Rev B 11/16/15 12 FEMTOCLOCK™ CRYSTAL/LVCMOS-TO-3.3V, 2.5V LVPECL
FREQUENCY SYNTHESIZER
843001I-22 DATA SHEET
Crystal Input Interface
The 843001I-22 has been characterized with 18pF parallel 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.
Figure 2. 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 3. 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. By overdriving the crystal oscillator, the device will
be functional, but note, the device performance is guaranteed by
using a quartz crystal.
Figure 3. General Diagram for LVCMOS Driver to XTAL Input Interface
XTAL_IN
XTAL_OUT
X1
18pF Parallel Crystal
C1
22pF
C2
22pF
XTAL_IN
XTAL_OUT
Ro Rs
Zo = Ro + Rs
50
Ω
0.1µf
R1
R2
V
DD
V
DD
