User`s guide

118 Agilent E5505A User’s Guide
5 Absolute Measurement Fundamentals
Changing the PTR
The peak tuning range (PTR) for the phase lock loop measurement is set by the
tune range entered for the VCO and the VCO’s tuning constant. (If the
calibration technique is set to measure the VCO tuning constant, the measured
value is used to determine the system’s PTR.)
PTR= VCO Tuning Constant X Voltage Tuning Range
From the PTR, the phase noise software derives the capture and drift tracking
Ranges for the measurement. These ranges set the frequency stability
requirements for the sources being used.
The PTR also determines the phase lock loop (PLL) bandwidth for the
measurement. An important attribute of the PLL bandwidth is that it
suppresses the close-in noise which would otherwise prevent the system from
locking the loop.
The Tuning Qualifications
Changing the PTR is accomplished by changing the tune range of VCO value or
the VCO tuning constant value or both. There are several ways this can be
done. However, when considering these or any other options for changing the
PTR, it is important to remember that the VCO source must always meet the
following tuning qualifications.
• The tuning response of the VCO source must always remain monotonic.
• The VCO source’s output level must remain constant across its tuning
range.
Figure 77 Peak tuning range
Total peak-to-peak tuning range of VCO
System
peak tuning range
Drift
tracking range
Capture
range
VCO Source center frequency
24% 24%
5% 5%
E5505a_capt_drift_trk_range
26 Feb 04 rev 1