4.2 Performance Guidelines for Outputs
Whenever a number of high frequency, fast rise time, large amplitude signals are located close to one another, the laws of physics
dictate that there will be some amount of crosstalk. Use of integer-related output frequencies reduces the opportunity for crosstalk.
However, the phase noise of the Si5386 is so low that crosstalk may still be detected in certain cases. Crosstalk occurs at both the
device level, as well as the PCB level. It is difficult (and possibly irrelevant) to allocate the crosstalk contributions between these two
sources since it can only be measured, while the Si5386 is mounted on a PCB.
In addition to following the PCB layout guidelines given in
9. XO and Device Circuit Layout Recommendations
, crosstalk can be
minimized by modifying the placements of the different output clock frequencies. For example, consider the following lineups of output
clocks in the table below. The “Clock Placement Wizard ...” button on the “Define Output Frequencies” page of ClockBuilder Pro
provides an easy way to change the frequency placements by either Manual or Automatic means.
Table 4.2. Comparison of Output Clock Frequency Placement Choices
Output
Not Recommended (Frequency MHz)
Recommended (Frequency MHz)
0A
—
155.52
0
—
—
1
100
100
2
155.52
125
3
156.25
156.25
4
122.88
—
5
125
—
6
245.76
983.04
7
983.04
491.52
8
491.52
245.76
9
—
122.98
9A
—
—
Using this example, a few guidelines are illustrated:
1. Avoid adjacent frequency values that are close in frequency. A 156.25 MHz clock should not be placed next to a 155.52 MHz clock
as crosstalk will be observed at 0.73 MHz offset from each frequency. If the jitter integration bandwidth or spur range goes up to 20
MHz then keep adjacent frequencies at least 20 MHz apart.
2. Frequency values that are integer multiples of one another should be grouped together. Noting that 983.04 MHz = 2 x 491.52 MHz
= 4 x 245.76 MHz = 8 x 122.88 MHz, it is okay to place each of these frequency values next to one another.
3. Unused outputs can also be placed to separate clock outputs that might otherwise show crosstalk.
4. If some outputs have tighter spur requirements while others are relatively loose, rearrange the clock outputs so that the critical
outputs are the least susceptible to crosstalk.
5. 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 when-
ever 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
Si5386 Rev. E Reference Manual • Output Clocks
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com
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Rev. 1.2 • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice • December 9, 2021
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