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Tracking Frequency Drift
Evaluating Beatnote
Drift
Document Part No. E5500-90024Ed. 1.0
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 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.
G
By Allowing sources to warm-up sufficiently.
G
By Selecting a different reference source with less drift.
2
Increase the capture and drift tracking Ranges.
E5500 Phase Noise Measurement System Version A.02.00
Absolute Measurement Fundamentals
Tracking Frequency Drift
6
6-13
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