How Transition Density Can Affect Jitter Measurements

In some clock recovery designs, the PLL's open-loop gain is affected by transition density. This dependence of loop gain on transition density causes the PLL's JTF and OJTF responses to vary dynamically as the transition density of data pattern changes. You can choose to model either a transition density dependent or a transition density independent PLL by selecting or clearing the Transition Density Dependent check box.

When Transition Density Dependent is checked, the PLL's response depends on the input data pattern's transition density for a given set of PLL parameters you specify. If you specify a JTF BW of 1 MHz and peaking of 2 dB, then the PLL's JTF response will have a -3 dB BW of 1 MHz and a peaking of 2 dB when measuring an input data signal with a transition density of 50%. If you then measure different input signal with a different transition density using those same parameters, the PLL's JTF will not have JTF BW of 1 MHz and peaking of 2 dB.

When specifying the characteristics of a transition density dependent PLL, ensure that you specify them for a transition density of 50%. Failing to do so can result in unexpected and erroneous results. If you would like to specify these parameters at a different transition density than 50%, see Adjusting PLL Specifications for Transition Densities Other Than 50% for help in converting the PLL parameters to different transition densities.