Automatic Calibration   

A Calibration Unit is an integrated solution for automatic system error correction of vector network analyzers of the R&S ZVAB family. Rohde & Schwarz provides different types of calibration units; for an overview refer to the System Overview chapter. The use of the calibration units is described below.

• Connecting the calibration unit

• Performing an automatic calibration

• Calibration procedure

• Manual characterization of calibration unit

Connecting the Calibration Units to the Network Analyzer

The calibration units provide the following connectors:

• Calibration units R&S ZV-Z51/.../-K55

  • A USB type B connector at the rear is used to power-supply and control the unit. A USB cable for connection to the network analyzer is provided with the calibration unit.

  • Two or four RF connectors numbered 1 to 4 are to be connected to the test ports 1 to 4 of the analyzer. The connector type is equal for all ports. Depending on the calibration unit model, it is either type N (f), 3.5 mm (f), 2.92 mm (f) or 2.4 mm (f).  

• Calibration units R&S ZV-Z58/-K59

  • A USB type B connector at the front is used to control the unit. A USB cable for connection to the network analyzer is provided with the calibration unit.

  • The RF connectors numbered 1 to 8 / 6 are to be connected to the corresponding test ports of the analyzer. The connector type is type N (f) or 3.5 mm (f) for all connectors.  

To connect the unit...

1. Switch on and start-up your network analyzer.

2. To protect your equipment against ESD damage use the wrist strap and grounding cord supplied with the instrument and connect yourself to the GND connector at the front panel.

3. Connect the USB type A connector of the USB cable to any of the USB type A connectors on the front or rear panel of the analyzer. Connect the USB type B connector of the USB cable to the USB type B connector of the calibration unit.  

4. Wait until the operating system has recognized and initialized the new hardware. When the unit is connected for the first time, this may take longer than in normal use.  

The unit is ready to be used, see Performing an automatic calibration.

Performing an Automatic Calibration

After connection and initialization, you can use the calibration unit as follows:  

1. Connect n analyzer ports (n = 1 to 8, depending on your analyzer and calibration unit model and the number of ports to be calibrated) to n arbitrary ports of the calibration unit. Terminate all unused ports (no. n + 1 to 2, 4, or 8) with a 50 Ω match.

2. Perform the automatic calibration for the selected number of ports using the Measurement Wizard or the Start Calibration menu.

3. Remove the test cables from the unit, connect your DUT instead and perform calibrated measurements.

The assignment between the analyzer ports and the cal unit ports is detected automatically. If auto-detection fails (e.g. because of a high attenuation in the signal path), connect matching port numbers and click Default Port Assignment at the beginning of the calibration process.

Accuracy considerations

The calibration type depends on the number of ports. If a single port is calibrated, the analyzer uses full one-port calibration. For 2 and more ports, the analyzer provides the following additional options:

• A single full n-port calibration for n calibrated ports.

• n full one-port calibrations for n calibrated ports.

• n/2 * (n – 1) separate full two-port (TOSM or UOSM) calibrations for n calibrated ports (all possible 2-port combinations). This calibration type is required for isolation measurements on mixers where a full 3-port calibration is inappropriate.

• (n – 1) one path two port calibrations for n calibrated ports (all possible 2-port combinations from the Node Port to any other port). The node port is the source port for each one path two port calibration (fully calibrated port).

• (n – 1) transmission normalizations (bidirectional, forward or reverse) for n calibrated ports (all possible 2-port combinations from the first port to any other port). "Forward" transmission normalization means that the signal direction is from the ports with the lower numbers to the port with the higher numbers.   
  Example: Select ports no. 1, 2, 3. A forward transmission normalization calibrates the S-parameters S21, S31, and S32. A backward transmission normalization calibrates the S-parameters S12, S13, and S23. A bidirectional transmission normalization calibrates all six S-parameters.  

Performing an automatic calibration

Calibration Procedure

The automatic calibration is started on clicking Calibration Unit in the Start Calibration menu. When the test port connectors in the Port Configuration dialog do not fit to the connectors of the calibration unit, a warning appears:

OK changes the connector type assignment of the test ports and starts the calibration. The Calibration Unit dialog shows the active cal unit type and characterization, the calibrated ports, and the progress of the calibration procedure.

While a user characterization is active, the Calibration Unit dialog shows an information message. In general, user characterizations are required for modified test setups (e.g. cal units with external adapters); see Manual Characterization of Cal Units. For an unmodified calibration unit, the factory calibration is adequate.  

In the dialog, you can:

• Click Change Active Calibration Unit or Characterization in case that several cal units are connected to the analyzer and you wish to select another unit or characterization. For manual characterization see Characterize Cal Unit.

  

• Click Modify Calibration Settings to change the calibration type and node port (for One Path Two Port calibrations), the detector settings (Normal or AVG detector, primarily for noise figure measurements with option R&S ZVAB-K30), the calibrated ports and the port assignment.

  

  In the Calibration Settings dialog, you can:

  • Select the network analyzer ports to be calibrated (Calibrate Ports).

  • Select the Calibration Type: See Calibration Typesand note on One Path Two Port calibration below. 

  • Detectthe current port assignment between the analyzer and the connected calibration unit in order to overwrite the Port Assignment table.

  • Restore the Default Port Assignment (connect port no. n of the analyzer to port no. n of the cal unit). This is important if the automatic detection of the port assignment fails.

  A One Path Two Port calibration requires two-port (Through) characterization data for the cal unit, which is not available on older calibration units. If a tooltip indicates missing two-port characterization data, simply perform a new characterization of your cal unit. In the first dialog of the Characterization Wizard, make sure the cal unit is characterized for all calibration types including One Path Two Port calibration.  

• Click Take Cal Sweep to start the necessary calibration sweeps. The calibration is started automatically if automatic port detection is successful and the cal unit temperature is within the validity range.

• Abort an ongoing calibration sweep.

The Calibration Unit dialog is closed automatically when all standards have been measured, the system error correction data has been calculated, and the result is applied to the active channel. The network analyzer is ready to perform calibrated measurements.

The full two-port and full n-port calibration types require a maximum frequency step size Δfmax which ensures that the difference in phase of the internal through-connection between any two consecutive sweep points is below 180 deg.  
Condition: |Δφmax| = maxn|φn – φn-1| < 180 deg; |Δφmax| is proportional to Δfmax.
The analyzer automatically inserts additional sweep points to reduce the maximum frequency step size if the condition above is not met. After the calibration, the measurements are performed with the original sweep points.