User`s guide

108 Agilent E5505A User’s Guide
5 Absolute Measurement Fundamentals
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 72, the ranges calculated in the previous example are marked to show
their relationship to the beatnote frequency.
If the beatnote does not remain within the drift tracking range during the
measurement, the out of lock detector is set and the System stops the
measurement. If this happens, you 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 provides 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.
Specifying your VCO source
When you set up your PLL measurement, you need to know four things about
the tuning characteristics of the VCO source you are using. The System
determines the VCO source’s peak tuning range from these four parameters.
• Tuning Constant, estimated tuning sensitivity (Hz/V)
• Center Voltage of Tuning Range, (V)
• Tune Range of VCO, (±V)
Figure 72 Capture and drift-tracking ranges and beatnote frequency
Drift tracking range
Capture
range
0 Hz 50 Hz 240 Hz 1000 Hz
5% of the PTR
24% of the PTR
Beatnote