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Evaluating Beatnote
Drift
Absolute Measurement Fundamentals
Tracking Frequency Drift
Tracking Frequency Drift
The system's frequency drift tracking capability for the phase lock loop
measurement is directly related to the tuning range of the VCO source being
used. The system's drift tracking range is approximately 24% of the peak
tuning range (PTR) of the VCO.
PTR= VCO Tuning Constant X Voltage Tuning Range
This is the frequency range within which the beatnote signal created by the
Agilent/HP 70420A phase detector must remain throughout the
measurement period. In addition, the beatnote signal must remain within the
system's Capture Range (5% of the PTR) during the time it takes the system
to calibrate and lock the phase lock loop.
The stability of the beatnote is a function of the combined frequency
stability of the sources being used for the measurement. If beatnote drift
prevents the beatnote from remaining within the Capture Range long enough
for the system to attain phase lock, the computer will inform you by
displaying a message. If the beatnote drifts beyond the drift tracking range
during the measurement, the computer will stop the measurement and
inform you that the system has lost lock.
The Checking the Beatnote section included in each phase lock loop
measurement example in this chapter provides a procedure for adjusting the
beatnote to within the Capture Range set for the measurement. If you have
not done so already, verify that the beatnote signal can be tuned to within the
Capture Range and that it will remain within the range.
Continue to observe the beatnote and verify that it will not drift beyond the
drift tracking range (24% of the PTR) during the measurement period. The
length of the measurement period is primarily a function of the frequency
offset range specified for the measurement (Start to Stop Frequency).
Action
If beatnote drift exceeds the limits of the Capture or drift tracking ranges set
for your measurement, the system will not be able to complete the
measurement. You have two possible alternatives.
1. Minimize beatnote drift.
•
By Allowing sources to warm-up sufficiently.
•
By Selecting a different reference source with less drift.
2. Increase the capture and drift tracking Ranges.
6-12 Agilent Technologies E5500 Phase Noise Measurement System
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