Hardware designers seeking assistance with the selection of reference crystals for the Si5315 and Si5317 jitter attenuating clocks from Silicon Labs will find this article helpful.  The Si5315 accepts fundamental mode crystals in the frequency range from 37 to 41 MHz. The Si5317 accepts 37 to 41 MHz fundamental mode crystals and third overtone 114.285 MHz crystals. A number of different crystals have been tested in the lab and approved for use with these devices. This document lists these crystals and assists in their selection.

For cost-sensitive applications that do not have the most demanding jitter requirements, all of the narrowband members of the Silicon Labs Any-Frequency family of devices can use fundamental mode crystals that are in the frequency range of 37–41 MHz (corresponding to RATE = LL). The Si5315 and Si5327 are the only narrowband members of the family that can only use fundamental mode crystals in the 37–41 MHz band. The remaining Silicon Labs Any-Frequency devices that can use either a fundamental or 3rd overtone crystals are: Si5316, Si5317, Si5319, Si5323, Si5324, Si5326, Si5366, Si5368, Si5369.

When choosing a crystal the usual factors such as price and availability should be considered.  When choosing a crystal for the Si5315 and Si5317, be sure the reference frequency does not result in the output frequency being an integer or a near integer multiple of the crystal frequency. For example, if the output frequency is 200 MHz, using a 40 MHz crystal will result in higher jitter due to increased mid-band spurs. Instead, choosing a crystal frequency of 38.4 or 39 MHz would have better jitter performance. When comparing the two phase noise plots and their associated RMS jitter values, you see that 200 MHz/40 MHz is an integer value and its performance is worse.

With regard to PCB layout, the two loading capacitors that normally go from each of the crystal pins to ground are not needed for any of the devices in the Silicon Labs Any-Frequency family. This means that the PCB layout should not include footprints for these resistors. In the event that they exist on a PCB, these capacitors should not be populated. This is true for both fundamental and 3rd overtone crystals and for the Si5315 and Si5317.

It is recommended that the ground plane underneath the Any-Frequency device, the crystal, and its etch runs be solid and uninterrupted.  For more details on this topic, please visit the Silicon Labs website at www.silabs.com/Timing and search for application note AN591: Crystal Selection for the Si5315, Si5317 and Other Si53xx Any-Frequency Jitter Attenuating Clocks.