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Adding Isolation
Increasing the PLL
Bandwidth
NOTE
Absolute Measurement Fundamentals
Minimizing Injection Locking
Minimizing Injection Locking
Injection locking occurs when a signal feeds back into an oscillator through
its output path. This can cause the oscillator to become locked to the injected
signal rather than to the reference signal for the phase locked loop.
Injection locking is possible whenever the buffering at the output of an
oscillator is not sufficient to prevent a signal from entering. If the injection
locking occurs at an offset frequency that is not well within the PLL
bandwidth set for the measurement, it can cause the system to lose phase
lock.
The best way to prevent injection locking is to isolate the output of the
source being injection locked (typically the unit-under-test) by increasing
the buffering at its output. This can be accomplished by inserting a low noise
amplifier and/or an attenuator between the output of the source being
injection locked and the Agilent/HP 70420A. (For information on
determining the effect that the amplifier noise will have on the measurement
noise floor, refer to Inserting a Device in this section.)
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.
The PTR for the measurement is set by the tuning characteristics of the VCO
source you are using. Notice in
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.
To estimate the PTR needed to prevent injection locking from causing the
system to lose lock:
1. Determine the injection locking bandwidth. Tune the beatnote toward 0
Hz using the procedure described in the Checking the Beatnote section
of each phase lock loop measurement example in this chapter. When the
injection locking occurs, the beatnote will disappear. The injection
locking bandwidth is the frequency of the beatnote just prior to where
the injection locking occurs as the beatnote is tuned toward 0 Hz.
6-16 Agilent Technologies E5500 Phase Noise Measurement System
Figure 6-8 on page 6-17
that increasing the
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