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Measurement Definition
Measurement Parameter Definition
Absolute Polarity Test
The absolute polarity test is used to detect the pinned layer reversal of the GMR head.
When pinned layer reversal occurs the polarity of the read pulse reverses indicating a
damaged head. TAA measurement of the 1-3 bit data pattern (i.e. combined pattern of
isolated pulse and tribit pulses) is the simplest way of detecting pinned layer reversal.
Heads with pinned layer reversal can be detected only by measuring the TAA of an isolated
pulse pattern and TAA of a bit pattern. The 1-3 bit data pattern proposed below is used for
measuring the pulse polarity and, consequently, for detecting pinned layer reversal. This
simple method can be used as a substitute from the traditional time asymmetry method, a
method that requires more complex measurement equipment and longer measurement
time. In time asymmetry, if the transition time from positive pulse to negative pulse is less
than the transition time from negative pulse to positive pulse, the head is said to be normal.
If it's the opposite, the head is considered damaged.
When using E5022A, the user must measure the "Absolute Polarity" of the standard head
(i.e, a good head) beforehand. The result of the standard head will serve as the basis in
determining the condition of the head to be tested. If the result is positive then a positive
value of 'Absolute Polarity' must be followed for normal pinned layer of the head. If the
result is negative then a negative value of 'Absolute Polarity' must be followed for normal
pinned layer of the tested head. Consequently, if the tested head yields a value (i.e, absolute
polarity) whose sign is opposite to the result of the standard head then the pinned layer of
the tested head is reversed. Without a standard reference, the [Absolute Polarity
Measurement] of E5022/E5023 cannot determine the pinned layer condition of the head.
This is because the head coil can be winded in either directions such that the resulting
initial pulse of the read signal changes accordingly. The direction of the resulting initial
pulse is also affected by the output polarity of the Head Amplifier.
In general, when the polarity of the output pulse is in the same direction as the media's
magnetization, the head's pinned layer is said to be normal. On the other hand, if the
polarity of the output pulse is in opposite direction to the media's magnetization, then the
head's pinned layer is reversed.
The explanation of "Absolute Polarity" written in this section (shown in Figure 5-12)
assumes the positive initial pulse of the read signal for normal pinned layer.
Figure 5-12
Normal and Reversed read signal
254
Chapter 5
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