MAX3676 Maxim, MAX3676 Datasheet - Page 11

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

The limiting amplifier is biased independently from the
clock recovery circuitry. Grounding INSEL turns off the
limiting amplifier and selects the PECL DDI inputs.
Many of the MAX3676’s specifications relate to input-
signal amplitude. When working with fiber optic
receivers, the input is usually expressed in terms of
average optical power and extinction ratio. The rela-
tions given in Table 2 and Figure 6 are helpful for con-
verting optical power to input signal when designing
with the MAX3676.
In an optical receiver, the input voltage to the limiting
amplifier can be found by multiplying the relationship in
Table 2 by the photodiode responsivity and transim-
pedance amplifier gain.
Power and hysteresis are often expressed in decibels.
By definition, decibels are always 10log (power). At the
inputs to the MAX3676 limiting amplifier, the power is
V
by a factor of two, and the preamplifier is linear, then the
voltage at the input to the MAX3676 also increases by a
factor of two.
The optical power increase is:
At the MAX3676, the voltage increase is:
Table 2. Optical-Power Relations*
*Assuming a 50% average input-data duty cycle
10
IN
PARAMETER
Average
Power
Extinction
Ratio
Optical Power
of a “1”
Optical Power
of a “0”
Signal
Amplitude
2
log
/R. If a receiver’s optical input power (x) increases
2V
V
10log(2x / x) = 10log(2) = +3dB
IN
IN
Converting Average Optical Power
2
/ R
2
/ R
Without the Limiting Amplifier
SYMBOL
Reduced Power Consumption
Data-Retiming IC with Limiting Amplifier
______________________________________________________________________________________
P
P
AVG
P1
P0
r
e
IN
10
log(
P
P
r = 1 / P0
P0 2P
P
2
e
1 2
AVG
IN
2
to Signal Amplitude
622Mbps, 3.3V Clock-Recovery and
)
Optical Hysteresis
P
P
P
= P0 + P1 / 2
AVG
1
AVG
20
RELATION
P
log( )
0 2
r
e
/ r
r
e
e
2
1
P
AVG
1
r
r
e
e
6
dB
1
1
In an optical receiver, the decibel change at the
MAX3676 always equals 2x the optical decibel change.
The MAX3676’s typical voltage hysteresis is 3.0dB. This
provides an optical hysteresis of 1.5dB.
Timing jitter, edge speeds, aberrations, optical disper-
sion, and attenuation all impact the performance of
high-speed clock recovery for SDH/SONET receivers
(Figure 7). These effects decrease the time available
for error-free data recovery by reducing the received
“eye opening” of nonreturn-to-zero (NRZ) transmitted
signals.
Figure 6. Optical Power Relations
Figure 7. Eye Diagram With and Without Timing Jitter
P
AVE
P1
P0
EFFECTS OF TIMING JITTER ON EYE DIAGRAM
EYE DIAGRAM WITH NO TIMING JITTER
Jitter in Optical Receivers
TIME
MIDPOINT
MIDPOINT
TIME
TIME
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