User calibration refers to the process of characterizing systematic errors in VNA measurements. Error terms in the correction model are found by comparing known and measured S-parameters of calibration standards—specialized electrical reference devices.

A collection of calibration standards that can be used to perform user calibration is called a calibration kit or simply calkit.

Perform user calibration to identify error terms in the correction model and apply these user calibrations to correct future VNA measurements.

Note Use a VNA calibration kit to eliminate systematic errors from ratio-based measurements, such as S-parameters. This type of calibration is called RF calibration. For taking accurate measurements not expressed in ratios, for example wave amplitudes, use power calibration.

Calibration Procedures

Short Open Load (SOL)

SOL cal requires short, open, and load calibration standards. The exact standard values are provided to the VNA in the form of a calibration kit file. The connection point for calibration standards is called the reference plane or calibration plane. This calibration plane can be a port, cable end, or test fixture. SOL cal is a one-port calibration procedure and is suitable for performing error corrected measurements of one-port DUTs.

Short Open Load Thru (SOLT)

For two-port VNA calibration, in addition to performing SOL calibrations on each port, you must measure an insertable through connection. Use a male-to-female cable or similar connection during SOLT calibration. Avoid external adapters or devices when completing the through connection. The inserted thru can be known or unknown.

  • Known Thru—Known Thru calibration is a type of SOLT calibration. The Known Thru uses a 2-port insertable standard with known S-parameters for the Thru step. Inserting any component during calibration and not using it in the post calibration measurement results in measurement error. If you cannot make a through connection, the component is referred to as "non-insertable." There are several methods to handle non-insertable devices. The simplest method is to use a set of phase-equal adapters along with shorts, opens, and loads of each gender. These adapters are included in most calibration kits. Use one adapter to complete the through connection during calibration. Swap it with an appropriate adapter for the DUT connection during post calibration measurements.
  • Unknown Thru—Known Thru calibration is a type of SOLT calibration. Known Thru calibration uses a 2-port insertable standard with unknown S-parameters, assumed to be reciprocal (S₂₁ = S₁₂).

    • Known Thru and Unknown Thru calibrations are abbreviated as SOLT and SOLR respectively. The R in SOLR stands for Reciprocal Thru, emphasizing the reciprocity assumption stated above.

    Thru Reflect Line (TRL)

    TRL is one type in a family of two-port calibrations that better support noncoaxial environments such as on-wafer measurements and test fixturing. In this family, the name of the calibration is an acronym that identifies the following necessary calibration standards.

    • Through-Reflect-Line (TRL)
    • Line-Reflect-Match (LRM)
    • Line-Reflect-Line (LRL)
    • Through-Reflect-Match (TRM)

    These calibrations derive error terms for the same error model as SOLT calibrations.

    Compared to SOLT, TRL requires fewer calibration standards. Typically, three standards are required for performing a TRL calibration. In noncoaxial environments, standards are often harder to obtain but still easier to fabricate than performing a calibration and measurement with coaxial connections. Calibration standards must be precisely made, as measurement accuracy depends on their quality and repeatability. These calibrations generally use combinations of through, line, reflection, and match standards. The following table defines the standards that are used in these calibrations.

    Standard Description
    Through Ideally, a zero length through line where the measurement ports are directly connected. An alternative would be a line with electrical length, but this line must be electrically distinct from any other line standard. The intrinsic impedance of the through and line standards set the impedance of the calibration.
    Reflection A highly reflective device (most often a short or open). The absolute reflection coefficient comes from extra standard measurements. Phase knowledge must be within ±90° (¼ wavelength) to correctly identify the standard. It is important that you use the equivalent reflection standards to calibrate both ports.
    Line A nonzero length through line with the same intrinsic impedance as the through standard. The difference between the phase shifts of the line and through standards requires a minimum of 10 degrees to 170 degrees. Alternatively, a preferred 20 degrees to 160 degrees for each frequency. Knowledge of the propagation constant is not necessary because you can derive it from the extra measurement information. The required phase relationship between through and line standards limits the measurable frequency span. Each standard pair supports only a specific frequency range. If you need larger frequency spans, use multiple through and line pairs.
    Match If a match standard is used in place of a line standard, you can use its impedance to set the impedance of the calibration. The match standard should be identical for each port and should generally use characterized data or assume data to be a perfect match.

    TRL is a calibration form that makes use of fewer standards, requiring less knowledge about the standards. TRL calibration is highly suited for noncoaxial environments where traditional standards may be hard to fabricate and fully characterize. TRL calibration requires high-quality, repeatable standards to ensure accurate calibrations. Phase-based standard identification limits the frequency span. You can extend it by adding more calibration standards as needed. Much of the standard characterization is extracted during measurement. This improves calibration accuracy compared to SOLT.

    Electronic Calibration

    Electronic calibration (vCal) units simplify SOL and SOLT calibrations. They offer faster setup, better repeatability, and greater ease of use. vCal units also remove most manual interaction, greatly reducing the chance of a human error during calibration. These vCal units typically house several one-port and or two-port calibration standards that can be switched electronically.

    The VNA uses the standards of the vCal module to take RF measurements for calibration. VNA then automatically switches to the next standard. During calibration, the VNA measures vCal standards. VNA compares these results with known data to calculate the error terms.