When multiple calibration algorithms are needed (covering different frequency ranges) and the usual calibration approaches are not adequate, any calibrations of the same type may be combined. Historically, a common example was the broadband coaxial W1 calibration where SOLT is used at low frequencies and SSST is used at high frequencies. In more recent software versions (2.3.1 and later), that particular case is handled with an integrated calibration for the 3739-based broadband modes but there are still a number of examples where a particular calibration algorithm may not be appropriate over the entire frequency range of interest. Some examples include:
• LRL/LRM calibrations where more than 5 bands are of interest.
• The use of a defined standards calibration (e.g., SOLT) at low frequency and transitioning to LRM at high frequency (an on-wafer scenario that is sometimes employed).
• A broadband calibration where it may be necessary to transition between a number of SSST or SSLT kits.
• A 3738-based broadband SOLT/SSST calibration in the 1 mm coaxial connector.
This starts off as merging a pair of calibrations but the operation may be repeated any number of times. The Cal Merge utility dialog (shown in Figure: CAL MERGE Dialog Box) simply combines the error coefficient arrays of the various calibrations while rationalizing the frequency lists as best as possible. Some points to keep in mind:
• Frequencies
If all frequencies are distinct, all points will be maintained.
• Frequency Points
If any frequency points are in both calibrations, the coefficients from the first calibration (see Figure: CAL MERGE Dialog Box) will be used.
• Number of Points
The combined number of frequency points may not exceed the current limit of the instrument which is either 25,000 or 100,000 points depending on the VNA mode.
• Calibration Small Steps
Since different calibration algorithms can be used, small steps may appear in the data where the calibrations overlap (assuming the cals were all of good quality).
• Segmented Sweep
The combined calibration will have its frequency list and other setup information loaded in a segmented sweep structure but it is not editable.
In the case of waveguide calibrations, an overriding cutoff frequency should be specified (for reference plane rotations and other dispersion processing events). When the merge is complete, the combined calibration will be resident in memory but will not automatically be saved. The user must save the combined calibration manually if desired.
