SI5347_16 Datasheet, PDF (7/55 Pages) Silicon Laboratories – Dual/Quad DSPLL Any-Frequency, Any-Output Jitter Attenuators

Si5347/46 Rev D Data Sheet
Functional Description
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
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.
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
XTAL or REFCLK is controlled by register configuration. The internal crystal loading capacitors (C_L) are disabled in the REFCLK mode.
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-
ance. A P_REF divider is available to accommodate external clock frequencies higher than 54 MHz. Although the REFCLK frequency
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.
48-54MHz
XO
48-54MHz
XO
48-54MHz
XTAL
XA
XB
2xCL
OSC
2xCL
÷PREF
100
XA
XB
2xCL
OSC
2xCL
÷PREF
XA
XB
2xCL
OSC
2xCL
÷PREF
Si5347/46
Crystal Resonator Connection
Si5347/46
Differential XO Connection
Si5347/46
Single-Ended XO Connection
Figure 3.3. Crystal Resonator and External Reference Clock Connection Options
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