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PNA-X noise figure standard receivers has spiky behavior

Question asked by rok on Mar 12, 2013
Latest reply on May 5, 2014 by rok

I believe the PNA-X I am using might have some internal hardware problems, but I wanted to verify here before I send it out for repair.

I am measuring the noise figure of a downconverter using a PNA-X with standard receivers. (do not have the noise receiver option). Previously, I was able to test units on that PNA-X in question with no issues on the noise figure. However, recently, its been exhibiting spiky behavior in the noise figure waveform. (see attached). I moved the DUT to another PNA-X and it didn't have the spikes issue. I also swapped the test cables on the PNA-X in question and retried the measurement, but still have those spikes appearing in the waveform. The strange part is the gain waveforms and return loss waveforms look good and have no spiky behavior at all. 

My question is as such. Is my assumption that it is a internal hardware issue correct? Or is there something that I can try prior to sending it back for repair? And what could be happening internally inside the PNA-X to cause such behavior?