Application specific waveform generator

Details Application specific waveform generator Application specific waveform generator

1999-05-03

2002-05-28

Frejd, Russell (Department: 2123)

Data processing: structural design, modeling, simulation, and em

Simulating electronic device or electrical system

Target device

C703S002000

Reexamination Certificate

active

06397173

ABSTRACT:

This invention relates generally to electronic test equipment, and more particularly to test equipment including a waveform generator that converts digital signals to analog pulses representing actual physical waveforms through the use of mathematical modeling of a physical system.
BACKGROUND OF THE INVENTION
Conventional arbitrary waveform generators such as the ones manufactured by Tektronix, Hewlett Packard, LeCroy, and others are used to output analog signals from a digital sequence. One application for arbitrary waveform generators is in the magnetic storage industry where devices include electronics operating at extremely high data rates. Engineers in the magnetic storage industry use arbitrary waveform generators to help with the design of the channel electronics (circuitry used to translate the analog signals output from the magnetic disks and read heads into digital signals). Arbitrary waveform generators made by Tektronix dominate the magnetic storage industry since they have the fastest rated signal speeds available today. The fastest speed available today in is one billion samples per second (1 Gs/s).
In disk drive systems, a transducing, or read/write, head writes incoming digital data onto a magnetic storage medium (the disks or platters). The incoming digital data serve to modulate the current in a coil of the read/write head so that a sequence of corresponding magnetic flux transitions are written as user data onto the magnetic medium in concentric tracks. The user data is typically divided into fixed lengths and stored in sectors of that length. Each sector of user data is stored with header information associated with it. In addition, servo information is also recorded on the disks to allow the drive to position the heads correctly over the data sectors. To read this recorded data (user data as well as headers and servo information), the read/write head passes over the magnetic medium and converts the magnetic transitions into pulses in an analog signal. These analog pulses are then decoded by the read channel circuitry to reproduce the digital data.
A disk drive is typically composed of a Head/Disk Assembly (HDA), and a Printed Circuit Board (PCB). The HDA includes an enclosed structure that contains the head and disk stacks, a disk drive motor, an actuator, and a pre-amplifier. The analog signals from the heads exit the HDA assembly and are sent through the PCB.
The PCB includes read channel circuitry on a chip that decodes the analog signals from the HDA into digital data. In order to do this, additional auxiliary signal lines (markers or gates) are also generated to keep track of whether header, servo or data fields are being read. The decoding of data has to be done very accurately in order to reproduce correctly the original digital data that was stored on the magnetic disks. The accuracy of disk drive systems is measured in error rates. An error rate of 1×10
−10
, for example, would mean only one bit was decoded incorrectly for every 1×10
10
(ten billion) bits processed. Although the details described above are those embodied in a magnetic disk storage system, they are also applicable to other magnetic recording systems such as magnetic tape recording systems, communication systems, and to other systems that have a physical basis onto which data is stored.
One limitation in development and testing of the channel circuitry design is that the designer has to develop the channel circuitry before the actual drive PCB (printed circuit board) and HDA (head/disk assembly) are available, due to time-to-market constraints. Another limitation is that the channel electronics are typically manufactured by a different company than the one manufacturing the disk drive system. Since the heads and disks are not available during the initial phases of the drive channel design, the engineers in both the drive company and the chip company that manufactures the channel chip use an arbitrary waveform generator and a noise source to produce an analog signal that is meant to represent the signal from the drive.
First, the engineer creates various sequences of digital bit streams that would be typically found in the final manufactured disk drive, with the assistance of a computer. These sequences are intended to represent all the aspects of signals from the disks/heads, including but not limited to the headers and data fields associated with each sector, the servo fields that allow the drive to position the head accurately within the disk surface, as well as the various tracks in the drive which typically operate at different areal densities (bit density x track density).
Once the digital bit sequence is constructed, linear superposition of an analytical function such as a Lorenzian function is typically used to create a series of ASCII characters corresponding to voltage levels to represent the analog waveform. This series of ASCII characters is stored in a data file in the computer.
The data file is provided as an input to an arbitrary waveform generator (typically manufactured by Tektronix) through a General Purpose Interface Bus (GPIB), such as an EEE 488 standard parallel interface, or any other suitable interface. Here, the ASCII characters from the data file are converted to a corresponding analog waveform.
The analog signal output of the arbitrary waveform generator is summed with the signal from a random noise generator generating “white noise,” which is random noise evenly distributed (constant amplitude) across the applicable frequency range. A white noise generator from NoiseCom is the one typically used. This final analog signal, the combination of the noise and the analog waveform is used by the engineer to study and improve the design of the read channel chip. Since the PCB which houses the read channel chip in the final disk drive system is typically not available during the design phase, the engineer uses a test fixture to house the read channel chip being designed.
At some later date in the disk drive development phase, when initial prototype HDAs are available, the analog signal from the prototype HDAs are used as a better representation. However, this usually does not represent the true distribution of signals from HDAs in production. This is because early samples of heads and disks used to make the initial HDAs are often from hand picked prototype parts from the suppliers rather than from all suppliers and manufacturing sites.
There are several shortcomings to this currently-used method of channel design. First, due to the cumbersome nature of sequence generation and input into the arbitrary waveform generator, the number of sequences of pulses that can be studied in a timely fashion are limited. Further, due to the limited amount of memory, full tracks are not easily studied since that would involve too long a sequence. Typically a single data input file, representing less than a full track length, is used through a GPIB interface to the arbitrary waveform generator.
Second, the analog pulse shape characteristics do not represent true pulse shapes of the heads and disks used in today's disk drive systems. Features that are unique to the magnetic storage system significantly affect channel error rates. Specifically, the error rates at today's and at future densities are significantly affected by the non-linear distortion associated with the write process and the distortions due to the magnetoresistive read head, that are commonly used today. None of these are taken into account when an analytical function is used to represent the analog signal output from the waveform generator.
Third, the channel design cannot be optimized for the final distribution of analog signals since, at best, a few hand-picked early head/disk combinations can be studied from the early HDAs built.
Fourth, the white noise that is used is of uniform amplitude at all frequencies. In reality, the actual noise in a disk drive system is not evenly-distributed white noise, but is instead colored noise. This means that the amplitude of the noise is correlated t

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