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Measurement Definition
Measurement Parameter Definition
Bit Error Test
System of Read
The Agilent E5039A/B/C is a Bit Error Rate Test Module designed to allow users to
Channel IC
measure the bit error rate characteristics of the head and media with a customer specified
read channel IC. The parameters of the read channel IC is programmable according to test
requirements.
Bit error test is used to determine what fraction of the total bits are in error. The read
channel IC is the main electronic component, it determines how much data can be stored in
a given area on a media. The amount of data stored per track is constant, so bits are more
tightly packed on the inner track than the outer track. As data storage becomes more dense,
data bits begin to interfere with their neighbors (known as intersymbol interference or ISI).
Highly advanced signal processing techniques are required to recover the data and the
recording clock.
An illustration of the write signal flow is shown below. The scrambler is added into the
data path in order to minimize the probability of recording repetitive patterns onto the
media. After scrambling, the user data is supplied to an encoder. An encoder adds
additional spacings between transitions in order to limit distortions caused by intersymbol
interference (ISI). After encoding the user data is transformed into a channel data. The
channel data is the actual data written on the media but needs to be transformed into some
means of magnetization such as NRZ (non-return to zero). The rate at which encoded data
bits are written on a track is called channel data rate. Write precompensation is used to
reduce or eliminate the magnetic non-linearity effects. Precoding shapes the signal before
it enters the read channel IC in order to limit the occurrence of ISI. Since precoding in the
magnetic channel is limited to shifting the positions of the transitions around, it does not
allow for full equalization at the receiver. Thus the equalizer must be optimized.
Figure 5-17
Write Signal Flow
Because data are written in analog waveform the readback signal must also be read in
analog before converting it to digital for error detection. The read channel IC must have an
advanced digital filtering, data processing and data feedback techniques in order to modify
and reshape the readback signal; so that the data written on the disk is accurately detected
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Chapter 5
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