Keysight Technologies N5511A, Руководство пользователя

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76 N5511A Phase Noise Test System User’s Guide
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.
Adding Isolation
The best way to prevent injection locking is to isolate the output of the source
being injection locked (typically the DUT) 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
test set. (Refer to “Inserting a Device” in this section.
In N5511A, one can troubleshoot isolation issues through an oscilloscope
connected to the Monitor outputs of the phase detector modules.
Figure 5-15 shows the beat notes from an absolute phase noise measurement
of a 10 MHz OCXO. Notice the impurity of the signal present at the output of
the phase detector. This reflects isolation issues. By adding isolation in each
channel, in this case by using amplifiers, the issue is improved.
Figure 5-15 Beat Notes from an Absolute Phase Noise Measurement of a 10 MHz OCXO