HMC7044 Datasheet, PDF (22/72 Pages) Analog Devices – JESD204B clock generation

HMC7044
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 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 the energy of the sine wave in the
frequency domain spreading out, 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.
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 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 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
Data Sheet
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 is
attributable to the device or subsystem being measured.
The phase noise of any external oscillators or clock sources is
subtracted, which 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. 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 is attributable to
the device or subsystem being measured. The time jitter of any
external oscillators or clock sources is subtracted, which 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.
Rev. B | Page 22 of 72