SI5315 Datasheet, PDF (39/54 Pages) Silicon Laboratories – Provides jitter attenuation and frequency translation between SONET/PDH and Ethemet

Si5315
7.1. Crystal/Reference Clock Selection
The Si5315 requires either a low-jitter external oscillator or a low-cost fundamental mode crystal to be connected to
its XA/XB pins. This serves both as a jitter reference for jitter attenuation and as a reference oscillator for stability
during holdover. The frequency the reference is not directly related to either the input or the output clock
frequencies. The range of the reference frequency is from 37 to 41 MHz. For recommendations on the selection of
the reference frequency and a list of approved crystals, see the application note AN591 which can be downloaded
from www.silabs.com/timing/.
In holdover, the DSPLL remains locked to this external reference. Any changes in the frequency of this reference
when the DSPLL is in holdover will be tracked by the output of the device. Note that crystals can have temperature
sensitivities. Table 18 shows how the XTAL/CLOCK pin is used to select between a crystal and an external
oscillator.
Table 18. XA/XB Reference Sources
XTAL/CLOCK
M
L
Type
37–41 MHz external clock
40 MHz crystal
Because the crystal is used as a jitter reference, rapid changes of the crystal temperature can temporarily disturb
the output phase and frequency. For example, it is recommended that the crystal not be placed close to a fan that
is being turned off and on. If a situation such as this is unavoidable, the crystal should be thermally isolated with an
insulating cover.
7.1.1. Reference Drift
During holdover, long-term and temperature related drift of the reference input result in a one-to-one drift of the
output frequency. That is, the stability of the any-frequency output is identical to the drift of the reference frequency.
This means that for the most demanding applications where the drift of a crystal is not acceptable, an external
temperature compensated or ovenized oscillator will be required. Drift is not an issue unless the part is in holdover.
Also, the initial accuracy of the reference oscillator (or crystal) is not relevant.
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