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MAX3675 Datasheet(PDF) 12 Page - Maxim Integrated Products

Part No. MAX3675
Description  622Mbps, Low-Power, 3.3V Clock-Recovery and Data-Retiming IC with Limiting Amplifier
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Maker  MAXIM [Maxim Integrated Products]
Homepage  http://www.maxim-ic.com
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MAX3675 Datasheet(HTML) 12 Page - Maxim Integrated Products

 
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622Mbps, Low-Power, 3.3V Clock-Recovery
and Data-Retiming IC with Limiting Amplifier
12
______________________________________________________________________________________
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
clock and data-recovery (CDR) blocks. However, these
stages also add distortions to the midpoint crossing,
contributing to timing jitter. Timing jitter is one of the
most critical technical issues to consider when devel-
oping 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)
Random Jitter (RJ)
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
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:
RJ (rms) = (rms noise) / (slew rate)
Pattern-Dependent Jitter (PDJ)
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 limiting preamplifier with a highpass fre-
quency response, select the input AC-coupling capaci-
tor, CIN, to provide a low-frequency cutoff (fC) one
decade lower than the preamplifier low-frequency cut-
off. 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 MAX3675, the following equation provides a
good starting point for choosing CIN:
where tL = duration of the longest run of consecutive
bits of the same value (seconds); PDJ = maximum
C
-t
1.25k
In
PDJ BW
IN
L
() ()( )
.
1
05
MIDPOINT
MIDPOINT
RANDOM
JITTER
ACTUAL
MIDPOINT
CROSSING
DESIRED
MIDPOINT
CROSSING
0–1
TRANSITION
WITH RANDOM
NOISE
TIME
Figure 8. Random Jitter on Edge Transition
TIME
MIDPOINT
LONG
CONSECUTIVE
BIT STREAM
0-1-0 BIT STREAM
LF DROOP
LF PDJ
Figure 9. Pattern-Dependent Jitter Due to Low-Frequency
Cutoff


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