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Minimizing Injection Locking
Adding Isolation
Increasing the PLL
Bandwidth
Document Part No. E5500-90024Ed. 1.0
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 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.
NOTE: The PTR for the measurement is set by the tuning
characteristics of the VCO source you are using.
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.
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
E5500 Phase Noise Measurement System Version A.02.00
Absolute Measurement Fundamentals
Minimizing Injection Locking
Figure 6-8
6
shows
6-17
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