Various 2-port calibrations are available in the Vector Network Analyzer. Transmission response is the simplest and requires only one connection during calibration, but it does not correct for test port match errors. 1‑Path 2‑Port calibration requires four calibration connections and corrects for the transmit port match, but does not correct for the receive port. Full 2-port calibration requires seven calibration connections and corrects for both test port match errors (refer to Figure: Full 2‑Port Calibration on MS2028C). The full 2-port calibration technique offers the most accuracy.

For accurate results, the instrument must be calibrated at the ambient temperature after allowing for warm up time (approximately 15 minutes) and before making any measurements. The instrument must be recalibrated whenever the setup frequency changes, whenever the ambient temperature changes by an amount that has more than likely rendered the calibration invalid, or whenever a test port extension cable is added, removed, or replaced. Refer to Calibration Data and Indications.

You can save a Setup with calibration (refer to the file management instructions in the user guide for your instrument). In the Save menu, press the Change Type soft key to open the Select File Type list box. Scroll to Setup (with CAL) and press the Enter key.

When you recall a setup, the calibration remains valid if instrument conditions (such as temperature) remain within the calibration tolerance.

 

When you perform a calibration, the correction coefficients are calculated for specific S‑parameters (depending on the type of calibration chosen) and for instrument settings (frequency range, number of points, and power level). The term “calibration correction” refers to the measurement correction coefficients that are applied to measurements as a result of your calibration.

When calibration correction is On, the correction is applied to all applicable S‑parameters. For example, if a Full S11 (1‑port) calibration is performed, then only traces that measure S11 have a valid calibration. For those traces, the calibration information data in the Instrument Settings Summary (described in your instrument user guide) shows “CAL: ON (OK)”. All other traces that do not measure S11 will display “CAL: --” to indicate that no valid calibration is available for those traces. The calibration correction can also be turned off manually under the Calibration menu by toggling the Cal Correction soft key from On to Off. In that case, the display will show “CAL: OFF” for all traces that have valid correction data available.

Note that “CAL: OFF” means that a calibration correction has been created, but it is not currently being used. This is different from “CAL: --”, which means that no valid calibration correction is available for the current setting.

When you have Cal Correction on, you cannot increase the frequency range or the number of points. You can, however, reduce the frequency range or decrease the number of points without forcing the calibration to become invalid. When reducing the frequency range, the VNA Master uses the appropriate points within the new frequency range that have correction coefficients applied to them. In that case, you can observe that the number of points that are being used for calibration correction are automatically reduced.

If you reduce only the number of points, then the frequency range is not changed. The VNA Master finds a subset of the original points in the sweep that can be used. You can therefore notice that the instrument may not use the exact number of points that you have entered. It picks a specific number of points that allow the calibration correction to continue to be valid. If you use the rotary knob, then you will more easily find the available number of points that can be set. For example, if you calibrated with 201 points, then you can observe that you can reduce the number of points to 101, 68, 51, 41, and so forth.

If you change the source power setting, then the calibration status will be changed to “CAL: ON (?P)”, which indicates that source power has changed since the instrument was calibrated (from Low to High, or from High to Low). Refer to Source Power Menu (MS20xxB and S412E only). In this case, the calibration may still be valid, but a new calibration is recommended.

Another status information display that you may see is “CAL: ON (?T)” which indicates that the instrument temperature has deviated by more than a set amount since the time that the calibration was conducted. The calibration is most likely still valid, but a new calibration is recommended. If you see “CAL: ON (X)” on the display, this indicates that the instrument temperature has deviated (since the time the calibration was conducted) by an amount that has more than likely rendered the calibration invalid. When this occurs, a new calibration is highly recommended before further measurements are conducted.

Only one calibration is available at one time. Performing a new calibration overwrites any existing calibration. You can, however, store a measurement setup (with CAL), which also stores the calibration. You can therefore have multiple calibrations available (as long as the calibration settings and conditions continue to apply).

The Cal Type soft key is found in the Calibration Menu (shownCalibration Menu). The Calibration Type list box provides the complete selection of available calibration types. Refer to section Calibration Types