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AD9574

Data Sheet

Rev. B | Page 20 of 35

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 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 the energy of the sine wave spreading out

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 is 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 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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