideally, digital bandgap referenced signal  19  is injected via a switch  20 , such as a multiplexer, at any one of several nodes (i.e.,  23   a ,  23   b ,  23   c ,  23   d , or  23   e ) in an analog path  23  of the channel chip. Preferably, analog path  23  comprises a coarse gain stage  22  the output of which is coupled to node  23   b  that is also connected to the input of an AC coupling capacitor  24  to remove DC offset from the signal path. The output of AC coupling capacitor  24  is in turn coupled to node  23   c , also connected to the input of a voltage gain amplifier (VGA)  26 . VGA  26  comprises an adaptive amplifier for amplifying the resulting signal from capacitor  24 . The output of VGA  26  is then coupled to node  23   d  that is also coupled in turn to an input of a continuous time filter (CTF)  28 . CTF  28  removes excess noise and operates in essence as an antialiasing filter. The output of CTF  28  is then coupled to node  23 e that is in turn also coupled to an input of FIR filter  30 . It is also envisioned, that in some PRML chips, the FIR filter is placed after the A/D converter. 
     FIR filter  30  comprises an adaptive filter to maintain a desired constant pulse shape at its output. The output of FIR filter  30  is then coupled to an input of A/D converter  32 , with resulting output calibration reference signal  33  of A/D converter  32 . Output calibration reference signal  33  can then be used as a signal input to a harmonic sensor circuit (not shown) during calibration to measure the fluctuations in the transfer function of the analog portion of the PRML chip due to environmental changes. Since variations in this programmable frequency reference signal  33  are attributable to changes in the transfer function of the PRML filter and gain stages, calibration reference signal  33  can thus be used to measure and characterize changes in the transfer function of analog signal path  23  in the PRML chip, and thus separate from the environmental sensitivities of the channel chip filters. 
     Additionally, different frequencies created by the divider circuit (or the synthesizer) allow the user to characterize changes in the gain of the analog transfer function at different frequencies. Moreover, the steep antialiasing roll off of the CTF minimizes harmonic contamination due to folding of the square wave&#39;s spectrum. 
     During calibration, the nominal input signal to the channel chip is disabled from its normal input to the chip. Calibration signal  19  then passes through analog path  23  of the chip and is sampled by A/D converter  32 . Digital samples provided by calibration reference signal  33  at output of A/D converter  32  are then provided to a typical harmonic sensor of the channel chip, such as described in U.S. Pat. No. 5,784,296, to detect an amplitude of the resulting output signal  33  at a particular frequency. Moreover, current implementations of the harmonic sensor provide that A/D output signal  33  signal be sampled in multiple passes with a known phase relationship between signal  33  and the harmonic sensor in each pass. In an alternative embodiment, the A/D clock can be selected from the recovered read clock through multiplexer  34 . 
     Thus, the calibration circuit of this invention allows calibration of analog and digital signal processing blocks so that absolute, rather than relative head/media measurements can be made. In one example, the calibration circuit enables the harmonic sensor to measure absolute changes in fly height over the life of a drive. Accordingly, the current invention presents a circuit and method for distinguishing variations in the PRML filters and isolating these measurements from the head media parameters that one desires to measure. By including the ability to characterize filter VGA and other systemic variations in the channel chip, one can monitor the relative fly height of heads in the drive over the entire life time of the drive. Consequently, this invention provides selfregulating disk drives, to identify problem disk drives prior to data loss. 
     Foregoing described embodiments of the invention are provided as illustrations and descriptions. They are not intended to limit the invention to precise form described. In particular, it is contemplated that functional implementation of invention described herein may be implemented equivalently in hardware, software, firmware, and/or other available functional components or building blocks. Other variations and embodiments are possible in light of above teachings, and it is thus intended that the scope of invention not be limited by this Detailed Description, but rather by Claims following.