The present invention relates generally to data storage systems and, more particularly, to a system and method for estimating and altering the frequency of a pole of a readback signal amplifier circuit.
A typical data storage system includes a magnetic medium for storing data in magnetic form and a transducer used to write and read magnetic data respectively to and from the medium. A typical disk storage device, for example, includes one or more data storage disks coaxially mounted on a hub of a spindle motor. The spindle motor rotates the disks at speeds typically on the order of several thousand revolutions-per-minute (RPM).
Digital information is typically stored in the form of magnetic transitions on a series of concentric, spaced tracks formatted on the surface of the magnetizable rigid data storage disks. The tracks are generally divided into a number of sectors, with each sector comprising a number of information fields, including fields for storing data, and sector identification and synchronization information, for example.
An actuator assembly typically includes a plurality of outwardly extending arms with one or more transducers and slider bodies being mounted on flexible suspensions. The slider body lifts the transducer head off the surface of the disk as the rate of spindle motor rotation increases, and causes the head to hover above the disk on an air bearing produced by high speed disk rotation. The distance between the head and the disk surface, which is typically on the order of 40-100 nanometers (nm), is commonly referred to as head-to-disk clearance or spacing.
A read element of a transducer reads digital information provided on the disk surface and, in response, produces a readback signal which is communicated to amplification circuitry. The amplification circuitry, which is often provided in an arm electronics (AE) module, typically exhibits a highpass filtering behavior due to the presence of a low pole in the AE amplifier transfer function. The presence of the low pole causes both an attenuation and a phase shift in the readback signals processed by the AE module out beyond the low pole frequency. It is known that any appreciable degree of phase distortion in readback signals processed by an AE module can result in read channel errors of varying severity.
In disk drive systems that employ magnetoresistive (MR) transducers, it is well known that the highpass pole frequency of an individual AE amplifier can vary by up to xc2x150% relative to other AE modules of the disk drive system. In addition, the AE module filter characteristics typically change as a function of temperature. Depending on the particular AE module design, and assuming that an MR transducer is employed, large variations in pole frequency often result from differences in the nominal resistances of the MR transducers that are employed in a given disk drive system.
Generally, a large variation in the AE amplifier pole frequency will occur for each individual MR head of a disk drive system. This variation in pole frequency is very detrimental to the dynamic equalization effort of the recording channel. Moreover, various techniques that attempt to compensate for pole frequency variations in an AE amplifier generally require an estimation of the pole frequency. Absent the ability to reliably estimate the pole frequency with a sufficient degree of accuracy, such compensation approaches are met with little success.
There is a need for an apparatus and method for accurately estimating the pole frequency of a readback signal amplifier circuit used in a data storage system. There exists a further need for a system and method for compensating for the AE pole associated with each head of a data storage system, and for providing the ability to alter the pole frequency depending on the particular function to be performed. The present invention fulfills these and other needs.
The present invention is directed to an apparatus and method for estimating a pole frequency of an amplifier circuit, such as a readback signal amplifier circuit implemented in an arm electronics module of a data storage system. The amplifier circuit exhibits a highpass filter frequency behavior due to the presence of a low pole, the frequency of which may be determined in accordance with the principles of the present invention.
Estimating the pole frequency of a readback signal amplifier circuit that exhibits a highpass filter frequency behavior involves modulating a bias signal that biases the read transducer. The bias signal, which may be a current signal or a voltage signal, is modulated at a first modulation frequency and at a second modulation frequency while a readback signal is obtained from a data storage medium using the read transducer, preferably from an erased location of the medium. The readback signal is communicated to the amplifier, and an output signal of the amplifier circuit, such as a output voltage signal, is measured at the respective first and second modulation frequencies. The pole is determined using the output signal measurements.
Modulating the bias signal involves modulating the bias signal using a first perturbing signal at the first modulation frequency and modulating the bias signal using a second perturbing signal at the second modulation frequency. The first modulation frequency is preferably a frequency less than the frequency of the pole, and the second modulation frequency is a frequency greater than the frequency of the pole. The first and second perturbing signals are preferably square waves. The second modulation frequency is preferably an odd harmonic of the first modulation frequency. For example, the second modulation frequency may be a third harmonic of the first modulation frequency.
Measuring the output signal involves digitizing the output signal and performing a discrete Fourier transform on the digitized output""signal at the first and second frequencies. In one embodiment the discrete Fourier transform operations are performed using Goertzel""s algorithm. Determining the pole frequency of the amplifier circuit involves computing a square impedance measurement ratio using the square magnitudes of the bias signal and the amplifier output signal respectively measured at the first and second modulation frequencies.
In accordance with one embodiment, the numerator of the square impedance measurement ratio is computed using the magnitude of the DFT component of an output voltage signal at a second modulation frequency, f2, multiplied by the magnitude of the DFT component of a modulating bias current at a first modulation frequency, f1. The denominator of the square impedance measurement ratio is computed using the magnitude of the DFT component of an output voltage signal at the first modulation frequency, f1, multiplied by the magnitude of the DFT component of a modulating bias current at the second modulation frequency, f2. The pole frequency is determined using the square impedance measurement ratio and values of the first and second modulation frequencies, f1 and f2.
The pole frequency of the readback signal amplifier may be altered subsequent to a determination of the pole frequency. The amplifier pole may be selectively altered according to selection of a particular mode of operation or function to be performed, such as a recording function or diagnostic function.
Altering the overall pole frequency involves digitally filtering a sampled amplifier output signal so as to alter the pole frequency to a preestablished frequency. Alteration of the overall pole frequency may be accomplished using a pole-zero cancellation technique. The pole frequency of the amplifier circuit may be estimated, altered, and stored for each of a plurality of read transducers when each is coupled to the amplifier circuit.
A circuit for estimating a frequency of a readback signal amplifier pole includes a modulator coupled to a read transducer, such as a read transducer comprising an MR element. The modulator produces a modulated bias signal at respective first and second modulation frequencies for purposes of biasing the read transducer during a pole frequency determination procedure. A converter is coupled to the amplifier and receives an output signal representative of the readback signal from the amplifier. The converter converts the amplifier output signal to a digitized output signal. A processor estimates the pole frequency of the readback signal amplifier using a measurement of the digitized output signal at the respective first and second modulation frequencies in accordance with the above described methodology. In one embodiment, the circuit is provided in a data storage system.
The above summary of the present invention is not intended to describe each embodiment or every implementation of the present invention. Advantages and attainments, together with a more complete understanding of the invention, will become apparent and appreciated by referring to the following detailed description and claims taken in conjunction with the accompanying drawings.