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Si53xx-RM
120 Rev. 0.5
Fundamental Mode Crystals
For cost sensitive applications that do not have the most demanding jitter requirements, all of the narrow band
devices can use fundamental mode crystals that are in the lowest frequency band ranging from 37 to 41 MHz
(corresponding to RATE = LL). Unlike the other narrowband members of the family, the Si5327 is only capable of
using fundamental mode crystals that are in this range. For a more detailed discussion of the trade-offs associated
with this approach and a list of approved low frequency crystals, please see the application note AN591, which can
be downloaded from www.silabs.com/timing.
Reference Drift
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 60.
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)
Power (dB)
Jitter Xfer