84021 Data Sheet

260MHz, CRYSTAL-TO-LVCMOS/LVTTL FREQUENCY SYNTHESIZER

84021 Rev E 9/23/15

13

©2015 Integrated Device Technology, Inc

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 2A. 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.

Figure 2A. General Diagram for LVCMOS Driver to XTAL Input Interface

Figure 2B. General Diagram for LVPECL Driver to XTAL Input Interface

VCC

XTAL_OUT

XTAL_IN

R1

100

R2

100

Zo = 50 ohms

Rs

Ro

Zo = Ro + Rs

C1

.1uf

LVCMOS Driver

XTAL_OUT

XTAL_IN

Zo = 50 ohms

C2

.1uf

LVPECL Driver

Zo = 50 ohms

R1

50

R2

50

R3

50