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5. Output Clocks
5.1 Outputs
The Si5341 supports ten differential output drivers which can be independently configured as differential or LVCMOS. The Si5340
supports four output drivers independently configurable as differential or LVCMOS.
5.2 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 Si5341/40 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 Si5341/40, 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
an Si5341/40 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 the table below.
Table 5.1. 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
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 acceptable to place 155.52 MHz close to 622.08 MHz and
156.25 MHz close to 625 MHz.
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 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. It is
highly recommended that you consult
AN862: Optimizing Si534x Jitter Performance in Next Generation Internet Infrastructure Systems
The ClockBuilder Pro Clock Placement Wizard is an easy way to reduce crosstalk for a given frequency plan. This feature can be
accessed on the “Define Output Clocks” page of ClockBuilder Pro in the lower left hand corner of the page. It is recommended to use
this tool after each project frequency plan change.
Si5341, Si5340 Rev D Family 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. 1.3 • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice • July 26, 2021
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