FEMTOCLOCK
®
CRYSTAL-TO-3.3V, 2.5V LVPECL CLOCK GENERATOR 10 REVISION B 10/29/15
843251I-15 DATA SHEET
Overdriving the XTAL Interface
The XTAL_IN input can be overdriven by an LVCMOS driver or by
one side of a differential driver through an AC coupling capacitor.
The XTAL_OUT pin can be left floating. The amplitude of the input
signal should be between 500mV and 1.8V and the slew rate
should not be less than 0.2V/ns. For 3.3V LVCMOS inputs, the
amplitude must be reduced from full swing to at least half the swing
in order to prevent signal interference with the power rail and to
reduce internal noise. Figure 3A shows an example of the interface
diagram for a high speed 3.3V LVCMOS driver. This configuration
requires that the sum of the output impedance of the driver (Ro)
and 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
changing R2 to 50. The values of the resistors can be increased
to reduce the loading for a slower and weaker LVCMOS driver.
Figure 3B shows an example of the interface diagram for an
LVPECL driver. This is a standard LVPECL termination with one
side of the driver feeding the XTAL_IN input. It is recommended
that all components in the schematics be placed in the layout.
Though some components might not be used, they can be utilized
for debugging purposes. The datasheet specifications are
characterized and guaranteed by using a quartz crystal as the
input.
Figure 3A. General Diagram for LVCMOS Driver to XTAL Input Interface
Figure 3B. General Diagram for LVPECL Driver to XTAL Input Interface
V
D
XTA L_OUT
XTA L_IN
R1
100
R2
100
Zo = 50 ohm
R
Ro
Zo = Ro + Rs
.1u
LVCMOS Driver
XTA L_ OU T
XTA L_ I N
Zo = 50 ohms
C2
.1uf
LVPECL Driver
Zo = 50 ohms
R1
50
R2
50
R3
50
