AD9508/PCBZ Analog Devices, AD9508/PCBZ Datasheet - Page 20

• Manufacturer Part Number: AD9508/PCBZ
• Description: Clock & Timer Development Tools 1
• Manufacturer: Analog Devices
• Type: Clock Buffersr

Specifications of AD9508/PCBZ

• Rohs: yes
• Product: Evaluation Boards
• Tool Is For Evaluation Of: AD9508
• Frequency: 250 MHz, 1.65 GHz
• Operating Supply Voltage: 6 V
• Description/function: High performance 1
• Interface Type: I2C, SPI, USB
• Factory Pack Quantity: 1

TERMINOLOGY
Phase Jitter and Phase Noise
An ideal sine wave can be thought of as having a continuous
and an even progression phase with time from 0 degrees to
360 degrees 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,
characterized statistically as being Gaussian (normal) in
distribution.
Phase jitter leads to a spreading out of the energy of the sine wave
in the frequency domain, producing a continuous power spec-
trum. 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.
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 contained 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 with time jitter. When
observing a sine wave, the time of successive zero crossings
AD9508
Rev. A | Page 20 of 40
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 units of seconds root mean square (rms) or one
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 that is
attributable only to the device or subsystem being measured.
The residual phase noise system makes use of two devices
operating in perfect quadrature. The correlated noise of any
external components common to both devices (such as clock
sources) is not present. This makes it possible to predict the
degree to which the device is going to affect 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 refers to the amount of time jitter that is
attributable to the device or subsystem being measured. It is
calculated by integrating the additive phase noise over a specific
range. This makes it possible to predict the degree to which the
device is going to impact 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.
Data Sheet