AD9545 datasheet(78/157 Pages) AD

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AD9545

Data Sheet

Rev. A | Page 78 of 157

DPLL. The feedback path persistance provides some time for

the DPLL to compare the new reference signal to the old one.

As shown in Figure 67, the reference likely exhibits jitter. The

presence of jitter implies uncertainty in the measured time

offset between the feedback and reference signals. This

uncertainty, in turn, leads to a potential error in the

determination of the correct phase buildout value in the DPLL.

To help mitigate jitter induced errors in the assessment of the

phase buildout value, the AD9545 provides a phase skew

refinement feature. To enable the phase skew refinement

feature, program a non-zero value into the 8-bit unsigned

Profile x phase skew refinement steps bit field (where x is 0

to 7) in Register 0x0811, Register 0x0831, Register 0x0851,

The default value is zero, which disables the skew refinement

function. With the skew refinement function disabled, the

phase buildout value is a unfiltered snapshot of the reference

and feedback time offset at a single sampled edge, which

includes the contribution of any jitter present on the reference

signal.

Programming a nonzero value, K, sets the number of phase

samples the AD9545 analyzes as part of the skew refinement

process. That is, instead of taking the first phase sample (jitter

included) as the phase buildout value, the skew refinement

feature processes the first K phase samples to assess the

reference jitter and to determine the phase buildout value. As

such, the skew refinement feature extends the time required to

determine a phase buildout value following a reference

switchover, but results in the application of a more robust phase

buildout value.

The skew refinement process operates under the assumption

that the feedback and reference clocks are of the same

frequency, or at least very close. If not, the frequency offset

appears as a linear phase slew, which quickly becomes the

dominant phase contributor and masks out any jitter that may

be present on the reference signal. Therefore, a reference

switchover between references of dissimilar frequency results in

degraded performance of the skew refinement feature.

REFERENCE JITTER

FEEDBACK

SIGNAL

REFERENCE

SIGNAL

SUPPRESSED

JITTER

NOMINAL TIME OFFSET BETWEEN OLD REFERENC SIGNAL FEEDBACK AND NEW REFERENCE SIGNAL

THE OLD REFERENCE SIGNAL TEMPORARILY PERSISTS IN THE FEEDBACK SIGNAL

REFERENCE

SWITCHOVER

UNCERTAINTY

Figure 67. DPLL Reference and Feedback Signals

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