
843081AGI-01
www.idt.com
REV. C JULY 25, 2010
8
ICS843081I-01
FEMTOCLOCKSCRYSTAL-TO-
3.3V, 2.5V LVPECL CLOCK MULTIPLIER
TERMINATION FOR 3.3V LVPECL OUTPUT
The clock layout topology shown below is a typical termina-
tion for LVPECL outputs. The two different layouts mentioned
are recommended only as guidelines.
FOUT and nFOUT are low impedance follower outputs that
generate ECL/LVPECL compatible outputs. Therefore, termi-
nating resistors (DC current path to ground) or current sources
must be used for functionality. These outputs are designed
FIGURE 3B. LVPECL OUTPUT TERMINATION
FIGURE 3A. LVPECL OUTPUT TERMINATION
to drive 50
Ω transmission lines. Matched impedance tech-
niques should be used to maximize operating frequency and
minimize signal distortion.
Figures 3A and 3B show two dif-
ferent layouts which are recommended only as guidelines.
Other suitable clock layouts may exist and it would be rec-
ommended that the board designers 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