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To measure changes in the flying height of a magnetic head relative to the surface of a rotating magnetic disk, a simple constant Q circuit is employed.
English (United States)
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Capacitance Probe And Constant Q Circuit For Measuring Head Flying
To measure changes in the flying height of a magnetic head relative to the
surface of a rotating magnetic disk, a simple constant Q circuit is employed.
With reference to the block diagram of Fig. 1 and the implementation
illustrated in the schematic circuit diagram of Fig. 2, a bias source 1, comprising
a resistor 2, provides a voltage through a high resistance 3 to a flying magnetic
head, which comprises half of a capacitor, the other half being the metal disk.
(The ferrite head is an electrical conductor.)
The change in capacitance C resulting from changes in the flying height
causes a change in the voltage V across the plates of the capacitor, according to
the relation V equals Q/C. Some error occurs because of the presence of the
resistor 3, which will bleed off some of the charge from the capacitor; however,
as R approaches infinity, this error becomes negligible. Since the resistance is
very high, the current I will be very small, in accordance with Ohm's law, I =
(As R approaches Infinity, the low frequency cutoff approaches DC). The signal
is passed through a high impedance isolation stage 5, acting as a follower, to a
low impedance amplifier having a set gain. The output signal e provides an
indication of relative changes in spacing between the head and the disk, i.e.,
changes in flying height.
A varactor cannot be used to calibrate this circuit because of the AC