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Measurement Definition
Measurement Parameter Definition
Bathtub Measurement
This test is done by optimizing the BER at the track center then moving a small step and
optimizing and measuring BER. This is done towards the ID (inner diameter) then back to
the track center and again towards the OD (outer diameter). Assuming that only the data
track is written and no adjacent track, then only signal noise from the media will limit the
read/write offset. If an adjacent track is written closer to the data track, the affordable offset
becomes limited since the head picks up the interference signal of the adjacent track. As a
result, it will narrow the otc. From the illustration below 'otcPos' and 'otcNeg' are
measured from the track center to the right and left sides of the bathtub curve. As the head
is shifted off the track center the bit error rate becomes poorer and gets better when the
head is closer to the track center.
When performing the bathtub test, first write the test track. The read head which is centered
on the test track reads the written data pattern. The read data pattern is evaluated and
compared with the original data sequence. The measured error rate at the center of the track
is relatively low and is referred to as the "on-track error rate". As head is gradually
displaced from the center of the track by a short distance, the data is read again and bit
error measurement is made. This procedure is repeated with step-by-step displacements of
the read head and measurements of the bit error rate. The displacement of the head that
results in this bit error rate is referred to as the OTC (off-track capability).
Figure 5-20
Bathtub Characteristic Curve
Chapter 5
267
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