User`s guide

340 Agilent E5505A User’s Guide
14 Evaluating Your Measurement Results
Small angle line
Caution must be exercised where L(f) is calculated from the spectral density
of the phase modulation
S
φ
(f) /2 because of the small angle criterion. Refer to
Figure 271. Below the line, the plot of
L(f) is correct; above the line, L(f) is
increasingly invalid and Sf(f) must be used to accurately represent the phase
noise of the signal. To accurately plot noise that exceeds the small angle line,
select the Spectral Density of Phase Modulation (dB/Hz) graph type (
S
φ
(f) ).
L(f) raises the noise floor by 3 dB.
The –10 dB per decade line is drawn on the plot for an instantaneous phase
deviation of 0.2 radians integrated over any one decade of offset frequency. At
approximately 0.2 radians, the power in the higher order sideband of the
phase modulation is still insignificant compared to the power in the first order
sideband. This ensures that the calculation of cal L(f) is still valid.
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.)
Table 54 Actions to eliminate spurs (continued)
Spur Sources Description Recommended Action