Author Topic: Electronic cable length compensation for VNA in the R&S FPC1500  (Read 175 times)

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Offline RBBVNL9

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In several (excellent) reviews of the Rohde & Schwarz FPC1500 spectrum analyzer, like here and here, considerable attention was paid to the calibration procedure, the electrical length of calibration units, and attempts to make homebrew test cables of a precise, calculated length so that the reference plane would be exactly in the right spot for specific chosen connectors or components.

Playing around with this instrument, I came across a possibility in the FPC itself to compensate electrically for any cable length differences, both positive and negative, and even a setting to do this fully automatic. So no more need to trim cable length manually. This electronic cable length compensation parameter is ‘hidden’ in the Phase menu, and it is not apparently clear that this creates a global setting that allows all other measurements, including S21 Smith Charts etc., get appropriately compensated. Since no one seems to have written about this, I thought it might be helpful to share here.

My own use case is as follows: I have the R&S ZN-Z103 Calibration kit, but it has an N connector, while most of the DUTs I want to measure are equipped with SMA connectors. So I need to compensate for different lengths of cables, adapters, or both. So what can be done is:

  • Connect the ZN-Z103 Calibration Kit via a 1 test meter lead  N(m) to SMA (m) cable, plus an SMA(f) to N(f) adapter. Then perform ecal calibration.
  • Disconnect the Calibration Kit and the adapter from the test lead.
  • At the end of the test lead, connect a short plug (I have a simple one lying around that came for free with a NanoVNA).
  • ______ Optionally, perform a VNA Smith Chart test now and see it is wrong because of the uncompensated length.
  • Go to the "VNA | Reflection s11 | Format | Phase" menu and activate ‘electrical length’. This length parameter is now shown on the screen.
  • Use the “Auto Length feature” and the measured compensation length is stored and displayed. Alternatively, use “Length offset”, and set the parameter yourself.
  • ______ Optionally, perform a VNA Smith Chart test and see that is now producing proper result as the length is properly compensated.

A more detailed discussion on electronic length compensation, with pictures and screen prints, can be found here.

Who knows how many other nice surprises the FPC has in store!
« Last Edit: May 05, 2021, 08:23:53 pm by RBBVNL9 »
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