ICS844S42I Datasheet, PDF (16/27 Pages) Integrated Device Technology – On-chip crystal oscillator for reference frequency generation

ICS844S42I Data Sheet
DUAL OUTPUT RF FREQUENCY SYNTHESIZER
Crystal Input Interface
The ICS844S42I has been characterized with 18pF parallel resonant
crystals. The capacitor values, C1 and C2, shown in Figure 2 below
were determined using an 18pF parallel resonant crystal and were
chosen to minimize the ppm error. These same capacitor values will
tune any 18pF parallel resonant crystal over the frequency range and
other parameters specified in this data sheet. The optimum C1 and
C2 values can be slightly adjusted for different board layouts.
X1
18pF Parallel Crystal
C1
22pF
XTAL_IN
C2
22pF
XTAL_OUT
Figure 2. Crystal Input Interface
Overdriving the 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 3A. The XTAL_OUT pin can be left floating. The
maximum amplitude of the input signal should not exceed 2V and the
input edge rate can be as slow as 10ns. 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.
3.3V
Ro ~ 7 Ohm
RS 43
Driv er_LVCMOS
3.3V
R1
100
Zo = 50 Ohm
R2
100
C1
0.1uF
XTAL_IN
XTAL_OUT
Cry stal Input Interf ace
Figure 3A. General Diagram for LVCMOS Driver to XTAL Input Interface
VCC=3.3V
LVPECL
Zo = 50 Ohm
Zo = 50 Ohm
C1
XTAL_IN
R1
0.1uF
50
XTAL_OUT
Cry stal Input Interf ace
R2
50
R3
50
Figure 3B. General Diagram for LVPECL Driver to XTAL Input Interface
ICS844S42BKI REVISION A FEBRUARY 21, 2012
16
©2012 Integrated Device Technology, Inc.