622Mbps, 3.3V Clock-Recovery and

Data-Retiming IC with Limiting Amplifier

______________________________________________________________________________________

11

Reduced Power Consumption

Without the Limiting Amplifier

The limiting amplifier is biased independently from the

clock recovery circuitry. Grounding INSEL turns off the

limiting amplifier and selects the PECL DDI inputs.

Converting Average Optical Power

to Signal Amplitude

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.

Optical Hysteresis

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

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:

10log(2x/x) = 10log(2) = +3dB

At the MAX3676, the voltage increase is:

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.

Jitter in Optical Receivers

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.

10

10

2

20

2

2

2

2

log

log(

)

log(

2V

/ R

V/ R

=

IN

IN

()

=

))

=+6dB

TIME

P0

P1

Figure 6. Optical Power Relations

Table 2. Optical-Power Relations*

*Assuming a 50% average input-data duty cycle.

SYMBOL

RELATION

Average

Power

Extinction

Ratio

PARAMETER

Optical Power

of a “1”

P1

Optical Power

of a “0”

P0

Signal

Amplitude

P

= P0 + P1 / 2

AVG

()

r =

1 / P0

e

P

PP

r

r

AVG

e

e

12

1

=

+

P0

2P

/ r

1

AVG

e

=+

()

PP

P

P

r

r

IN

AVG

e

e

=−

=

−

()

+

10

2

1

1

EYE DIAGRAM WITH NO TIMING JITTER

EFFECTS OF TIMING JITTER ON EYE DIAGRAM

TIME

MIDPOINT

TIME

MIDPOINT

Figure 7. Eye Diagram With and Without Timing Jitter