
ICS84314-02
700MHZ, CRYSTAL-TO-3.3V/2.5V LVPECL FREQUENCY SYNTHESIZER W/FANOUT BUFFER
IDT / ICS 3.3V/2.5V LVPECL FREQUENCY SYNTHESIZER
12
ICS84314AY-02 REV. A MARCH 24, 2009
Crystal Input Interface
The ICS84314-02 has been characterized with 18pF parallel
resonant crystals. The capacitor values, C1 and C2, shown in
Figure 3 below were determined using an 18pF parallel resonant
crystal and were chosen to minimize the ppm error. The optimum
C1 and C2 values can be slightly adjusted for different board
layouts.
Figure 3. Crystal Input Interface
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 4. The XTAL_OUT pin can be left floating. The
input edge rate can be as slow as 10ns. For LVCMOS signals, 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
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
applications, R1
and R2 can be 100
. This can also be accomplished by removing
R1 and making R2 50
.
Figure 4. General Diagram for LVCMOS Driver to XTAL Input Interface
XTAL_IN
XTAL_OUT
X1
18pF Parallel Crystal
C1
22p
C2
22p
XTAL_IN
XTAL_OUT
Ro
Rs
Zo = Ro + Rs
50
0.1f
R1
R2
VCC