Automatic Calibration (AutoCal)

Automatic calibration (AutoCal) units, or external automatic calibration modules, are popular because of their speed, repeatability, and ease of use. Moreover, they remove most human interaction, greatly reducing the chance for a mistake during calibration. These units typically contain an electronic component, such as a diode, termination, or other standard, with a corresponding detailed electrical description encoded in an AutoCal characterization file. When connected to the vector network analyzer, the automatic calibration unit is set to different states. The states measured during calibration are compared with the corresponding known states inside the characterization file to derive the correction values.

Automatic calibration offers a convenient calibration technique that avoids manual connection of standards, which can lead to errors (such as the wrong standard being connected), lost standards, damaged standards, and lost time. In an automatic calibration process, the standards are usually connected using a switch, so you only need to make one cable connection.

Note  When you perform an automatic calibration, NI-VNA sets the power at each device port to -10 dBm during the calibration process, regardless of any settings you may have provided.

Note  The automatic calibration unit uses an internal through connection to perform the calibration. However, you can also use an external through standard to perform the calibration. This configuration is known as true through auto-calibration. When performing a true through auto-calibration, you must connect the test cables to complete the calibration. The benefit of this method is a higher accuracy calibration balanced against a longer calibration time, though it takes more user interaction to perform.

Automatic Calibration Validation

The automatic calibration unit performs an additional measurement after calibration that validates if the error correction has been correctly applied. The unit does so by measuring a standard within the unit and then comparing that measurement to the characterized measurement. If they match, the automatic calibration is said to be assured.

Note  If assurance fails, the calibration may still be valid. In cases where power is low or the number of sweep points is small and the frequency range is narrow, the automatic calibration assurance may not have enough data to assure the calibration. In these situations, always verify by measuring a DUT with a known response.