ICS84320-01 Datasheet, PDF (10/21 Pages) Integrated Circuit Systems – 780MHZ, CRYSTAL-TO-3.3V DIFFERENTIAL LVPECL FREQUENCY SYNTHESIZER

ICS84320-01
780MHZ, CRYSTAL-TO-3.3V DIFFERENTIAL
LVPECL FREQUENCY SYNTHESIZER
CRYSTAL INPUT INTERFACE
A crystal can be characterized for either series or parallel mode
operation.The ICS84320-01 has a built-in crystal oscillator circuit.
This interface can accept either a series or parallel crystal without
additional components and generate frequencies with accuracy
suitable for most applications. Additional accuracy can be
achieved by adding two small capacitors C1 and C2 as shown in
Figure 3.
X1
18pF Parallel Crystal
25
C1
18p
XTAL1
24
C2
22p
XTAL2
ICS84320-01
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
inputs, 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 50Ω applications, R1
and R2 can be 100Ω. This can also be accomplished by
removing R1 and making R2 50Ω.
VDD
VDD
R1
Ro
Rs
50Ω
0.1µf
XTAL_IN
Zo = Ro + Rs
R2
XTAL_OUT
84320AY-01
FIGURE 4. GENERAL DIAGRAM FOR LVCMOS DRIVER TO XTAL INPUT INTERFACE
10
REV. C OCTOBER 22, 2007