ICS8430002 datasheet(16/27 Pages) IDT

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ICS8430002AY REVISION C NOVEMBER 12, 2009

ICS8430002 Data Sheet

HIGH-PERFORMANCE FRACTIONAL-N FREQUENCY SYNTHESIZER

Crystal Input Interface

The ICS8430002 has been characterized with 18pF parallel resonant

crystals. The capacitor values, C1 and C2, shown in Figure 4 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.

Figure 4. 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 5. 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 50

Ω applications, R1 and R2

can be 100

Ω. This can also be accomplished by removing R1 and

making R2 50

Ω. By overdriving the crystal oscillator, the device will

be functional, but note, the device performance is guaranteed by

using a quartz crystal.

Figure 5. General Diagram for LVCMOS Driver to XTAL Input Interface

XTAL_IN

XTAL_OUT

18pF Parallel Crystal

27p

XTAL_IN

XTAL_OUT

Zo = Ro + Rs

Ω

0.1µf

VCC

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