Problem of Phase Lock Lost In VNA

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Hi, I have encountered a problem where I try to measure a RF Amp with a very low power input. But the VNA keep showing an warning message "Phase Lock Lost". I cant measure anything when I see this message. Would like to know what is the problem? What lead to this problem?
 

In a VNA, oscillators are phase-locked to a 10 or 100 MHz reference. Indication of Lock Lost means that either the reference source went bad or check that "internal reference" is selected. For some tests an external reference can be connected. Check specifications, such source must be an OCXO, stable frequency and within certain power limits to achieve phase lock.

The worst case is that one or two oscillators in VNA failed and need repair.
 
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It mostly happens when your Amp is oscillating, check with spectrum analyzer.
 
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I agree with johnjoe, "source unleveled" is also a possible warning in case of oscillating amplifier.
 

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