What causes a CBPro warning about coupling between output clocks?
CBPro gives a coupling warning for outputs when
An example of such a warning in CBPro is “OUT1 [131 MHz] and OUT2 [125 MHz] may have coupling”
The phase noise plot of Fout1 and Fout2 will show a spur at Fspur. If Fspur is above 20 MHz it will not cause a coupling warning because most jitter specs do not integrate the phase noise above 20 MHz.
Outputs that are adjacent but “around the corner” from each other can cause a coupling warning as defined above but in practice these outputs have very little coupling. Outputs around the corner have about the same or less coupling than outputs that are spaced apart by 1 driver. The driver in between can be outputting a clock if that clock frequency is a benign clock frequency. A benign clock frequency does not generate a coupling issue to an adjacent clock.
Outputs that are spaced apart by 1 driver can still have measurable coupling but the magnitude is much less and can be ignored unless the jitter requirements are below ~180 fs rms.
The above does not apply to LVCMOS outputs because CMOS outputs, by their very nature, cause much more coupling than differential outputs like LVDS and LVPECL.
See AN862: Optimizing Jitter Performance in Next-Generation Internet Infrastructure Systems.
Fout1 = 131 MHz, Fout2 = 125 MHz
Then Abs(Fout1-Fout2) = 6 MHz; therefore a spur at 6 MHz will be on the phase noise plot of both Fout1 and fout2 and a coupling warning will be issued by CBPro.
Fout1 = 156.25 MHz, Fout2 = 125 MHz; therefore a spur at 21.25 MHz will exist but this will not cause a warning in CBPro.
For Fout1 = 131 MHz and Fout2 = 375 MHz
Then Abs(3*Fout1 – Fout2) = 18 MHz
Hence a spur at 18 MHz can be expected on the phase noise plot of both the 131 and 375 MHz outputs. CBPro will issue the warning “OUT1 [131 MHz] and OUT2 [375 MHz] may have coupling”.
Generally, the larger A + B, the smaller the spur amplitude and the lower is the jitter.
On a device with 10-12 outputs the coupling from OUT3 to OUT4, and from OUT6 to OUT7, is much less than other adjacent outputs because these outputs are around the corner from each other.
If the very lowest coupling is required, the following rules should reduce coupling effects to be less than ~50 fs rms.
The Clock Placement Wizard in CBPro only takes into account the following rules to minimize the coupling between output clocks