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Simulating high-speed disk drive signals

Posted: 16 May 2003     Print Version  Bookmark and Share

Keywords:disk drive 

use of disk drives to store and

retrieve information, or to play

their favorite CD or DVD.

/PDF document p>

By Don Commare

Product Marketing Manager

Tektronix Inc.

People around the world make

use of disk drives to store and

retrieve information, or to play

their favorite CD or DVD. As

the need for more storage ca-

pacity and faster date transfer

rates explode, manufacturers

address the challenge by mak-

ing incremental improvements

in drive assembly and in the

read/write channel. Current

channelspeedsareontheorder

of 800Mbps, but future chan-

nels will offer 1Gbps, 1.2Gbps,

1.5Gbps and 2Gbps data rates.

To develop next-generation

disk drives, engineers require

test signals capable of simulat-

ing high-speed channel rates.

Higher data rate signals are

necessary to insure that proto-

type components and hardware

meet target specifications.

Without these signals, design-

ers face great difficulty per-

forming advanced component

andsystemleveltesting.Today,

engineers rely on a signal

source instrument called an

Arbitrary Wave Generator

(AWG)astheirtoolfordevelop-

ing next-generation read/write

channels.

In principle, the AWG can

be compared to a CD playing

your favorite song. The song,

stored on the CD in binary for-

mat (1s and 0s), is read by the

player's detection hardware.

The data (1s and 0s), although

not arbitrary, passes through a

DAC and is amplified to pro-

duce musical tones. When

played continuously, the data

produces the song that is heard

throughanalogspeakers.Inthe

case of the AWG, Figure 1,

waveform data resides in

memory in binary form. The bi-

nary values are "sampled" by

the master clock. This "sam-

pling clock" drives the high-

speed shift registers and DAC.

Thedatastoredinmemorycon-

tains amplitude and position

data representing the specific

waveform shape when plotted.

The sampling clock's rate and

Simulating high-speed disk drive signals

total number of data points

sampled determine the signal's

output frequency.

Since all waveforms are cre-

atedfromaseriesofdatapoints,

determining the output signal

frequency is expressed by divid-

ing the sample rate (clock) by

the total number of waveform

points. For multiple cycle wave-

forms, the output frequency is

expressedas[sampleratexnum-

ber of cycles/total waveform

points]. An AWG, with a maxi-

mum sample rate of 4Giga-

samples per second (GS/s), is

capable of generating wave-

forms by using two points at a

frequency of 2GHz. As such, a

waveform constructed using

three points per cycle will de-

liver a maximum frequency of

1.3GHz, while four-point data

delivers a maximum frequency

of1GHz andsoon.Drivedevel-

opers must understand this

trade-off because the quality of

the waveform relates directly to

the total number of waveform

points and selected sampling

rate. Luckily, some AWGs in-

cludebuilt-inapplicationmenus

whichallowuserstoquicklycre-

ate standard disk drive signals

that simplify the process.

Tektronix'AWG710isanex-

Clock

Frequency

control

PLL

High-speed

shift

registers

D/A

converter

Low

pass

filter

Output

amplitude

Output

outlet

Waveform

outputs

Analog

Markers

Digital patterns

Waveforms loaded from...

Front panel, HDD, LAN, OPSU or floppy

RAM

Figure 1: In the case of AWG, waveform data resides in memory in binary form.

Figure 2: AWG710 creates 50/50 Lorentz/Gaussian pulse to create the read/write channel transition response.

cellent example of this kind of

utility. The utility relies on the

fact that virtually all magnetic

disk drives utilize saturated re-

cording to read/write data. In

this case, each bit on the disk is

polarized in one of two states.

That is, the data is encoded by

simply changing the sign of the

current in the write head,

which flips the polarity of the

magneticmediabelowit.Later,

thereadheadsensesthefluxre-

versals on the disk and gener-

ates a response to the transi-

tion. An effective model of this

transitionresponseisamixture

ofLorentzandGaussianpulses.

By taking various ratios of

the Lorentz and Gaussian

pulse,aread/writechannelsig-

Figure 3: Superposed 50/50 Lorentz/Gaussian pulses concatenated with clock and NRZ1 (x7

+x3

+1) pattern data.

Figure 4: Cursor positions identify the pulse (or pulses) targeted for impairment. Marker 1 identifies pulse location.

nal can be modeled. The mod-

eling corresponds to a string of

transitions that represent the

output voltage of the read/

writeheadinresponsetoaniso-

latedtransitionoftherecording

media. The sign of the pulse al-

ternates from successive flux

reversals. Figure 2 illustrates

the 50/50 blended Lorentz/

Gaussian function used to cre-

ate the read/write channel sig-

nal.TheAWG710alsosupports

PR4, EPR4, E2PR4 and user-

defined pulses.

To properly simulate a chan-

nel signal, the combination

pulse is superimposed with a

pattern that indicates read or

write data timing. The data pat-

tern, shown in Figure 3, de-

fines the location of the transi-

tions. This pattern can range

from a simple 1F or 2F single

frequency pattern to a 2n-1

maximum length pseudo-

random bit stream (PRBS) or

user-defined data pattern. The

combination pulse and pattern

data are then concatenated

within the AWG to create a

read/write channel signal. Ad-

ditional parameters available

for edit include track average

amplitude (TAA), pulse width

(PW),nonlineartransitionshift

(NLTS) and signal asymmetry.

Once the read/write chan-

nel signal is created, the

graphic editor can be used to

add amplitude and timing

variations. Figure 4 and Fig-

ure 4a show the concatenated

read/write signal with one

pulse targeted for impairment.

Missing or extra bits are easy to

insert using the graphic editor.

Decreasing the vertical scale

(amplitude) between the cur-

sors eliminates an entire seg-

ment from the simulation sig-

nal. Figure 4 also shows

marker 1 and 2. Marker 2 rep-

resents the time for each data

point,whilemarker1identifies

the impaired pulse within the

entire waveform record. Mark-

ers are typically used as "read

gate" and "write gate" signals;

or to identify trigger positions

for acquisition equipment like

oscilloscopes and logic analyz-

ers. Figure 5 and Figure 5a il-

lustrate the pulse amplitude af-

ter impairment.

The designers' ability to add

timingimpairmentshelpsthem

debug and verify margin toler-

ance in downstream hardware.

Timing impairments are added

in the same way as missing (or

extra) bits. Cursors are used to

identify the region of interest

andassociatedwaveformpoints

are "scaled" to provide a speci-

fied timing shift. Scaling is dif-

ferent than zoom because each

target data point is multiplied

or divided by the scaling factor.

Scalingallowstimingshiftsthat

surpass the apparent 250ps

limit of a 4GS/s clock (T=1/f).

In the AWG710's case, the

minimum waveform-timing

shift is 400fs.

AWGs also offer disk drive

developers the ability to se-

quence a series of waveforms

togetherinreal-time.Real-time

sequencing (RTS) serves two

important functions. First, it

allows multiple waveforms to

be seamlessly connected with-

outinterruptingtheoutputsig-

nal. Secondly, RTS allows long

signals or records to be deliv-

ered to the device under test

(DUT). Both features allow

drive developers to assemble a

series of waveforms to conduct

long-term evaluation.

The key behind RTS is in the

allocation of memory within

the AWG. An AWG without

RTS must store each waveform

in its entirety. For example, to

store a sequence consisting of

three different, 500-cycle sig-

nals with 1,000 points each re-

quires 1.5Mpoints of RAM

(3x500x1,000). An AWG with

RTS requires only one wave-

form cycle to be stored because

Figure 5. Cursor positions identify impaired transition response pulse. Marker 1 identifies pulse location.

the real-time sequencer auto-

matically repeats each segment

of the waveform without inter-

ruption. Storing the same se-

quenceusingRTSrequiresonly

3kilopoints of RAM. RTS is im-

portant because AWGs have

limited waveform storage ca-

Figure 5A. AWG710 output read/write channel signal showing impaired transition response pulse.

pacity. A high-speed AWG, like

the AWG710, is equipped with

up to 32Mpoints of wave-

form memory. In a more prac-

tical example, simulating

10ms of write time at 4GS/s

(250ps interval) would require

40Mpoints of waveform

memory. In practice, a se-

quence may contain hundreds

of waveforms with conditional

jumps and branches deter-

mined by logic or software

states that, when met, call the

next line of the sequence.

(Waveform memory is separate

fromharddiskcapacityusedfor

long-term waveform storage.)

AWGs offer disk drive devel-

opers multiple ways to create

simulation waveforms, includ-

ingnominalandworstcasecon-

ditions. The first, a built-in

graphic editor, provides an in-

ternal workspace to construct

and manipulate waveforms. An

example of the graphic editor

waspreviouslyshowninFigure

4 and Figure 5. The second

methodprovidesmorecomplex

waveforms by creating the sig-

nal using an equation editor.

The equation editor compiles

complex mathematical func-

tions into their analog and digi-

tal equivalents. Figure 6 illus-

trates a portion of the equation

Figure 6. Equation editor showing a partial formula used to create spread spectrum clock signal.

used to generate a spread spec-

trum clock. Another highly

popular method uses the re-

cording capabilities of a digital

storage oscilloscope to transfer

the waveform record into the

AWG's memory. Typically, this

transfer occurs over external

control using GPIB or LAN.

Floppy disk transfer is also an

option.Oncethewaveformisin

the AWG's memory, it can be

edited and replayed as if it was

created in the graphic editor.

Downloading ASCII data from

simulation programs like

MATLAB or MathCAD is also

possible.

With its ability to output a

variety of waveforms, the AWG

is an indispensable design and

debugtool.Byutilizingthistest

and measurement instrument,

disk drive developers can bring

the AWG's power to bear upon

their products and give new

meaning to the storage and re-

trieval of the ever-increasing

volumes of information.





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