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IDT / ICS 3.3V, 2.5V LVPECL CLOCK GENERATOR
8
ICS843031AG-01 REV. A NOVEMBER 11, 2008
ICS843031-01
FEMTOCLOCKS CRYSTAL-TO-3.3V, 2.5V LVPECL CLOCK GENERATOR
LVCMOS TO XTAL INTERFACE
The XTAL_IN input can accept a single-ended LVCMOS signal
through an AC coupling capacitor. A general interface diagram is
shown in
Figure 3. The XTAL_OUT pin can be left floating. The
input edge rate can be as slow as 10ns. For LVCMOS inputs, it is
recommended that the amplitude be reduced from full swing to
half swing in order to prevent signal interference with the power
rail and to reduce noise. This configuration requires that the output
FIGURE 3. GENERAL DIAGRAM FOR LVCMOS DRIVER TO XTAL INPUT INTERFACE
impedance of the driver (Ro) plus the series resistance (Rs) equals
the transmission line impedance. In addition, matched termination
at the crystal input will attenuate the signal in half. This can be
done in one of two ways. First, R1 and R2 in parallel should equal
the transmission line impedance. For most 50
Ω applications, R1
and R2 can be 100
Ω. This can also be accomplished by removing
R1 and making R2 50
Ω.
R2
Zo = 50
VDD
Ro
Zo = Ro + Rs
R1
VDD
XTAL_IN
XTAL_OUT
.1uf
Rs
TERMINATION FOR 3.3V LVPECL OUTPUTS
The clock layout topology shown below is a typical termination
for LVPECL outputs. The two different layouts mentioned are
recommended only as guidelines.
FOUT and nFOUT are low impedance follower outputs that gen-
erate ECL/LVPECL compatible outputs. Therefore, terminating
resistors (DC current path to ground) or current sources must be
used for functionality. These outputs are designed to drive 50
Ω
FIGURE 4B. LVPECL OUTPUT TERMINATION
FIGURE 4A. LVPECL OUTPUT TERMINATION
transmission lines. Matched impedance techniques should be
used to maximize operating frequency and minimize signal dis-
tortion.
Figures 4A and 4B show two different layouts which are
recommended only as guidelines. Other suitable clock layouts
may exist and it would be recommended that the board design-
ers simulate to guarantee compatibility across all printed circuit
and clock component process variations.
V
CC - 2V
50
Ω
50
Ω
RTT
Z
o = 50Ω
Z
o = 50Ω
FOUT
FIN
RTT =
Z
o
1
((V
OH + VOL) / (VCC – 2)) – 2
3.3V
125
Ω
125
Ω
84
Ω
84
Ω
Z
o = 50Ω
Z
o = 50Ω
FOUT
FIN
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