ICS843004-02 Datasheet, PDF (8/12 Pages) Integrated Circuit Systems – CRYSTAL-TO-3.3V LVPECL FREQUENCY SYNTHESIZER

Integrated
Circuit
Systems, Inc.
PRELIMINARY
ICS843004-02
CRYSTAL-TO-3.3V LVPECL
FREQUENCY SYNTHESIZER
CRYSTAL INPUT INTERFACE
The ICS843004-02 has been characterized with 18pF paral-
lel resonant crystals. The capacitor values shown in Figure 2
below were determined using a 26.5625MHz 18pF parallel
resonant crystal and were chosen to minimize the ppm error.
X1
18pF Parallel Crystal
XTAL2
C2
33p
XTAL1
C1
27p
ICS843004-02
Figure 2. CRYSTAL INPUt INTERFACE
TERMINATION FOR 3.3V LVPECL OUTPUT
The clock layout topology shown below is a typical termi-
nation for LVPECL outputs. The two different layouts men-
tioned are recommended only as guidelines.
FOUT and nFOUT are low impedance follower outputs that
generate ECL/LVPECL compatible outputs. Therefore, ter-
minating resistors (DC current path to ground) or current
sources must be used for functionality. These outputs are
designed to drive 50Ω transmission lines. Matched imped-
ance techniques should be used to maximize operating
frequency and minimize signal distortion. Figures 3A and
3B show two different layouts which are recommended
only as guidelines. Other suitable clock layouts may exist
and it would be recommended that the board designers
simulate to guarantee compatibility across all printed cir-
cuit and clock component process variations.
Z_o = 50Ω
FOUT
FIN
Z_o = 50Ω
50Ω
RTT =
1
((V_OH + V_OL) / (V_CC – 2)) – 2
Z_o
50Ω
V_CC - 2V
RTT
FOUT
3.3V
125Ω
125Ω
Z_o = 50Ω
FIN
Z_o = 50Ω
84Ω
84Ω
FIGURE 3A. LVPECL OUTPUT TERMINATION
FIGURE 3B. LVPECL OUTPUT TERMINATION
843004AG-02
www.icst.com/products/hiperclocks.html
8
REV. A JULY 20, 2005