Standard RF/microwave vector network analyzers (VNAs) are designed for testing twoport devices, such as amplifiers and, in some cases, components with as many as four ports, such as a power divider or combiner. But in the case of a device under test (DUT) with as many as 62 ports, special modifications are required. Such a specialized VNA-based system was developed recently by In-Phase Technologies (Clarksburg, NJ) for a custom requirement. At the heart of the system is a high-performance switch matrix and power combiner assembly capable of routing as many as 72 different ports of a device under test into the two test ports of a two-port microwave VNA. Designed for use through 11 GHz, the switched VNA system is supported by proprietary software that provides instrument control, data acquisition, and signal processing to provide test results in a variety of formats.

Traditional microwave VNAs are designed for evaluating the amplitude and phase responses of two-port devices, such as filters or amplifiers, in the form of transmission and reflection scattering (S) parameters. Forward S-parameter measurements are made by connecting the VNA’s builtin source to the DUT’s input port, and the DUT’s output port to the VNA’s test port for analysis. For reverse Sparameter measurements, the connections must be reversed. By adding a switching S-parameter test set, a single set of connections is all that is needed to acquire all four S-parameters for a two-port DUT.

Of course, the number of S-parameters can grow quite large for more complete DUTs, increasing as the square of the number of ports. While a two-port device has four S-parameters, a four-port component has 16 S-parameters, and measurements of transmission and reflection between ports can also grow in complexity as signals must be routed and switched from port to port. For S-parameters, the numbering convention is that the first number following the S is the port at which the energy emerged while the second number is the port at which the energy enters. The S21 parameter is therefore a measure of forward transmission for energy entering port 1 and leaving port 2. Reflection measurements are denoted by S-parameters with two of the same number, such as S11.

Commercial microwave VNAs are available for two- and four-port S-parameter measurements, but the test setups, as well as the S-parameter numbering conventions, become quite unwieldy as the number of ports grow. More complex DUTs require special measurement solutions. By using a custom switch matrix (see table) and proprietary RF test head, a VNAbased measurement system developed by In-Phase Technologies can make S-parameter measurements on DUTs with as many as 72 signal ports, using a single set of test-port connections.

The switch matrix employs a modular design in which a pair of 2 x 36 port switch matrices in the test head are connected to a 2 x 12 port switch matrix chassis, which then connects to the VNA. The 2 x 36 pole matrices are constructed from a half dozen low-loss single-pole, six-throw (SP6T) electromechanical switches. The matrix was designed for expandability. By adding four more 2 x 36 port matrices, a total of 72 + 72 + 72 or 216 ports can be handled by the switching system and fed to a two-port VNA for extreme measurement flexibility.

A full multiport measurement system consists of the model M144 2 x 72 port test interface described above, a commercial microwave VNA, the 2 x 12 port switch matrix combiner chassis, two 2 x 36 port switch matrix combiner chassis, a system personal computer (PC) running proprietary control, data acquisition, and dataprocessing software, and a thermal control system. The system components are housed in a single 19-in. rack-mount enclosure with casters for portability (Fig. 1). The system can be configured with commercial microwave VNAs from leading suppliers, including Agilent Technologies, Anritsu Co., and Rohde& Schwarz.

The initial test system configuration was constructed for a specific customer, with primary interface test fixturing customized to permit simultaneous mating and demating of the customer’s DUT. The test system configuration was also designed for real-time temperature cycling during testing. Because a microwave VNA requires test channels that are precisely matched in amplitude and phase for accurate S-parameter measurements, the semi-rigid RF/microwave coaxial cabling within the combiner chassis had to be configured with the shortest and most consistent lengths possible. To maintain consistent amplitude and phase throughout the many signal paths, each cable-to-connector junction was formed with a similar bend shape (Fig. 2).

The test head attaches to the combiner chassis by means of 36 screw-on connectors with semi-rigid cables. The test head was designed for maximum accessibility to tuning rotors on the DUT for real-time port calibration capability. The test head features camdriven clamps to seat or unseat the test head onto the DUT (Fig. 3). When the test head is lowered, it simultaneously mates 62 ports on the customer’s DUT to the measurement system. In order to improve the connector density with so many signal paths, the interconnecting semi-rigid cable assemblies alternately flare to opposite sides of the top plate. Again, the short and consistent lengths of these cable assemblies help minimize test system phase errors. For temperature cycling, a thermal plate is mounted underneath the DUT. During testing, hold-down plates are retracted, allowing access to tuning rotors. The hold-down plates are moved into position and clamped in place to secure the DUT during demating.

One of these multiport measurement systems is already at work in the field. That system can evaluate DUTs with as many as 63 coaxial ports. The system is currently being used to make measurements on a power combiner assembly with 60 coaxial input ports and two coaxial output ports. Without the multiport measurement system, and using a conventional two- or four-port microwave VNA, the task of testing the performance of such a complex passive assembly becomes a long and tedious process.

For example, the electrical requirements for this power combiner call for extremely tight amplitude and phase matching among input-to-output signal paths. Performing electrical measurements with a standard twoor four-port microwave VNA, and manually switching ports at the VNA, is not trivial. Manually, with 60 ports to be tested, and for S-parameter measurements referenced from each port to every other port, it is necessary to remove a termination from one port, attach a test cable that was connected at the other end to the source power port on the VNA, make the measurements, and then replace the termination after the measurements.

The process requires weeks of test time, not to mention the labor from operator involvement. Because connectors must be mated and demated from different coaxial ports on the DUT each time a different set of S-parameters are measured, measurement repeatability can also be an issue.

With the In-Phase Technologies multiport measurements system, a full set of accurate and repeatable measurements on the 60-input power combiner can be performed in about six minutes. During the measurements, the switch matrix automatically terminates all required ports prior to a measurement.

The custom-designed 2 x 72 port switch matrix makes it possible to apply a signal from a VNA’s test source to any port of a complex DUT and connect any other port to the VNA’s test port under automated control. In that way, forward and reverse Sparameters can be measured from any one port to any other port of a complex DUT using a standard twoport VNA. The multiport measurement system is designed to eliminate the manual coupling and decoupling of coaxial connectors to a test set for forward and reverse S-parameter measurements which, for multiport devices, grow rapidly more complex with the number of ports. For example, applying the “squared port” rule, a device with as many as 72 different RF/ microwave signal ports would yield as many as 5184 different S-parameters, and quite a few test and source port changes for manual measurements.

SOPHISTICATED SOFTWARE
Precision hardware is only part of the multiport measurement system. The system is supported by flexible software that performs a number of functions, including automatically controlling the switch matrices, coordinating measurements with the VNA, and acquiring and formatting test data to provide meaningful measurement results.

Given the complexity of making S-parameter measurements on a DUT with a large number of ports, such as a 60-input power combiner, naming those S-parameters for DUT with so many ports can become unwieldy. The proprietary software that drives the multiport measurement system allows an operator to set up a switch map to simplify measurements on complex DUTs and better visualize the signal-routing plan. The software enables a user to set up signal paths and switch closures that are defined by easy-to-remember names or in the convention of a particular product or company’s nomenclature. The software allows completely arbitrary naming of ports. Users can fall back on conventional port naming used in two-port systems, such as S21 and S11, or can call one port the “input port” and another output port 1, 2, 3, etc. The port-naming convention can be relevant to the particular unit that is being tested. Any number of test configurations can be set up and saved to a file for later use.

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