4.2.1 Optimizing Output Phase Noise
To obtain the best phase noise performance for RF and other demanding applications, it is important to configure the Si5386 device
optimally. Using integer dividers for P, M, and N will provide the highest level of performance. Integer mode dividers are optimized to
support LTE, JESD204b and other integer-ratio derived frequencies.
Tips for optimizing phase noise performance, with suggestions listed most important to least important:
1. Use an Integer-N output divider.
2. Follow the crosstalk guidelines given above in all cases. Where possible, leave an unused output between all-integer outputs
and outputs using fractional N output dividers. ClockBuilder Pro provides a means for manually choosing DSPLL N dividers for
each output on the "Define Output Frequencies" page. Also, the "Clock Placement Wizard" allows for manual or automatic output
placement to reduce the likelihood of crosstalk.
3. Use Integer-P input dividers.
4. Use Integer-M feedback divider. In many cases fractional M performance is indistinguishable from integer performance. However, it
is possible that there may be some cases where this can measurably increase phase noise.
4.3 Output Signal Format
The differential output amplitude is fully programmable and covers a wide variety of signal formats including LVDS, LVPECL, HCSL.
For CML applications and other differing amplitude requirements, see
13. Appendix—Custom Differential Amplitude Controls
. To save
power, the HCSL differential format uses a low power format which has an output impedance that is much higher than 100 Ω. In
addition to supporting differential signals, any of the outputs can be configured as LVCMOS (3.3, 2.5, or 1.8 V) drivers providing up
to 24 single-ended outputs, or any combination of differential and single-ended outputs. Note also that CMOS output can create much
more crosstalk than differential outputs so extra care must be taken in their pin replacement so that other clocks that need best spur
performance are not on nearby pins and most preferably separated by a corner of the Si5386. See
AN862: Optimizing Si534x Jitter
Performance in Next Generation Internet Infrastructure Systems
. Note that output frequencies > 1474.56 MHz are restricted to a High
Speed Differential format and that only 2.5 V and 3.3 V options are allowed.
Table 4.3. Output Signal Format Registers
Register Name
Hex Address
[Bit Field]
Function
OUT0A_FORMAT
0x0104[2:0]
Selects the output signal format as differen-
tial or LVCMOS mode.
0: Reserved
1: Normal Differential
2: Low-Power Differential
3: Reserved
4: LVCMOS
5: LVCMOS (OUTx pin only)
6: LVCMOS (OUTxb pin only)
7: Reserved
OUT0_FORMAT
0x0109[2:0]
OUT1_ FORMAT
0x010E[2:0]
OUT2_ FORMAT
0x0113[2:0]
OUT3_ FORMAT
0x0118[2:0]
OUT4_ FORMAT
0x011D[2:0]
OUT5_ FORMAT
0x0122[2:0]
OUT6_ FORMAT
0x0127[2:0]
OUT7_ FORMAT
0x012C[2:0]
OUT8_ FORMAT
0x0131[2:0]
OUT9_ FORMAT
0x0136[2:0]
OUT9A_FORMAT
0x013B[2:0]
Si5386 Rev. E Reference Manual • Output Clocks
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