Datasheet

AD9510 Data Sheet
Rev. B | Page 24 of 56
TERMINOLOGY
Phase Jitter and Phase Noise
An ideal sine wave has a continuous and even progression of
phase with time from 0 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 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 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.
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 the performance of
analog-to-digital converters (ADCs), digital-to-analog
converters (DACs), and signal input (RF) mixers. It lowers the
achievable dynamic range of the converters and mixers,
although they are affected in 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 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.
Since 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.
Time jitter that occurs on a sampling clock for a DAC or an
ADC decreases the 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 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
contribute their 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 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 contribute their
own time jitter to the total. In many cases, the time jitter of the
external oscillators and clock sources dominates the system
time jitter.