Port Subset Correction (Devolve Calibration)

It is often convenient to calibrate all the ports of the instrument so that corrected data is available at every port. However, applying the resulting calset results in every port being swept. This level of correction is appropriate if every port is connected to the DUT. However, sweeping ports that are disconnected unnecessarily slows down measurement throughput. To remedy this situation, the user can tell the instrument to exclude selected ports from the correction process. This is called "port sub-setting" or "devolve calibration". This process does not modify the calset in any way. There are two settings associated with port sub-setting: an on/off state, and the list of ports that should be included in the correction for the channel.

Port sub-setting values are independent of calset selection. They are essentially a mask that is applied to the calset.

Restricting correction to Enabled Ports

For example, on a 4 port instrument, the user is measuring two DUTs. Device #1 is connected to ports 1 and 2. Device #2 is connected to ports 3 and 4. The two devices are not interconnected in any way. Channel 1 is used to measure device #1. Channel 2 is used to measure device #2. If you apply a 4 port calset to each of these channels, both channels will sweep all 4 ports. Port sub-setting can be used to reduce the level of the correction for each channel.

Channel	Port subset values	Measurements	Correction applied
Channel 1	ON, ports 1 and 2 enabled	S11, S21, S12, S22	Full 2 Port (1,2)
Channel 2	ON, ports 3 and 4 enabled	S33, S43, S34, S44	Full 2 Port (3,4)

In this condition, when the user performs a 4 port calibration and applies the same user calset to both channels. Channel 1 sweeps ports 1 and 2. Channel 2 sweeps ports 3 and 4.

Best Effort on Disabled Ports

If measurements are added to the channel that utilize ports that are disabled in port sub-setting, those measurements will be corrected on a "best effort" basis: some correction may be applied depending on the contents of the calset. The level of correction is limited to enhanced response calibration or simple response calibration.

Channel	Port subset values	Measurements	Correction applied
Channel 1	ON, ports 1 and 2 enabled	S11, S21, S12, S22	Full 2 Port (1,2)
Channel 1	ON, ports 1 and 2 enabled	S43	Enhanced Response(4,3)
Channel 2	ON, ports 3 and 4 enabled	S33, S43, S34, S44	Full 2P(3,4)
Channel 2	ON, ports 3 and 4 enabled	S11	1 Port (1)

This "best effort" correction cannot be turned off using the dialog. But there are SCPI commands for disabling this feature.

See SENS:CORR:METH:PORT:SUBS:FULL:VAL and SENS:CORR:METH:PORT:SUBS:RESP:VAL.

Click Cal > Main > Correction Methods.....

Correction Methods dialog box help

Enabling match correction on power measurement Apply match correction on all receivers used on this channel. See Matching

Enable port subset correction Enabling port subset correction to reduce the number of corrected ports.

Define port subset Selects which ports should be included in a full N-port correction. Un-selected ports will be corrected on a “best effort” basis: In other words, these ports will be corrected with an enhanced response calibration if the error terms are available in the calset.

Exclude all ports Excludes all ports from correction. The button will change to Select all ports to include all ports for correction.

The correction pop up pane, accessed from the status bar, indicates port by port correction methods for VNA with 12 or less test ports. This table is updated when the port subset correction is turned on to reflect the correction methods being applied. In the image below, the pane indicates a full 4 Port calibration. On the right, the table indicates the methods after the correction was devolved to ports 1,2,3.