76 N5511A Phase Noise Test System User’s GuideAbsolute Measurement FundamentalsMinimizing Injection LockingMinimizing Injection LockingInjection locking occurs when a signal feeds back into an oscillator through itsoutput path. This can cause the oscillator to become locked to the injectedsignal rather than to the reference signal for the phase locked loop.Injection locking is possible whenever the buffering at the output of anoscillator is not sufficient to prevent a signal from entering. If the injectionlocking occurs at an offset frequency that is not well within the PLL bandwidthset for the measurement, it can cause the system to lose phase lock.Adding IsolationThe best way to prevent injection locking is to isolate the output of the sourcebeing injection locked (typically the DUT) by increasing the buffering at itsoutput. This can be accomplished by inserting a low noise amplifier and/or anattenuator between the output of the source being injection locked and thetest set. (Refer to “Inserting a Device” in this section.In N5511A, one can troubleshoot isolation issues through an oscilloscopeconnected to the Monitor outputs of the phase detector modules.Figure 5-15 shows the beat notes from an absolute phase noise measurementof a 10 MHz OCXO. Notice the impurity of the signal present at the output ofthe phase detector. This reflects isolation issues. By adding isolation in eachchannel, 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
