The instrument must be re-calibrated whenever the internal instrument temperature exceeds the calibration temperature window (±20 ºC) or when the test port extension cable is removed or replaced. Unless the calibration type is Flex Cal, the instrument must also be re-calibrated every time the frequency range changes.

For all measurement types other than the Transmission (Ext. Sensor), the instrument can be manually calibrated with an external precision OSL (Open-Short-Load) calibration tee/discrete components or with the standard internal InstaCal. The benefit of the InstaCal module is that it is much faster, requires no connection changes, and eliminates the need to use three different terminations (open, short, load) for calibration. The trade-off is that the specified corrected directivity is 38 dB instead of 42 dB.

For transmission measurements with an external sensor, a different calibration method is used – Transmission (Ext. Sensor). For this method, a normalization process is used to calibrate the frequency response of the sensor and any test port cables/adapters used. The calibration method uses the sensor itself and does not require any additional components. An optional zeroing step is performed to remove any residual noise in the sensor, and then, a normalization step is performed by connecting the sensor directly to the port (or test port cables).

To support both types of measurements (1-port reflection and 2-port transmission), the S331L provides a combined calibration method: OSL + Transmission (Ext. Sensor). Choosing this calibration method allows the instrument to be calibrated for all the types of measurements that it supports. This enables the user to switch between multiple types of measurements (Return Loss and Transmission, for example) without having to recalibrate the instrument.

While OSL Cal tee, InstaCal, or Tranmission (Ext. Sensor) provide alternatives for the tools needed to perform the calibration, Standard Cal or Flex Cal determines the calibration frequency range. The default standard calibration is for a selected frequency range, and is no longer valid if the frequency is changed. Flex Cal is a broadband frequency calibration that remains valid if the frequency is changed.

Flex Cal calibrates the instrument over the entire frequency range and interpolates data points if the frequency range is changed. This method saves time as it does not require the user to re-calibrate the system for frequency changes. The trade-off is that the accuracy is not the same (fewer data points) as it would be with the standard calibration. If the frequency range will not be changed often, standard calibration is recommended for troubleshooting purposes. Table: Summary of Calibration Methods and Tools has a summary comparing the OSL and InstaCal methods for both Standard and Flex Cal settings.