RF switches for PXI begin proliferation

Growing interest in PXI as an RF test platform has stimulated demand for high-density RF switching modules. The result has been a jump in the range of switch configurations available, with one drawback: The limitations of the PXI panel size can quickly force system developers into a tradeoff between performance and density.

Even with the limited panel space, high-density connectors allow single-slot PXI modules to provide RF multiplexers with as many as 16 channels. Courtesy of Pickering Interfaces.

This need has prompted a dramatic increase in the number of offerings and range of configurations available for PXI RF switches. National Instruments, for example, nearly tripled its offerings during 2007. “We went from four switches to 15 last year,” said NI's switches product manager Jaideep Jhangiani. “At 3 GHz, for instance, we started 2007 with only a 4x1 switch module. We have now added a dual 4x1, an 8x1, SPDT [single pole, double throw], and terminated dual-SPDT configurations.”

Pickering Instruments is offering an even more extensive range of products. “We have 30 to 40 products in a variety of configurations,” said the company's marketing manager, Bob Stasonis, “and that's a big enabler for building RF test systems. A few years ago, we didn't have that capability.” Other vendors of PXI RF switches include Aeroflex, Geotest, and Dow-Key Microwave.

Further growth in RF switching options faces a significant challenge, however, because of the space constraints on PXI cards. “PXI gives you a lot of functional density in a rack,” said Mike Dewey, Geotest's senior product manager, “but it also has 0.8-in. spacing.” This presents a particular problem for switching signals in the microwave end of the RF spectrum (typically considered to be frequencies above 5 GHz by switch vendors), where coaxial switches are the only option.

“You used to not be able to even fit a coaxial switch in a single-width PXI module,” said Dewey, “but that is now changing. Vendors are downsizing their switch modules so you can fit a 1x2 in single width and a 1x4 in two slots.”

The situation eases only slightly at frequencies in the 3-GHz range. Here, module vendors are able to use the smaller electromechanical relays in the signal path and are thus able to fit more switches onto a single-width module, but panel space still represents a significant limitation. This forces RF test engineers to choose between switching density and performance.

Two key performance metrics for RF switching are insertion loss through the circuit and return loss due to reflections at junctions within the switch. (Vendors remind developers to account for these losses throughout the entire circuit, not just in the switch module.) While careful layout and fabrication of the printed-circuit board within a switch module can help minimize these losses, the connectors themselves play a significant role.

Moving from (left) a BNC connector to something like (right) an MCX connector offers greatly increased front-panel connector density although at the cost of performance and repeatability. Courtesy of National Instruments.

Vendors have responded to these limitations in different ways. Geotest, for instance, has chosen to concentrate on higher performance, and thus primarily uses BNC and SMA connectors. “We try to stay away from exotic mechanisms,” said Dewey. “We may get lower density but also have a more robust connection.”

Pickering Interfaces, on the other hand, has embraced the more compact SMX and SMB connectors. “We moved to higher-density connectors to lower the cost per channel,” said Stasonis. “This now allows us to fit a 16-channel RF multiplexer onto a single 3U module.”

The use of SMX and SMB connectors to achieve higher-density systems can present a challenge for test engineers who are unfamiliar with them, however. The engineers may need to create custom cabling or interface panels to match the switch module with the more traditional connectors on their other instruments, without having the experience or tools necessary for the task. Module vendors can help by providing support in designing or even fabricating the necessary interface panels and cables.

Regardless of the connector approach a vendor has taken, the expectation is that more RF switching options will be forthcoming as application needs grow. In addition to 50-Ω switches for traditional RF, vendors are increasing their 75-Ω offerings for high-frequency video and set-top box test. Pickering has recently introduced a balanced 100-Ω switch for Gigabit Ethernet, as well.

The bottom line for test engineers is to check with their vendors when developing a test system to learn whether any new switch options are forthcoming. “Talk with the vendors with the specs in hand,” said Stasonis. “That way you can get the best of what is available.”