Wrong Measurements using PXI RF Multiplexer Switch Module

Updated Apr 11, 2018

Issue Details

I'm measuring different RF signals which I'm switching between them with a PXI RF Multiplexer Switch, after a short time I receipt wrong measurements and I need to change the module to a new one.


We need to verify that signals connected to the switch are properly terminated to ensure that the entire system has the same characteristic impedance.
In a typical RF application involving switches, your source, load, transmission line, and switch impedance may be 50 Ω. When the RF relay opens, the source and transmission line impedance will remain at 50 Ω, while your load impedance has jumped to a very large value. This impedance mismatch can cause RF signals to be reflected into the signal source and damage equipment or even the connectors on the switch causing a mismatch on the measurements expected.
When switching sensitive RF equipment, it may be necessary to ensure that the signal is terminated to fulfill the requirement of keeping identical characteristic impedance throughout the system. A terminated switch is essentially a broadband impedance (resistor) to ground that matches the characteristic impedance of the system when the switch is closed.
Normally wrong measurements on this kind of system is due the lack of terminated signal on the module as we can see on the following diagram of a PXI-2546.
To verify this, is necessary to apply a path resistance test and see the manual to check the value when is an End of Life.
Path resistance is a combination of relay contact resistance and trace resistance. Contact
resistance typically remains low for the life of a relay. At the end of relay life, the contact
resistance rises rapidly above 1.0 Ω.

If the result shows a possible End of Life, it may be necessary to contact NI Support and send it to RMA.
To avoid this problem in the future, we strongly recommend the use of a module with terminated signals as shown in the following diagram.

Additional Information

A switch that is closed with termination generally cannot handle the full power of the switch. See device documentation for additional information.


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