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PNA Drifting in time

Question asked by steveor on Nov 18, 2011
Latest reply on Nov 21, 2011 by Dr_joel

I have a fleet of 12 network analyzers under my control and have noticed that one analyzer (model N5230A) seems to drift much more than it's neighbors.  By the way, the unit in question just came back from annual cal, so it passed all it's performance tests.

First, a little background information about my setup.  Each network analyzer is connected to a wafer prober.  We use Agilent 11500F cables to connect the network analyzer to a Cascade Infinity probes.  The calibration is performed to the probe tips using the Cascade Impedance Standard Substrate.  The calibration is performed every 72 hours and immediately followed by running a reference wafer.  We check the reference wafer results against previous runs and look for changes in the performance.  We also re-run the reference wafer every 36 hours to insure the system is in good shape.

Recently, we found one station was consistently failing the 36 hour reference check.  We swapped the analyzer with another and found the performance followed the analyzer to it's new system.  That is, all the cables and probes were swapped and this unit still fails the 36 hour reference check.  To troubleshoot the problem we did the following:

1. Perform SOLT correction to the end of our cascade probe tips,
2. Dump the Error terms
3. Wait 36 hours
4. Calibrate to the end of our probe tips again
5. Dump the Error terms
6. Compare the Error terms from Step 2 and Step 5

The results show the shift in Reverse Transmission Tracking and Reverse Load Match are 0.03 and 0.01 respectively.  All of the other e-terms show shifts less than 0.005 with most less than 0.001. 

It appears one particular PNA has a drift issue and I'd like to get it repaired.  Only problem is I'm not sure which assemblies to replace.  What could cause this drift?  Have other users seen this problem?  If so, how'd you fix it?

Steve Ortiz