SI5348 Datasheet, PDF (8/54 Pages) Silicon Laboratories – Network Synchronizer for SyncE/ 1588 PTP Telecom Boundary (T-BC) and Slave (T-SC) Clocks

Si5348 Rev D Data Sheet
Functional Description
3.6 External Reference (XA/XB, REF/REFb)
The external crystal at the XA/XB pins determines jitter performance of the output clocks, and the external reference clock at the REF/
REFb pins determines the frequency accuracy, wander and stability during free-run or holdover modes. Jitter from the external clock on
the REF/REFb pins will have little effect on the output jitter performance, depending upon the selected bandwidth.
XTAL + OSC
Determines Output
Jitter Performance
48-54MHz
XTAL
XA
XB
OSC
TCXO
REF REFb
External Reference
Clock Determines
Output Frequency
Accuracy and Stability
Si5348
Figure 3.4. External Reference Connections
3.6.1 External Crystal (XA/XB)
The external crystal (XTAL) is used in combination with the internal oscillator (OSC) to produce an ultra low jitter reference clock for the
DSPLLs. The device includes internal XTAL loading capacitors which eliminate the need for external capacitors and also has the bene-
fit of reduced noise coupling from external sources. A crystal in the range of 48 to 54 MHz is recommended for best jitter performance.
Although the device includes built-in XTAL load capacitors (CL) of 8 pF, crystals with load capacitances up to 18 pF can also be accom-
modated. The Si5348 Reference Manual provides additional information on PCB layout recommendations for the crystal to ensure opti-
mum jitter performance. Although not recommended, the device can also accommodate an external clock at the XA/XB pins instead of
a crystal. Selection between the external crystal or clock is controlled by register configuration. The internal crystal loading capacitors
(CL) are disabled in this mode. Refer to Table 5.12 Crystal Specifications¹ on page 36 for reference clock requirements when using
this mode. The Si5348 Reference Manual provides additional information on PCB layout recommendations for the crystal to ensure
optimum jitter performance.
48-54 MHz
XTAL
XA
XB
2xCL
2xCL
OSC
Si5348
÷ PXAXB
Crystal Resonator
Connection
Figure 3.5. Crystal Resonator Connections
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