Hardware Reference
In-Depth Information
The vector y
A
contains the samples of the burst waveform and the vector d
0
consists of the samples from a ideal model signal of the burst.
Coherent Detection using Selective Harmonics :
A
coh
=
y
T
m
m
T
m
.
(2.22)
This is a special case of the maximum likelihood (ML) detection. The model
signal (d
0
) in the ML detector contains all harmonics of the nominal noise-free
signal. In this scheme, the model signal (m(k)) is created with weighted sum
of selected harmonics,
X
m(k)=r
0
+
(r
p
sin(pwkT
S
)),
(2.23)
p
where, ω is the fundamental frequency of the burst signal, and p and k are the
indices for harmonics and samples, respectively.
2.3.3 Recent Developments
For an HDD operating at 10,000 rpm with 120 servo sectors per track, the
sampling frequency is 20 kHz. This sampling frequency is good enough to
meet the specifications for the head positioning servomechanism during track-
following. However, the relentless effort in reducing track pitch demands for
ever improving precision and accuracy for the head positioning servomech-
anism. Since there is no end to the demand for higher bandwidth, the servo
engineers always remain in the pursuit of higher sampling frequency as it affects
the achievable bandwidth in a discrete-time control system.
Sampling frequency can be increased in the HDD head-positioning servo-
mechanism in two possible ways:
1. spinning the disks at higher RPM, and
2. adding more servo sectors per track.
Spinning the disk at higher RPM also reduces average latency. However, the
speed can not be increased arbitrarily. Increased RPM comes at the cost of
higher power consumption and generation of excessive heat inside drive en-
closure. Better design of the spindle motor and other mechanical features
inside the enclosure is an ongoing effort to solve these problems. There is
yet another problem with higher RPM drives that directly affects the head-
positioning servo. Many of the disturbances discussed later in this chapter are
synchronous to the spindle motor. Spinning the disk at higher RPM shifts the
spectrum of these disturbances to higher frequency. As a result, higher band-
width is required for satisfactory rejection of those disturbances. Increasing
sampling frequency with higher spindle speed is therefore not a solution that