Discontinuity In The Graph - Agilent Technologies E5500A User Manual

Discontinuity in the
Graph
Table 15-2
Circumstance
Break between segments where
closely spaced spurs are resolved
in one segment but not in the next.
Erratic Noise: One or more
segments out of line with the rest of
the graph.
Break at the upper edge of the
segment below PLL Bandwidth ³ 4.
Small Break at 100 kHz,
10 kHz, or 1 kHz
Evaluating Your Measurement Results
Problem Solving
Because noise distribution is continuous, a break in the graph is evidence of
a measurement problem. Discontinuity in the graph will normally appear at
the sweep-segment connections.
Table 15-2 identifies the circumstances that can cause discontinuity in the
graph.
Potential Causes of Discontinuity in the Graph
Description
Closely spaced spurs that are
resolved in one sweep-segment
but not in the next can cause an
apparent jump in the noise where
they are not resolved.
This occurs when the noise level of
the source being used is
inconsistent over time. The
time-varying noise level causes the
overall noise present when one
segment is being measured to
differ from the level present during
the period when the next segment
is measured.
Accuracy degradation of more than
1 or 2 dB can result in a break in
the graph at the internal
changeover frequency between the
phase detector portion of the
measurement and the voltage
controlled oscillator tune line
measurement. The accuracy
degradation can be caused by:
• An inaccurate Tuning or Phase
Detector Constant
• Injection locking, or
• Noise near or above the small
angle line at an offset equal to
the PLL Bandwidth for the
measurement.
15-14 Agilent Technologies E5500 Phase Noise Measurement System
Recommended Action
Use the Real-time Monitor to
evaluate the noise spectrum at the
break frequency on the graph. To
eliminate the break in the graph,
you may find it necessary to
change the Sweep-Segment
Ranges so that the measurement
resolution remains constant over
the frequency range where the
spurs are located.
Repeat the noise measurement
several times for the segment that
does not match the rest of the
graph, and check for a change in
its overall noise level.
Check the Parameter Summary list
provided for your results graph to
see if any accuracy degradation
was noted. If the Tuning constant
and Phase Detector constant were
not measured by the phase
detector system, verify their
accuracy by selecting the
Measured calibration method and
then initiating a New Measurement.
If you suspect injection locking or
noise above the small angle line,
refer to the Problem Solving
section of Chapter 3 for specific
actions.