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ZNBT Parallel Measurements Nov 4, 2015

Because a ZNBT can have up to 24 true VNA ports, it can do something that a VNA and a switch matrix cannot: measure multiple devices in parallel. The ZNBT interface makes this easy to do for identical DUTs. But what if you want to measure DUTs with a varying number of ports, or you want to set up these measurements in your own software?

Example Scenario

For this example we will assume the following:

  • A ZNBT with at least 8 ports
  • A channel, channel 1, configured for measurement
  • A device DUT1 with 3 ports
  • A device DUT2 with 4 ports
  • Logical port indexes are the same as the physical port

Common Gotchas

Before I begin, I'd like to clarify a few things:

Parallel measurements must share the same measurement settings
Internally the ZNBT has a limited number of sources being used by all the physical ports (and DUTs). As such, it is not possible to measure devices in parallel with arbitrarily independent measurement settings. In this example, both DUTs are measured in channel 1, and as such share channel 1's measurement settings.

Port assignments supersede channel settings
Another thing to keep in mind is that the port assignments for parallel measurement supersede any port assignments in the channel. So, if you have already created logical port assignments (for example, with balanced ports) they may get overwritten by these SCPI commands.

SCPI Command Menu

All of the SCPI commands for parallel measurements can be found in the SOURce:GROup command menu. Here is a screenshot of the documentation from the ZNBT help menu.

Note the location in help: Command Reference > SCPI Command Reference > SOURce Commands > SOURce:GROup

SCPI Command Menu

SCPI Commands

In the terminology of the SCPI Command Menu, each DUT will have it's own Port Group. The most straightforward command for defining each group is:

Set Port Group

Now we will create these port groups. I will assign DUT1 to ports 1-3, and DUT2 to ports 4-7.

"SOURce1:GROup1:PPORTs 1,2,3"
"SOURce1:GROup2:PORTs 4,5,6,7"

Optionally, you can also give these groups names

"SOURce1:GROup1:NAME 'DUT1'"
"SOURce1:GROup2:NAME 'DUT2'"

You can now use the normal SCPI commands to perform a sweep and query/save data.