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Data Sheet

AD9531

TERMINOLOGY

Phase Jitter and Phase Noise

An ideal sine wave has a continuous and even progression of

phase with time from 0° to 360° for each cycle. A sine wave as a

real-world signal, however, exhibits a certain amount of variation in

its phase progression over time relative to the ideal sine wave. This

variation is phase jitter. Although many causes can contribute to

phase jitter, one major cause is random noise, characterized

statistically by a normal (Gaussian) distribution.

In the frequency domain, an ideal sine wave exhibits a discrete

spectral line. Phase jitter, however, blurs the ideal spectral line

because it distributes some of the energy of the sine wave

throughout the frequency spectrum, resulting in a continuous,

rather than discrete, power spectrum. This power spectrum

usually appears in the literature as a table of values given in

units of dBc/Hz at various offset frequencies from the frequency

of the sine wave (carrier). The units, dBc/Hz, represent a ratio

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

bandwidth at some specified offset frequency from the carrier and

relative to the power in the carrier. In fact, the c in dBc is an abbrev-

iation for carrier and signifies decibels relative to the carrier.

It is important to integrate the total power contained within

some interval of offset frequencies (for example, 10 kHz to

10 MHz). This is integrated phase noise and relates phase noise

(a frequency domain parameter) over the given bandwidth to

jitter (a time domain parameter).

Phase noise has a detrimental effect on the performance of analog-

to-digital converters (ADCs), digital-to-analog converters (DACs),

and radio frequency (RF) mixers. Phase noise lowers the achievable

dynamic range of the converters and mixers, although it affects

these various devices in different ways.

Time Jitter

Phase noise is a frequency domain phenomenon. In the time

domain, the same effect appears as time jitter, which is a variation

of the instants of zero crossing of a sine wave (or a variation in the

occurrence of the edges of a square wave relative to their ideal

position in time). In both cases, timing jitter is variations relative to

the ideal timing instants. Because time jitter variations are random

in nature, they carry units of seconds root mean square (rms),

which corresponds to the standard deviation (σ) of a normal

(Gaussian) distribution.

Time jitter that occurs on a sampling clock for a DAC or an ADC

decreases the signal-to-noise ratio (SNR) and dynamic range of the

converter. A sampling clock with the lowest possible jitter allows

the highest possible performance from a given converter.

Additive Phase Noise

Additive phase noise is the amount of phase noise attributable

to the device or subsystem in question. That is, additive phase

noise is phase noise exhibited only by the device in question and

effectively disregards the phase noise contributions of other sources

(like external oscillators or clock sources). This disregarded phase

noise makes it possible to predict the impact of the device in

question on the total system phase noise when used in conjunction

with the various oscillators and clock sources (each contributing

its own phase noise to the total). In many cases, the phase noise

of one element dominates the system phase noise. When there

are multiple contributors to phase noise, the total is the square

root of the sum of squares of the individual contributors.

Additive Time Jitter

Additive time jitter is the same as additive phase noise, except it is

applicable to the time domain rather than the frequency domain.

Rev. 0 | Page 29 of 88

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