SI5375 Datasheet, PDF (17/54 Pages) Silicon Laboratories – 4-PLL ANY-FREQUENCY PRECISION CLOCK MULTIPLIER/JITTER ATTENUATOR

Si5375
5.5. Reference Oscillator Selection
Care should be taken during the selection of the external oscillator that is connected to the OSC_P and OSC_N
pins. There is no jitter attenuation from the OSC reference inputs to the output; so, to achieve low output jitter, a
low-jitter reference OSC must be used. Also, the output drift during holdover will be the same as the drift of the
OSC reference. For example, a Stratum 3 application will require an OSC reference source that has Stratum 3
stability (though Stratum 3 accuracy is not required).
The OSC frequency can be any value from 109 to 125.5 MHz. See the RATE_REG (reg 2) description. Alternately,
for applications with less demanding jitter requirements, the OSC frequency can be in the range from 37 to
41 MHz. For applications that use Free Run mode, the freedom to use any OSC frequency within these bands can
be used to select an OSC frequency that has an integer relationship to the desired output frequency, which will
make it easier to find a high-performance frequency plan.
If Free Run is not being used, an OSC frequency that is not integer-related to the output frequency is preferred. A
recommended choice for an external oscillator is the Silicon Labs 530HB121M109DG, which is a 2.5 V, CML
device with a temperature stability of 20 ppm. It was used to take the typical phase noise plot on page 14. For more
details and a more complete discussion of these topics, see the Si53xx Reference Manual.
5.6. Alarms
To assist in the programming of the IRQ_n pins, refer to the below diagram of the Si5375 alarm structure.
LOSx_INT
in _Sticky out
Write 0
to clear
LOSX_FLG
LOSX_MSK
INT_POL
LOS1_INT
in _Sticky out
Write 0
to clear
LOS1_FLG
LOS1_MSK
IRQ_PIN
E
IRQ_n
LOL_INT
in _Sticky out
Write 0
to clear
LOL_FLG
LOL_MSK
Figure 6. Si5375 Alarm Structure
Rev. 1.0
17