SI5347 Datasheet, PDF (29/59 Pages) Silicon Laboratories – DUAL/QUAD DSPLL ANY-FREQUENCY, ANY-OUTPUT JITTER ATTENUATORS

Si5347/46
5.5. 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 Figure 13. 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 12 for crystal specifications. A crystal in the range of 48 MHz to 54 MHz is recommended for best jitter
performance. Frequency offsets due to C_L mismatch can be 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.
The device can also accommodate an external reference clock (REFCLK) instead of a crystal. Selection between
the external XTAL or REFCLK is controlled by register configuration. The internal crystal loading capacitors (C_L)
are disabled in this mode. Refer to Table 3 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 performance. A P_REF divider is available to accommodate external clock frequencies higher than
54 MHz. Although the REFCLK frequency range of 25 MHz to 200 MHz is supported, frequencies in the range of
48 MHz to 54 MHz will achieve the best output jitter performance.
48-54MHz
XO
48-54MHz
XO
48-54MHz
XTAL
XA
XB
2xCL
OSC
2xCL
÷P_REF
100
XA
XB
2xCL
OSC
2xCL
÷P_REF
XA
XB
2xC_L
OSC
2xCL
÷P_REF
Si5347/46
Crystal Resonator Connection
Si5347/46
Differential XO Connection
Si5347/46
Single-Ended XO Connection
Figure 13. Crystal Resonator and External Reference Clock Connection Options
Rev. 1.1
29