Si5345-44-42-D-RM
Rev. 1.0
35
6.2. Performance Guidelines for Outputs
Whenever a number of high frequency, fast rise time, large amplitude signals are all close to one another, the laws
of physics dictate that there will be some amount of crosstalk. The jitter of the Si5342/44/45 is so low that crosstalk
can become a significant portion of the final measured output jitter. Some of the source of the crosstalk will be the
Si5342/44/45 and some will be introduced by the PCB. It is difficult (and possibly irrelevant) to allocate the jitter
portions between these two sources because the jitter can only be measured when a Si5342/44/45 is mounted on
a PCB.
For extra fine tuning and optimization in addition to following the usual PCB layout guidelines, crosstalk can be
minimized by modifying the arrangements of different output clocks. For example, consider the following lineup of
output clocks in Table 18.
Using this example, a few guidelines are illustrated:
1. Avoid adjacent frequency values that are close. A 155.52 MHz clock should not be next to a 156.25 MHz clock.
If the jitter integration bandwidth goes up to 20 MHz then keep adjacent frequencies at least 20 MHz apart.
2. Adjacent frequency values that are integer multiples of one another are okay and these outputs should be
grouped accordingly. Noting that because 155.52 x 4 = 622.08 and 156.25 x 4 = 625, it is okay to place these
frequency values close to one another.
3. Unused outputs can be used to separate clock outputs that might otherwise interfere with one another. In this
case, see OUT3 and OUT7.
If some outputs have tight jitter requirements while others are relatively loose, rearrange the clock outputs so that
the critical outputs are the least susceptible to crosstalk. These guidelines typically only need to be followed by
those applications that wish to achieve the highest possible levels of jitter performance. Because CMOS outputs
have large pk-pk swings, are single ended, and do not present a balanced load to the VDDO supplies, CMOS
outputs generate much more crosstalk than differential outputs. For this reason, CMOS outputs should be avoided
whenever possible. When CMOS is unavoidable, even greater care must be taken with respect to the above
guidelines. For more information on these issues, see AN862 “Optimizing Si534x Jitter Performance in Next
Generation Internet Infrastructure Systems.”
Table 18. Example of Output Clock Frequency Sequencing Choice
Output
Not Recommended (Frequency MHz) Recommended (Frequency MHz)
0
155.52
155.52
1
156.25
155.52
2
155.52
622.08
3
156.25
Not used
4
200
156.25
5
100
156.25
6
622.08
625
7
625
Not used
8
Not used
200
9
Not used
100