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SHNR (Sub-Harmonic Noise Ratio)
The SHNR measurement is closely tied with baseline popping measurement. This test is
used to detect baseline popping of MR/GMR heads by measuring the sub-harmonics noise
ratio. In general, some form of biasing is applied to the MR element in order to achieve
linearity of the output signal. However, the region where biasing is applied is inconsistent
with the MR response curve as shown in Figure 5-26. This inconsistency results in the
distortion of the output signal that affects the signal to noise ratio overtime. One of the
factors that causes distortion of the output signal is Barkhausen Noise. Barkhausen noise
are noise spikes that occur due to discontinuous irreversible domain wall motion of the
head.
Figure 5-26
MR Response Curve
It is also important to realize that even when optimum biasing is applied to the MR head its
output is inherently non-linear due to hysteresis. Thus an MR element will always have its
baseline+ and baseline- differ very much. As shown in Figure 5-27 the dashed line shows
the degree of deviation from the normal output pulse.
Figure 5-27
Noise Reference
Chapter 5
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Measurement Definition
Measurement Parameter Definition
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