AD9573ARUZ View Datasheet(PDF) - Analog Devices

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AD9573ARUZ

PCI-Express Clock Generator IC, PLL Core, Dividers, Two Outputs

Analog Devices 

AD9573

TERMINOLOGY

Phase Jitter

An ideal sine wave can be thought of as having a continuous

and even progression of phase with time from 0 degrees to

360 degrees for each cycle. Actual signals, however, display a

certain amount of variation from ideal phase progression over

time. This phenomenon is called phase jitter. Although many

causes can contribute to phase jitter, one major cause is random

noise, which is characterized statistically as gaussian (normal)

in distribution.

This phase jitter leads to a spreading out of the energy of the

sine wave in the frequency domain, producing a continuous

power spectrum. This power spectrum is usually reported as

a series of values whose units are dBc/Hz at a given offset in

frequency from the sine wave (carrier). The value is a ratio

(expressed in dB) of the power contained within a 1 Hz

bandwidth with respect to the power at the carrier frequency.

For each measurement, the offset from the carrier frequency is

also given.

Phase Noise

When the total power contained within some interval of offset

frequencies (for example, 12 kHz to 20 MHz) is integrated, it is

called the integrated phase noise over that frequency offset

interval, and it can be readily related to the time jitter due to the

phase noise within that offset frequency interval.

Phase noise has a detrimental effect on error rate performance

by increasing eye closure at the transmitter output and reducing

the jitter tolerance/sensitivity of the receiver.

Time Jitter

Phase noise is a frequency domain phenomenon. In the time

domain, the same effect is exhibited as time jitter. When observing

a sine wave, the time of successive zero crossings is seen to vary.

In a square wave, the time jitter is seen as a displacement of the

edges from their ideal (regular) times of occurrence. In both

cases, the variations in timing from the ideal are the time jitter.

Because these variations are random in nature, the time jitter is

specified in units of seconds root mean square (rms) or 1 sigma

of the gaussian distribution.

Additive Phase Noise

Additive phase noise is the amount of phase noise that is

attributable to the device or subsystem being measured. The

phase noise of any external oscillators or clock sources has been

subtracted. This makes it possible to predict the degree to which

the device impacts the total system phase noise when used in

conjunction with the various oscillators and clock sources, each

of which contributes its own phase noise to the total. In many

cases, the phase noise of one element dominates the system

phase noise.

Additive Time Jitter

Additive time jitter is the amount of time jitter that is

attributable to the device or subsystem being measured. The

time jitter of any external oscillators or clock sources has been

subtracted. This makes it possible to predict the degree to which

the device impacts the total system time jitter when used in

conjunction with the various oscillators and clock sources, each

of which contributes its own time jitter to the total. In many

cases, the time jitter of the external oscillators and clock sources

dominates the system time jitter.

Rev. 0 | Page 8 of 12

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