ADN2817 datasheet(18/35 Pages) AD

18 / 35 page

ADN2817/ADN2818

Preliminary Technical Data

Rev. PrA | Page 18 of 35

At medium jitter frequencies, the gain and tuning range of the

VCO are not large enough to track input jitter. In this case, the

VCO control voltage becomes large and saturates, and the VCO

frequency dwells at one extreme of its tuning range or the other.

The size of the VCO tuning range, therefore, has only a small

effect on the jitter accommodation. The delay-locked loop

control voltage is now larger, and so the phase shifter takes on

the burden of tracking the input jitter. The phase shifter range,

in UI, can be seen as a broad plateau on the jitter tolerance

curve. The phase shifter has a minimum range of 2 UI at all

data rates.

The gain of the loop integrator is small for high jitter

frequencies, so that larger phase differences are needed to make

the loop control voltage big enough to tune the range of the

phase shifter. Large phase errors at high jitter frequencies

cannot be tolerated. In this region, the gain of the integrator

determines the jitter accommodation. Because the gain of the

loop integrator declines linearly with frequency, jitter accom-

modation is lower with higher jitter frequency. At the highest

frequencies, the loop gain is very small, and little tuning of the

phase shifter can be expected. In this case, jitter

accommodation is determined by the eye opening of the input

data, the static phase error, and the residual loop jitter

generation. The jitter accommodation is roughly 0.5 UI in this

region. The corner frequency between the declining slope and

the flat region is the closed loop bandwidth of the delay-locked

loop, which is roughly 3 MHz at OC-48.

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