AD9525SLASHPCBZ datasheet(18/48 Pages) AD

18 / 48 page

AD9525

Data Sheet

Rev. 0 | Page 18 of 48

TERMINOLOGY

Phase Jitter and Phase Noise

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

and even progression of phase with time from 0° to 360° 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 being 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 decibels) 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.

It is meaningful to integrate the total power contained within

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

10 MHz). This is called the integrated phase noise over that

frequency offset interval; 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 the performance of ADCs,

DACs, and RF mixers. It lowers the achievable dynamic range of

the converters and mixers, although they are affected in somewhat

different ways.

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 varies. In a square

wave, the time jitter is 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 seconds root

mean square (rms) or 1 sigma of the 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 provides the highest performance from a given converter.

Additive Phase Noise

Additive phase noise is the amount of phase noise that can be

attributed to the device or subsystem being measured. The phase

noise of any external oscillators or clock sources is subtracted,

making 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.

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 amount of time jitter that can be attri-

buted to the device or subsystem being measured. The time jitter of

any external oscillators or clock sources is 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.

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