ADN2815 datasheet(13/24 Pages) AD

13 / 24 page

ADN2815

Rev. 0 | Page 13 of 24

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 at 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 therefore 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; therefore, larger phase differences are needed to

make the loop control voltage large 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 1.5 MHz at 1.25 Gb/s.

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