ACS8530 datasheet(25/152 Pages) SEMTECH

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ADVANCED COMMUNICATIONS

FINAL

DATASHEET

Revision 3.02/November 2005 © Semtech Corp.

Page 25

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ACS8530 SETS

Jitter and Wander Transfer

The ACS8530 has a programmable jitter and wander

transfer characteristic. This is set by the DPLL bandwidth.

The -3 dB jitter transfer attenuation point can be set in the

range from 0.5 mHz to 70 Hz in 18 steps. The wander and

jitter transfer characteristic is shown in Figure 8. Wander

on the local oscillator clock will not have a significant

effect on the output clock whilst in Locked mode, provided

that the DPLL bandwidth is set high enough so that the

DPLL can compensate quickly enough for any frequency

changes in the crystal.

In Free-run or Holdover mode wander on the crystal is

more significant. Variation in crystal temperature or

supply voltage both cause drifts in operating frequency,

as does ageing. These effects must be limited by careful

selection of a suitable component for the local oscillator,

as specified in the section See Local Oscillator Clock.

Phase Build-out

Phase Build-out (PBO) is the function to minimize phase

transients on the output SEC clock during input reference

switching. If the currently selected input reference clock

source is lost (due to a short interruption, out of frequency

detection, or complete loss of reference) the second, next

highest priority reference source will be selected, and a

PBO event triggered.

ITU-T G.813[11] states that the maximum allowable short-

term phase transient response, resulting from a switch

from one clock source to another, with Holdover mode

entered in between, should be a maximum of 1 µs over a

15 second interval. The maximum phase transient or

jump should be less than 120 ns at a rate of change of

less than 7.5 ppm and the Holdover performance should

be better than 0.05 ppm. The ACS8530 performance is

well within this requirement. The typical phase

disturbance on clock reference source switching will be

less than 5 ns on the ACS8530.The PBO requirement, as

specified in Telcordia GR-1244-CORE[19], Section 5.7, is

that a phase transient of greater than 3.5

µs occurring in

less than 0.1 seconds should be absorbed for Stratum 3E

level clocks. The ACS8530 can be configured to trigger a

PBO event on an input phase transient of between 1 and

3.5

µs, programmable, via Reg. 76.

The PBO operation can be set to operate automatically or

it can operate under external control. For example an

input phase jump of > 1 to 3.5

µs could be absorbed

automatically or just flagged by the device with an

interrupt raised, the external processor can then decide

when and whether to perform a PBO event to absorb the

phase disturbance. The monitoring block for detecting

Figure 8 TO DPLL Wander and Jitter Measured Transfer Characteristics (Jitter = 0.2 UI p-p)

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