AD9575

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TERMINOLOGY

Phase Jitter

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 phenome-

non 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 fre-

quency 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 measure-

ment, the offset from the carrier frequency is also given.

Phase Noise

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 and 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.