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Table 15-4
Spur
Description
Sources
Internal
Potential spur sources within the
measurement system include the phase
noise system, the unit-under-test, and the
reference source. Typical system spurs are
–120 dBc, and they occur at the power line
and system vibration frequencies in the
range of from 25 Hz to 1 kHz, and above 10
MHz.
External
Spur sources external to the system may be
either mechanical or electrical. When using
the Phase Lock Loop measurement
technique, the system's susceptibility to
external spur sources increases with
increases in the Peak Tuning Range set by
the VCO source.
Electrical
Electrically generated spurs can be caused
by electrical oscillation, either internal or
external to the measurement system. The
list of potential spur sources is long and
varied. Many times the spur will not be at the
fundamental frequency of the source, but
may be a harmonic of the source signal.
Some typical causes of electrical spurs are
power lines, radio broadcasting stations,
computers and computer peripherals (any
device that generates high frequency square
waves), and sum and difference products of
oscillators that are not isolated from one
another in an instrument such as a signal
generator.
Mechanical
Mechanically generated spurs are usually at
frequencies below 1 kHz. The source of a
mechanically generated spur is typically
external to the measurement system.
Agilent Technologies E5500 Phase Noise Measurement System 15-17
Evaluating Your Measurement Results
Recommended Action
If you do not have a plot of the system's
noise and spur characteristics, perform the
system Noise Floor Test. If you suspect that
the unit-under-test or the reference source
may be the spur source, check each source
using a spectrum analyzer or measuring
receiver (such as an Agilent/HP 8902A).
Also, if additional sources are available, try
exchanging each of the sources and
repeating the measurement.
Shorten coax cables as much as possible
(particularly the Tune Voltage Output cable).
Make sure all cable connections are tight. It
may be possible to identify an external spur
source using a spectrum analyzer with a
pick-up coil or an antenna connected to it.
The frequency of the spur and patterns of
multiple spurs are the most useful
parameters for determining the source of
spurs. The spur frequency can be estimated
from the graph, or pinpointed using either
the Marker graphic function which provides
a resolution of from 0.1% to 0.2% or by
using the spur listing function.
Try turning off or moving fans, motors, or
other mechanical devices that oscillate at a
specific frequency. (Temporarily blocking
the airflow through a fan may alter its speed
enough to discern a frequency shift in a spur
that is being caused by the fan.)
Problem Solving
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