Dynamic magnetic information storage or retrieval – Head mounting – Disk record
Reexamination Certificate
2000-05-08
2003-08-19
Cao, Allen (Department: 2612)
Dynamic magnetic information storage or retrieval
Head mounting
Disk record
Reexamination Certificate
active
06608736
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the field of read circuits for magnetic media recordings. More particularly, the present invention relates to an interconnect circuit for a magnetic media disk drive.
2. Description of the Related Art
FIG. 1
shows a high RPM disk drive
10
having a magnetic read/write head (or a recording slider) that is positioned over a selected track on a magnetic disk
11
for recording data using a servo system. The stage servo system includes a voice-coil motor (VCM)
13
for coarse positioning a read/write head suspension
12
and may include a microactuator, or micropositioner, for fine positioning the read/write head over the selected track.
FIG. 2
shows an enlarged exploded view of the read/write head end of suspension
12
in the case when a microactuator is also being used. An electrostatic rotary microactuator
14
is attached to a gimbal structure
15
on suspension
12
, and a slider
16
is attached to the microactuator. A read/write head
17
is fabricated as part of slider
16
.
A single-ended input (SE) preamplifier is preferred over a differential input (Diff.) preamplifier in a readout channel front-end for a disk drive because a single-ended preamplifier requires less chip area and costs less than a Diff. preamplifier for the same performance. For the same performance, the power dissipation of a single-ended preamplifier is lower than that of a Diff. preamplifier. Additionally, a single-ended preamplifier only requires a single power supply, whereas a Diff. preamplifier requires two supplies (a positive supply and a negative supply). For the same chip area, the signal-to-electronics-noise ratio of a single-ended preamplifier is higher. Nevertheless, a drawback associated with a single-ended preamplifier is that a single-ended preamplifier has an upper data rate that is limited by the“¼ wavelength effect” of the“return” transmission line on the head suspension. This limitation is illustrated by
FIGS. 3-5
.
FIG. 3
shows a schematic block diagram of a conventional readout channel front-end for a disk drive.
FIG. 4
shows a cross-sectional view of the conventional integrated trace suspension interconnect circuit shown in
FIG. 3
between the (G)MR sensor and the SE preamplifier. In
FIG. 3
, a magnetoresistive (MR) head is connected to a single-ended preamplifier through a conventional integrated trace suspension interconnect circuit
30
. The MR head is represented by a resistance R
mr
and signal source V
mr
connected in series. The MR head is connected to the SE preamplifier through traces, or lines, A and B. A stainless steel suspension SS is represented in
FIG. 3
along each trace A and B. To achieve high bandwidth, it is necessary for the input impedance of the SE preamplifier to be close to the characteristic impedance of the interconnect circuit
Z
0
=
L
C
.
Even when the input impedance of the SE preamplifier is close to the characteristic impedance of the interconnect circuit, the bandwidth of the readout channel is always limited by the ¼ wavelength notch of the shorted“return” transmission line, as described below.
FIG. 4
shows a cross-sectional view of the conventional integrated trace suspension interconnect shown in FIG.
3
. Traces A and B are each typically formed from copper, and disposed above stainless steel suspension SS adjacent to each other, that is, side-by-side. The widths of traces A and B are typically less than 50 &mgr;m each and are separated from each other by about 50 &mgr;m. Traces A and B are each separated from suspension SS by a dielectric material DM that is about 20 &mgr;m thick. The dielectric constant ∈
r
of the dielectric material separating traces A and B from suspension SS is typically about 2.7.
The cross-sectional dimensions of traces A and B and the ∈
r
of the dielectric are always such that the energy travels predominantly between line A and suspension SS and between line B and suspension SS. The energy transfer by the transmission path formed by the A and B lines can be neglected. Consequently,
FIG. 3
is an adequate representation of a conventional SE preamplifier readout channel. The “forward” transmission line A/SS is terminated in the SE preamplifier by the characteristic impedance Z
OA
for obtaining maximum transfer bandwidth. The return transmission line B/SS is grounded at the (grounded) input of the SE preamplifier. For a length l and a propagation velocity v
p
at a frequency f
n
=v
p
/4l, the return line B presents an open input to the MR head (R
mr
, V
mr
), thereby creating a null in the frequency transfer characteristic from the head voltage V
mr
to the SE preamplifier input voltage.
FIG. 5
is a graph showing the voltage transfer characteristic
51
as a function of frequency of the conventional interconnect circuit of
FIGS. 3 and 4
. A null
52
appears in the frequency transfer characteristic that is given by:
f
n
=V
p
/4
l
=¼&tgr;
p
=¼
l{square root over (LC)},
wherein &tgr;
p
is the propagation delay along return line B of length l, L is the distributed series inductance per meter of return line B, and C is the distributed parallel capacitance per meter of return line B. Additionally,
Z
0
B
=
L
C
and most often Z
0B
=Z
0A
, i.e., the line pairs are designed to be left/right symmetric, such as shown in FIG.
4
.
For frequencies below f
n
, the −3 dB point of the transfer characteristic shorted return line (trace B) constitutes a frequency-dependent inductor at the MR head input side. That is,
L
eq
=
Z
0
B
2
π
f
tan
(
π
f
2
f
n
)
.
This inductance in series with the MR head causes a frequency roll-off of the transfer characteristic, as shown by curve
51
in FIG.
5
. The presence of output-shored return line B, which is necessary for accommodating a single-ended amplifier, causes a−3 dB point in the extrinsic transfer (i.e., the signal transfer extrinsic to the electronics) given by:
f
-
3
d
B
=
(
R
mr
+
Z
0
A
)
/
2
π
L
eq
=
2
f
n
π
arctan
(
2
R
mr
Z
0
B
)
.
Consider a numerical example in which l=5 cm and v
p
=0.6 v
C
. Thus, f
n
=900 MHz. For R
mr
=Z
0A
=25 &OHgr;, f
−3 dB
=630 MHz. This is only the extrinsic bandwidth, that is, the signal transfer characteristic that is extrinsic to the electronics. The.overall bandwidth of the readout channel front-end (i.e., including the SE preamplifier) is narrower still.
The range for the characteristic impedance Z
0
for the interconnect circuit is determined by the range over which the width of the traces can be varied and by the range over which the thickness and the &egr;
r
of the dielectric layer between the stainless steel suspension and the traces can be varied. First, the stainless steel suspension SS is not nearly as conductive as copper traces A and B, thereby causing skin-effect losses for the high-frequency signal content of the readout signal. Stainless steel suspension SS must be interrupted at certain places along traces A and B in order to accommodate hinges and gimbals that are part of a leaf-spring head suspension. The interruptions cause reflection points that further deteriorate the high frequency signal transfer characteristic of the interconnect circuit. The suspension/hinge/gimbal arrangement is not always sufficiently wide for accommodating two side-by-side read lines having a preferably low characteristic impedance Z
0
(i.e., wide trace widths).
What is needed is a way to eliminate the ¼ wavelength effect associated with a readout channel front-end for a disk drive employing an SE preamplifier, thereby increasing the data rate that can be transferred over an interconnect circuit between an MR head and a single-ended preamplifier.
SUMMARY OF THE INVENTION
The present invention eliminates the ¼ wavelength effect a
Klaassen Klaas Berend
Van Peppen Jacobus Cornelis Leonardus
Banner & Witcoff , Ltd.
Cao Allen
Tran, Esq. Khan O.
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