March 24, 2020
Engineers typically use high-end vector network analyzers (VNAs) with broadband frequency coverage during development of high-speed devices operating at microwave and millimeter wave (mmWave) frequencies. When it comes to testing those devices on the production floor, however, many test applications do not need the level of performance provided by those expensive VNAs with banner specifications.
Finding the proper VNA for a specific production environment depends on several factors. Typically, manufacturing test is not as rigorous as in device characterization, making cost-of-test critical. On some manufacturing floors, VNAs are integrated into test systems where S-parameter measurements are a fraction of the test list. In these environments, only moderate performance is required.
Regardless of the production scenario, there are three overriding criteria that should be evaluated. In this post, we will discuss each – performance, durability/size, and ease-of-use.
A VNA must be able to make the measurements required in the respective production setting. Some applications do require the utmost in dynamic range and high-level noise performance to properly measure the device under test (DUT). For those applications, the right choice is a full featured, high-end VNA like the Anritsu VectorStar™ family of VNAs.
Many manufacturing applications do not need that performance level; VNAs simply must conduct pass/fail measurements to determine good devices from bad. In these scenarios, VNAs like the ShockLine™ family that provide very good performance with a targeted set of features can be used to reduce the cost-of-test without compromising quality.
Application techniques can help achieve better VNA measurement performance by optimizing setup parameters. For instance, a VNA’s dynamic range is typically maximized by using the narrowest available intermediate frequency bandwidth (IFBW) selection. Engineers must remember, though, that the narrower the IFBW the slower the frequency sweep. The result is reduced sweep speed and overall measurement throughput.
Other techniques include minimizing sweep frequency points and using only selected frequencies in a segmented sweep. These approaches can help offset the speed reduction when using a narrow IFBW but only to a limited extent. The VNA hardware must still have enough performance to allow the engineer to make the application tradeoffs to achieve the speed and measurements required.
Testing Close to DUT
Another way to improve the effective performance of a test setup is to reduce or eliminate the effect of interconnect and fixturing between the VNA and the DUT. One method is to bring the VNA port as close to the device as possible, minimizing or eliminating any cables used to interface to the DUT.
For example, the ShockLine MS46121B 6 GHz 1-port VNA (figure 1) is a power-sensor-sized device that easily connects directly to the DUT. This architecture eliminates the need for interfacing cables or fixtures - and their associated effects - between the VNA and the DUT. Measurement quality is improved as a result. Ultimately, this quickens device test time and reduces cost of test.
In a similar fashion, an instrument such as the ShockLine MS46522B E-band VNA (figure 2) tethers measurement modules to the VNA. The result is a single system whose architecture creates mobile VNA ports that can directly, or almost directly, connect to the DUT. This simplifies and improves the measurement quality by minimizing the detrimental effects of any interface cables, waveguides, or fixtures required to interface to the DUT, especially at mmWave frequencies.
When the VNA can’t be connected directly to the DUT, removing the effects of interface cables and fixtures will enhance S-parameter measurement performance and improve test margins in manufacturing. Simple techniques, like reference plane extension, can be used to remove the effects of a cable interface to the DUT. More complex setups require additional de-embedding.
Durability and Compactness
Manufacturing environments are typically tough on equipment. Buttons, screens, knobs, and any other human interface on instruments tend to get battered and broken, so ruggedness and durability are musts for a production VNA. Size is also a key factor, as space on a production floor is a valuable and scarce commodity. Instrumentation with a smaller footprint on a bench or in a rack lowers test cost.
Production VNAs with a headless architecture (no embedded keypads or screens) are more robust since the fragile human interface components are eliminated. They should also come standard with ruggedized test port connectors and a durable chassis, ensuring the unit can withstand a manufacturing floor.
Another criterion for a production VNA is how easy it is to program and use. For automated test setups using remote control programming, comprehensive SCPI commands and/or other remote interfaces are required. Reuse of program code is a key way to improve efficiency in transitioning devices from design to production as fast as possible.
VNAs that share a common GUI interface with comprehensive production functionality is also beneficial in manufacturing environments because operator training is minimized. GUI software should also be customizable to suit a particular production setting and the needs of various operators.
When looking at a production VNA, it is ideal to have software that enables expert users to create a guided, graphical step-by-step procedure for a given test setup and measurement. This will simplify complex VNA measurements by clearly guiding users through the process. An example is easyTest™ software from Anritsu (figure 3).
To optimize efficiency, engineers should be able to work offline with the VNA software. Running in simulation mode enables engineers to continue working on a project without using live hardware, freeing it for use by another engineer.
As noted here, there are many aspects to consider when properly selecting a VNA for microwave S-parameter testing in production. Cost-of-test needs must be balanced with performance requirements and different tradeoffs can be made depending on the specific application.
To learn more, download this application note entitled Factors in Choosing a VNA for Microwave Applications in Manufacturing.