S820E Microwave Site Master™ User Guide : Calibration, VNA : Calibration Considerations
Calibration Considerations
Various 2-port calibrations are available in the S820E in VNA Mode. 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. The full 2-port calibration technique offers the most accuracy. Figure: Cal Line Setup shows how calibration connections are displayed by the S820E.
The previously described calibration considerations omit isolation steps in which loads are connected to each test port. During the isolation step of the calibration procedure, the S820E (in VNA Mode) measures the isolation between test ports in order to achieve best dynamic range performance.
For accurate results, the S820E 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. For an example of measurement improvement after calibration, refer to Example of Calibration Benefits.
The calibration menu has a Thru Update submenu key (see Figure: Calibration Menu) that allows you to recalibrate the Thru component without repeating the entire calibration sequence. Refer to Thru Update.
Calibration data are saved when you save a Setup file. When you recall a setup, the calibration remains valid if instrument conditions (such as temperature) remain within the calibration tolerance.
Calibration Data and Indications
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 shows “CAL: OK”. All other traces that do not measure S11 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 shows “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 and Interpolation set to off, you cannot modify the frequency range or the source power level, or increase the number of points. You can, however, decrease the number of points without forcing the calibration to become invalid.
If you reduce only the number of points, then the frequency range is not changed. The S820E 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 Interpolation is set to on, then you can reduce the frequency range and modify the number of points without invalidating the calibration. In that case, the calibration coefficients are regenerated (interpolated) to match the new settings.
Another status information display that you may see is “CAL: ON (X)”, which 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, which also stores the calibration. You can therefore have multiple calibrations available (as long as the calibration settings and conditions continue to apply).
Example of Calibration Benefits
S11, S22, and S21 Measurements before Calibration
S11, S22, and S21 Measurements after Calibration