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6
Absolute Measurement Fundamentals
The Phase Lock Loop Technique
6-4
E5500 Phase Noise Measurement System Version A.02.00
Tuning Requirements
The peak tuning range required for your measurement will depend
on the frequency stability of the two sources you are using. The
signals from the two sources are mixed in the system's phase
detector to create a beatnote. In order for the loop to acquire lock,
the center frequencies of the sources must be close enough together
to create a beatnote that is within the system's Capture Range. Once
the loop is locked, the frequency of the beatnote must remain within
the drift tracking range for the duration of the measurement. In
Figure
6-3, the ranges calculated in the previous example are marked
to show their relationship to the beatnote frequency.
Figure 6-3 Relationship of Capture and Drift Tracking Ranges to Beatnote
Frequency
If the beatnote does not remain within the drift tracking range
during the measurement, the out of lock detector will be set and the
System will stop the measurement. If this happens, you will need to
increase the system's drift tracking range by increasing the system's
peak tuning range (if possible) or by selecting a VCO source with a
greater tuning range.
Selecting the VCO Source
Although you must select a VCO source that will provide a sufficient
tuning range to permit the system to track the beatnote, keep in mind
that a wide tuning range typically means a higher noise level on the
VCO source signal. When the VCO source for your measurement is
also the reference source, this trade-off can make reference source
selection the most critical aspect of your measurement setup.
Document Part No. E5500-90024 Ed. 1.0
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