Keysight Technologies N5511A, User Manual

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N5511A Phase Noise Test System User’s Guide 305
Evaluating Your Measurement Results
Problem Solving
Forest of spurs
A so called forest of spurs is a group of closely spaced spurs on the phase noise
plot. A forest of spurs is often caused by improper shielding that allows stray
RF energy to be picked up by the DUT wiring, etc. A breadboarded or prototype
circuit should be well shielded from external RF fields when phase noise
measurements are being made.
Table 14-4 shows actions to take to eliminate spurs.
Table 14-4 Actions to eliminate spurs
Spur Sources Description Recommended Action
Internal Potential spur sources within the
measurement system include the phase
noise system, the DUT, 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.
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 DUT or the reference
source may be the spur source, check each source
using a spectrum analyzer or measuring receiver
(such as an Keysight 8902A). Also, if additional
sources are available, try exchanging each of the
sources and repeating the measurement.
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.
Shorten coaxial 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.
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.
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.
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.
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.)