MAX3676 Maxim, MAX3676 Datasheet - Page 12

• Manufacturer Part Number: MAX3676
• Description: 622Mbps / 3.3V Clock-Recovery and Data-Retiming IC with Limiting Amplifier
• Manufacturer: Maxim

622Mbps, 3.3V Clock-Recovery and
Data-Retiming IC with Limiting Amplifier
Optical receivers, incorporating transimpedance
preamplifiers and limiting postamplifiers, can signifi-
cantly clean up the effects of dispersion and attenua-
tion. In addition, these amplifiers can provide fast
transitions with minimal aberrations to the subsequent
CDR blocks. However, these stages also add distor-
tions to the midpoint crossing, contributing to timing jit-
ter. Timing jitter is one of the most critical technical
issues to consider when developing optical receivers
and CDR circuits.
A better understanding of the different sources of jitter
helps in the design and application of optical receiver
modules and integrated CDR solutions. SDH/SONET
specifications are well defined regarding the amount of
jitter tolerance allowed at the inputs of optical receivers,
as well as jitter peaking requirements, but they do little
to define the different sources of jitter. The jitter that
must be tolerated at an optical receiver input involves
three significant sources, all of which are present in
varying degrees in typical receiver systems:
1) Random jitter (RJ)
2) Pattern-dependent jitter (PDJ)
3) Pulse-width distortion (PWD)
RJ is caused by random noise present during edge
transitions (Figure 8). This random noise results in ran-
dom midpoint crossings. All electrical systems gener-
ate some random noise; however, the faster the speed
Figure 8. Random Jitter on Edge Transition
12
CROSSING
MIDPOINT
MIDPOINT
ACTUAL
______________________________________________________________________________________
RANDOM
JITTER
DESIRED
MIDPOINT
CROSSING
MIDPOINT
Random Jitter
TIME
0–1
TRANSITION
WITH RANDOM
NOISE
of the transitions, the lower the effect of noise on ran-
dom jitter. The following equation is a simple worst-
case estimation of random jitter:
PDJ results from wide variations in the number of con-
secutive bits contained in NRZ data streams working
against the bandwidth requirements of the receiver
(Figure 9). The location of the lower -3dB cutoff fre-
quency is important, and must be set to pass the low
frequencies associated with long consecutive bit
streams. AC-coupling is common in optical receiver
design.
When using a preamplifier with a highpass frequency
response, select the input AC-coupling capacitor, C
to provide a low-frequency cutoff (f
than the preamplifier low-frequency cutoff. As a result,
the PDJ is dominated by the low-frequency cutoff of
the preamplifier.
When using a preamplifier without a highpass response
with the MAX3676, the following equation provides a
good starting point for choosing C
where t
bits of the same value (seconds); PDJ = maximum
Figure 9. Pattern-Dependent Jitter Due to Low-Frequency
Cutoff
LONG
CONSECUTIVE
BIT STREAM
L
= duration of the longest run of consecutive
RJ (rms) = (rms noise) / (slew rate)
C
IN
LF DROOP
1.25k In
LF PDJ
0-1-0 BIT STREAM
Pattern-Dependent Jitter
-t
1
L
IN
PDJ BW
:
C
) one decade lower
0 5
.
MIDPOINT
TIME
IN
,