The increasing importance of increasing ports: addressing the challenges of multiport test
01 May 2020
Figure 1. VNA with simple switch test set
Modern electronic devices are highly integrated. As a result, wireless and high-speed digital engineers often test devices with more than four ports.
This article was originally featured in EPDT's 2020 PXI for T&M supplement, included in the May 2020 issue of EPDT magazine [read the digital issue]. Sign up to receive your own copy each month.
In wireless RF, front-end modules (FEM) for multi-band operation and multiple-input, multiple-output (MIMO) antennas require multiport characterisation for all their components. Testing high-speed digital technologies like HDMI and USB 3.1 is even more tedious. Not only do digital cables have multiple internal cables and connectors, but each must undergo testing twice – once in the time domain and once in the frequency domain. The need for multiport test accelerated the development of switch-based solutions for traditional vector network analysers (VNAs). As Matt Campbell, Product Marketing Engineer at T&M specialist, Keysight Technologies tells us, when switch-based solutions were not adequate to keep up with multiport test, the VNA itself was re-imagined and optimised for multiport test.
The traditional solution for multiport test is switching matrixes for 2-port or 4-port VNAs. Simple switching test sets comprised of RF switches can route VNA ports to port pairs on the DUT. In this setup (see Figure 1), 12 ports branch from test port 1 of the VNA, and 12 ports from test port 2. Any path between the port 1 switches and the port 2 switches is measurable. However, many multiport devices require a measurement from each port to every other port. In this simple switch matrix, no path exists between switches that branch from the same VNA port. Therefore, this 24-port setup supports only 144 paths – but a 24-port device has 276 paths.
Full crossbar switching test sets measure between every port of a device, as well as apply a load termination to unused ports. However, this introduces new challenges. Each port has two possible switches to use as a load termination. The switch used for termination on each port varies depending on which ports are active. The varying termination of the ports makes full N-by-N calibration difficult, because you need to calibrate for every case.
External switches significantly affect measurement performance. The switching hardware is beyond the VNA’s directional couplers that sample the test signal. Interference from the switching hardware creates a mismatch between the signal reaching the DUT and what the VNA measures. Calibration can help, but a switch matrix can never achieve the dynamic range, temperature stability and trace noise performance of a standalone VNA.
Modern multiport devices demand faster and more accurate measurements than a switch-based solution can give you.
The new way: modular
Multiport modular vector network analysers (VNAs) are purpose-built to address the challenges of testing modern highly integrated devices. Based on the PCI Extensions for Instrumentation (PXI) architecture, multiport VNAs provide exceptional measurement performance, no matter how many ports you use.
Figure 2. Sweeps required for multiport devices
Modular VNAs have three key advantages over traditional solutions: scalability, speed and performance.
Traditional switch-based solutions require configuration at the factory, and are not easily customisable. Upgrading your switch setup means returning it to the manufacturer, or completely replacing the setup. This process increases your cost of test, due to downtime, and limits your flexibility.
Multiport solutions are fully scalable – meaning you buy only what you need, and then upgrade at any time. Modern multiport VNA chassis support up to 50 ports in a single chassis, and you can start with just two ports and add as many as you need.
In manufacturing or design verification test environments where throughput is crucial, you can configure multiport VNAs for multi-site operation. Multi-site VNAs provide parallel measurements of multiple devices at the same time.
Since every PXI card is an independent network analyser, you can custom configure multi-site measurements to fit your requirements. If you have a chassis with 24 ports, you can have two 12-port VNAs, twelve 2-port VNAs, or any combination in-between.
An N-port device has N2 S-parameters – meaning the number of S-parameters for a device increases significantly with the port count. A simple 2-port filter has only four S-parameters, but a 24-port antenna has 576 S-parameters. If you have a 24-port switch matrix to test a 24-port antenna, you’ll need hundreds of sweeps. The switch matrix must route test signals to every port on the device.
PXI-based solutions provide full network analysers on every PXI card, so each port of your device has its own network analyser test port, with a source and receiver. The multiport solution simultaneously measures all the paths through the device, including the S-parameters.
Figure 3. Measurement applications on multiport VNAs provide complete characterisation, without rearranging connections
Multiport setups require significant calibration time. Switched test sets with solid-state switches are easily affected by temperature and must undergo frequent calibration to ensure accurate measurements. Calibration downtime drastically impacts throughput; a PXI-based system with longer calibration intervals reduces your test time and your cost of test.
True multiport setups maintain their measurement performance, no matter how many ports you use. As frequencies trend higher and margins for error shrink, your test setup’s measurement performance can significantly impact your yield.
Removing switches from multiport measurements does more than speed up the measurements. Setups without switches provide full VNA measurement performance. Attenuation from the switches degrades the dynamic range of switch-based multiport measurements, particularly at higher frequencies. By 20 GHz, switch-based solutions retain only half the dynamic range of multiport solutions. Today’s engineers need exceptional accuracy at the high frequencies of modern devices.
Modern device characterisation requires more than just S-parameter measurements. For example, high-speed digital interconnects require time domain analysis, and active array antennas require pulsed RF signals. Performing specialised measurements through a switch-based setup is slow, and may require additional instruments.
Flexible multiport VNAs provide fast and complete characterisation, without additional instruments or changing connections. Applications like automatic fixture removal to remove adapters and fixtures from your measurements, time domain analysis, and pulsed-RF measurements improve your workflow by consolidating different measurements into one instrument and test setup.
As device characterisation becomes more unique, and modern technologies push the limits of network analysis, you need a unique network analyser to address your specific challenges. Modular network analysers scale with you and allow you to configure your test setup exactly how you need it. Tailoring your network analyser to your measurement gives you unmatched speed and performance from basic S-parameters to advanced measurements.
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