Calibrating Modules with Souriau Connector

    Modules such as 40-755 and 40-754 use a RF connector shell that can that accepts coaxial inserts. This type of arrangement is popular in some industries because it allows a high density of RF connections to be made to a switching system in a single operation with a reduced need for identifying each cable. Connectors of this type are available from a variety of suppliers with very similar characteristics, including Souriau and Positronics. The 40-755 for example uses the Souriau connector.

    These connectors are however not without their compromises. By their nature they are small, and have to use a crimp termination system on a flexible coaxial cable and this does have consequences for their RF performance. The manufacturers data suggest the performance of the connector alone degrades significantly above 500MHz.

    This does create an issue when products using this connector are tested. The connector is a significant source of VSWR at higher frequencies and there is a danger that users in characterizing the modules mistakenly also correct for the connector but in the application the connector is part of the test system.

    The standard practice is to characterize an RF switch at the switch interface (in this case the connector). Typically a cable is made up from the VNA to the switch interface and a back to back RF connection inserted to calibrate for insertion loss and a precision termination for return loss. This is not possible with these connectors, indeed doing so will typically produce highly optimistic measurements of the switch performance since the VNA will attempt to calibrate out part or all of the connector mismatch. There is no such thing as a calibration kit for this type of connector.

    The method used by Pickering Interfaces is much more representative. Two sets of cables are made up to test the switch, each using the same cable type and each being of the same length. One set of cables connects from the VNA to SMA terminations, the other set connects from the VNA to Souriau connectors. The VNA is calibrated using the SMA cables but measurements are performed using the Souriau connectors. Since the SMA terminated cables have much better performance than the Souriau terminated cables the net measurement essentially included the impairments introduced by the Souriau connectors at the input and the output of the switch module, but does not include the impairment of the cables in accordance with normal measurement practice.

    There is one other characteristic of note in the use of this type of connector. Most RF switching systems ground the connector shell at a bulkhead (the PXI module front panel). Multipole connectors of this type however do not. This does mean that the outer of the connector carries through to the RF switch without being grounded on the front panel.

    The version using the multipole connector is characterized to 500MHz rather than the 2GHz of the SMB version. Differences between the different manufacturers of multipole connectors are minor - at Pickering Interfaces we tested all versions and none had more than minor differences in performance and those differences were well within expected build variation expectations.

    The switch design for both the Souriau and SMB versions is identical on the 40-755 and the 40-754.

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