Digital Signal Processing Reference
In-Depth Information
a
output
data
d
[
k
]
Level/
swing
control
RX
amplifier
input data
d
[
k
]
Pre-emphasis
Driver
Channel
CDR
FR-4 trace, stubs,
vias, connectors
1/
T
reference
clock
TX-PLL
b
0Km
75Km
125Km
Transmitter
input data
d
[
k
]
Laser source
FEC Encoder
Modulator
Fiber
Fiber
Optical channel
optical amplifier
output
data
Optical
Filter
Photo
Detector
Electrical
Filter
FEC
Decoder
ˆ
d
[
k
]
CDR
Receiver
Fig. 1
High-speed link configurations for: (
a
) a back-plane serial link, and (
b
) an optical fiber link
A linear equalizer (LE) or decision feedback equalizer (DFE) may further reduce
ISI. The equalized output is sliced to make a decision. In an optical transmitter, an
information sequence
d
is used to modulate the intensity of a laser source. For
long-haul links (typically greater than 200 km), the optical signal will propagate
along multiple spans of fiber with repeated optical amplification to overcome
transmitted and received symbol patterns as they would appear on an oscilloscope at
various stages through the optical fiber. Note how an
open
transmit eye from which
correct decisions can easily be made using a simple slicing operation, begins to
close
due to dispersion and noise, as fiber length increases. Eye diagrams for back-plane
links similarly indicate significant eye closure at the receiver input.
As a result, high-speed links have recently began to employ sophisticated signal
processing and communication techniques such as equalization, and FEC. In back-
was introduced within the last decade. For optical links, electronic dispersion
[
k
]
