Keysight Technologies N5511A, Руководство пользователя

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N5511A Phase Noise Test System User’s Guide 83
Absolute Measurement Fundamentals
Evaluating Noise Above the Small Angle Line
Figure 5-20 Phase lock loop bandwidth provided by the peak tuning range
1. Once the beat note is displayed, Auto Tune the analyzer.
2. Set the span width on the signal analyzer to approximately 4 x PLL
bandwidth. Adjust the beat note to position it near the center of the
display.
a. Turn on trace averaging.
b. Perform Peak Search.
3. Set a Delta Marker.
On the analyzer, offset the marker by the PLL bandwidth. Read the offset
frequency and noise level indicated at the bottom of the display.
4. Compare the average noise level at the PLL bandwidth offset to the small
angle criterion level shown on the graph in Figure 5-21 . The average noise
level of the signal must remain below the small angle line at all offset
frequencies beyond the PLL bandwidth. (The small angle line applies only
to the level of the average noise. Spur levels that exceed the small angle
line do not degrade measurement accuracy provided they do not exceed —
40 dBc.)
If you are not able to tune the beat note to 2 X PLL bandwidth (center of display) due to frequency
drift, refer to Tracking Frequency Drift in this section for information about measuring drifting
signals. If you are able to locate the beat note, but it distorts and then disappears as you adjust it
towards 0 Hz, then your sources are injection locking to each other. Set the beat note to the
lowest frequency possible before injection locking occurs and then refer to “Minimizing
Injection Locking” on page 76 for recommended actions.