The tuner calibration has less gamma than expected (In Situ)

June 12, 2024


Symptoms / Solutions:

When performing an in situ tuner calibration, there are some important steps to follow.

When assembling the system, the loss of the probe and the cable between tuner and probe is one of the main factors that will determine the final gamma of the tuner. The user should select cables and probes such that the loss between tuner and DUT is reduced. This will result in higher gamma.

Before starting the system calibrations, the VNA must be calibrated to the ends of the cables. The frequency list of the VNA must be selected such that the list explicitly contains the frequencies that will be eventually calibrated on the tuner. FDCS does not interpolated between frequencies.

Before the in situ tuner calibration, the user will perform a TRL calibration at the probe tips to determine the CONA (input tuner, input probe cable and input probe) and CONB (output tuner, output probe cable and output probe) S-Parameters. The tuners must be initialized during this step. Always verify the CONA/CONB files on a THRU to confirm that they are correct.

When the user starts the in situ calibration, he may find that the tuner calibration has less gamma than expected. Before version 3.5, if the tuner calibration frequency is not included in the CONA/CONB file, then the software will perform the tuner calibration but not deembed any s-parameters as it should. In newer versions, there is a warning to say which file is missing frequencies.

If the CONA/CONB have the correct frequencies and there is still reduced gamma confirm that:

  • the VNA has been calibrated at the correct frequencies
  • the selected cables and probes are low loss
  • all connectors are properly tightened


Keywords: In-situ tuner calibration, low gamma, missing frequencies, TRL calibration, CONA, CONB

Last date modified:  Sept 27, 2017