Silicon Laboratories SI5375 - Manual (Page 111)

Si53xx-RM
Rev. 1.2 111
Reference Drift/Wander
During Digital Hold, 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 device is in Digital
Hold. Also, the initial accuracy of the reference oscillator (or crystal) is not relevant as long as it is within one of the
frequency bands described in Table 51.
For the Si5374/75/76 reference oscillator (or if there is a need to use a reference oscillator instead of a crystal for
any of the other narrow-band devices), Silicon Labs does not recommend using MEMS-based oscillators. Instead,
Silicon Labs recommends the Si530EB121M109DG, which is a very low-jitter/wander, LVPECL, 2.5 V crystal
oscillator. In addition, the very low loop BW of the Si5324, Si5327, Si5369, and Si5374 means that they can be
particularly susceptible to reference sources that have high wander. Experience has shown that in spite of having
low jitter, some MEMs oscillators have high wander, and these devices should be avoided. Contact Silicon Labs for
details. To meet the wander requirements of G.8262 for SyncE timing cards, a TCXO or OCXO should be used as
the reference oscillator for the Si5328. For details, see Si5328 data sheet and “AN776: Using the Si5328 in a
G.8262 compliant SyncE Application”.
Reference Jitter
Jitter on the reference input has a roughly one-to-one transfer function to the output jitter over the band from
100 Hz up to about 30 kHz. If the XA/XB pins implement a crystal oscillator, the reference will have suitably low
jitter if a suitable crystal is used. If the XA/XB pins are connected to an external reference oscillator, the jitter of the
external reference oscillator may also contribute significantly to the output jitter. A typical reference input-to-output
jitter transfer function is shown in Figure 59.
Figure 59. Typical Reference Jitter Transfer Function
38.88MHz XO, 38.88MHz CKIN, 38.88MHz CKOUT
-30
-20
-10
0
10
1 10 100 1000 10000 100000 1000000
Frequency (Hz)
Po
w
e
r (
d
B
)
Jitter Xfer