Keysight Technologies N5511A, User Manual

Page: 77 / 369

N5511A Phase Noise Test System User’s Guide 77
Absolute Measurement Fundamentals
Minimizing Injection Locking
Increasing the PLL Bandwidth
If the injection locking bandwidth is less or equal to the PLL bandwidth, it may
be possible to increase the PLL bandwidth sufficiently to complete the
measurement. The PLL bandwidth is increased by increasing the peak tuning
range (PTR) for the measurement.
To estimate the PTR needed to prevent injection locking from causing the
system to lose lock:
1. Determine the injection locking bandwidth. Tune the beat note toward
0 Hz using the procedure described in the Checking the beat note section
of each phase lock loop measurement example in this chapter. When the
injection locking occurs, the beat note disappears. The injection locking
bandwidth is the frequency of the beat note just prior to where the
injection locking occurs as the beat note is tuned toward 0 Hz.
2. Multiply the injection locking bandwidth by 2 to determine the minimum
PLL bandwidth required to prevent the injection locking from causing the
system to lose lock. (To prevent accuracy degradation, it may be
necessary to increase the PLL bandwidth to 4 X the injection locking
bandwidth. The computer informs you during the measurement if the
possibility of accuracy degradation exists.)
3. Locate the required PLL bandwidth in Figure 5-16 to determine the PTR
required for the measurement. (For details on increasing the PTR, refer to
“Changing the PTR” in this section.
The PTR for the measurement is set by the tuning characteristics of the VCO source you are
using. Figure 5-16 shows that increasing the PLL bandwidth can require a substantially larger
increase in the PTR. For information on the limitations of increasing the PTR, refer to “Changing
the PTR” in this section.