XRT91L81IB View Datasheet(PDF) - Exar Corporation
Part Name
Description
MFG CO.
'XRT91L81IB' PDF : 40 Pages View PDF
PRELIMINARY
XRT91L81
2.488/2.666GBPS OC-48/STM-16 SONET/SDH TRANSCEIVER
REV. P1.0.3
FIGURE 11. LOOP TIMING MODE USING AN EXTERNAL CLEANUP VCXO
VCXO
Loop Filter
VCXO_SEL
REFCLK0P
REFCLK0N
VCXO_INP
VCXO_INN
LPTIME_NO_JA
LOCKDET_CMU
REFFREQSEL
Charge
Pump
Phase
Detect
2.488/2.666GHz
CMU
a
XRT91L81
Div by 4
Div by 4
Clk
CDR
Data
CPOUT
LPTIME_JA
RXIP
RXIN
a
3.8 External Loop Filter
As shown in Figure 11, there is an internal charge pump used to drive an external loop filter and external
VCXO. The charge pump current is fixed at 250uA. Figure 12 is a simplified block diagram of the external loop
filter and recommended values.
FIGURE 12. SIMPLIFIED DIAGRAM OF THE EXTERNAL LOOP FILTER
21
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