Analog/RF Playbook Blog

Choosing between multiport VNA and modular VNA

Testing devices with a large port count (beyond 8 ports) or many separate devices/signal paths simultaneously is the new reality of many automated test and laboratory testing scenarios. The explosion of multi-input/multi-output (MIMO) and beamforming antennas systems are only part of the reasons behind this.

There has also been a massive increase in the devices within wireless modules, which ultimately need to be quality tested post-production as well. Also, many wireless communication technologies, such as 5G/6G and Wi-Fi, are expanding the number of bands. These multiband systems also need to be efficiently and accurately tested, not only for characterization, but also for quality and conformance testing.

Multiport testing solutions

The solution to some aspects of these testing challenges are vector network analyzers (VNAs) with more than the typical number of ports. On one hand, there are VNA systems available now that have a large number of ports. However, there is always the option of commissioning a custom VNA system with even more ports. Such massive VNA units simply require source and receive electronics and a measurement bridge (directional coupler) for each port, as well as the ability to process and store the data obtained from testing.

Figure 1 Vector network analyzers are available in various configurations regarding port count for specific testing solutions. Source: Rohde & Schwarz

Such a large port-count VNA can also be achieved using more recently available modular VNA architectures, the most common of which are based on a PCI/PXI system. These modular VNAs can be extended to whatever the maximum slot count is of the PCI/PXI base unit. Some of these modular VNAs also allow for multi-site operation, so they could theoretically be extended to many tens if not over 100 ports.

The other option is to extend the port count of an existing VNA using switch matrix solutions or VNA port extension units. VNA port extension units are just switch matrices that allow for full-crossbar operation, where there is a path among every port in the unit. This involves the inclusion of measurement bridges (directional couplers) at the output of each of the test ports of the switch matrix. In order to do this, a VNA must have accessible stimulus (source) and receiver ports available to connect to the VNA port extension unit.

Comparing multiport testing solutions

Both a true multiport VNA and a modular VNA equipped with a high port-count are generally expensive. However, these units are likely turnkey solutions and come with built-in software and methods that assist with intrinsically complex multiport VNA calibration. Such units are also necessarily large, as they must house and provide thermal management and interconnect for a stimulus, receiver, and measurement bridge for every port.

Figure 2 Both multiport VNAs and modular VNAs generally come equipped with built-in software. Source: Keysight

The true multiport VNA will most likely offer the best performance. Meaning, that a true multiport VNA will have the highest dynamic range, best measurement accuracy, and may possibly be the fastest test solution. A custom multiport VNA built specifically for extremely high port counts could theoretically outperform other multiport VNA solutions. However, a custom VNA would also require custom methods of capturing, processing, storing, and presenting data. A custom VNA unit is also potentially the most expensive solution and may also have the largest footprint.

Modular VNA units tend not to have as good of performance as true multiport VNAs, as trade-offs to reach a compact modular design often result in slightly lower performance than a big-box VNA. Also, for some modular VNA units, extending the port count beyond a certain amount may necessitate purchasing software licenses or requesting custom software to accommodate such large port counts.

The least expensive and likely most readily available solution is to extend the VNA ports using switch matrices or VNA port extension units. These units do require additional software and control methods to switch between all the necessary states and mandate significantly complicated calibration. Moreover, addition of switch matrices after the VNA ports measurement bridge add losses and will also degrade measurement stability.

Using a VNA port extension unit helps to mitigate some of the loss of measurement stability by placing the switch matrices behind the measurement bridge. However, some dynamic range is lost either way and there will be added noise in the measurement system.

Jean-Jaques (JJ) DeLisle, an electrical engineering graduate (MS) from Rochester Institute of Technology, has a diverse background in analog and RF R&D, as well as technical writing/editing for design engineering publications. He writes about analog and RF for Planet Analog.

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