3.5 External Reference (XA/XB)

An external crystal (XTAL) is used in combination with the internal oscillator (OSC) to produce an ultra-low jitter reference clock for the

DSPLLs and for providing a stable reference for the Free-run and Holdover Modes. A simplified diagram is shown in the figure below.

The device includes internal XTAL loading capacitors, which eliminates the need for external capacitors and also has the benefit of

reduced noise coupling from external sources. Refer to Table 5.12 Crystal Specifications1 on page 33 for crystal specifications. A

adjusted using the frequency adjustment feature, which allows frequency adjustments of ±200 ppm. The Si5347/46 Family Reference

Manual provides additional information on PCB layout recommendations for the crystal to ensure optimum jitter performance.

To achieve optimal jitter performance and minimize BOM cost, a crystal is recommended on the XA/XB reference input. For SyncE line

card PLL applications (e.g. loop bandwidth set to 0.1 Hz), a TCXO is required on the XA/XB reference to minimize wander and to pro-

vide a stable holdover reference. See the Si5347/46 Family Reference Manual for more information. Selection between the external

Refer to Table 5.3 Input Clock Specifications on page 24 for REFCLK requirements when using this mode. The Si5347/46 Family

Reference Manual provides additional information on PCB layout recommendations for the crystal to ensure optimum jitter perform-

range of 25 MHz to 54 MHz is supported, frequencies in the range of 48 MHz to 54 MHz will achieve the best output jitter performance.

OSC

XB

XA

48-54MHz

XTAL

2xCL

Crystal Resonator Connection

OSC

XB

XA

48-54MHz

XO

100

Differential XO Connection

OSC

XB

XA

48-54MHz

XO

Single-Ended XO Connection

Si5347/46

Si5347/46

Si5347/46

Figure 3.3. Crystal Resonator and External Reference Clock Connection Options

Si5347/46 Rev D Data Sheet

Functional Description

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