PXI multiport vector analyser for fast multiport network analysis

20 April 2015

More modern components are including different functionalities integrated into a single component.

Doing so greatly increases the number of ports on these devices; while also increasing their complexity and that makes multiport characterisation all the more critical to ensuring the components function as expected in the real world. 

The challenge for test engineers is to balance the need to perform that multiport network analysis with an increased number of measurement parameters, against the pressure from high-volume manufacturers to minimise total test time. 

Fortunately, modern instrumentation such as a PXI multiport Vector Network Analyser (VNA) is now offering test engineers a viable option for both performing accurate multiport measurements and improving throughput.

Understanding the measurement challenge

The conventional means of performing multiport network analysis involves the use of a VNA with an external switch matrix. With this multiport solution, signals from either a 2- or 4-port VNA are distributed via RF switches in an external switch matrix. Required measurement paths are then chosen sequentially by simply selecting the ports of the switches. The switches utilised may be either solid-state or electro-mechanical, depending on the level of measurement performance required. 

While this switch-based solution does provide test engineers with a low-cost way to increase the number of ports on the VNA, it also has a number of drawbacks. To begin with, it relies on sequential measurements to obtain all of the S-parameters in a multiport Device under Test (DUT). These measurements can take a great deal of time due to many frequency sweeps being required to measure the full set of S-parameters, and this significantly slows throughput during production testing. Even more problematic, however, is that the attenuation in the switch matrix degrades the test system’s performance (e.g., dynamic range, trace noise and temperature stability) as compared to a standalone VNA. That degradation is especially significant in high-frequency applications above 10GHz. With the trend toward multiport components showing no signs of abating, such drawbacks have left many test engineers eagerly seeking an alternate solution.

Eyeing a better solution

Thanks to advances in both VNA technology and modular PXI instrumentation, modern test engineers now have an alternate solution for fast and accurate multiport network analysis; one that offers improved throughput with more flexible text configurations than the conventional switch-based approach. The solution relies on a VNA, built on a PXI platform, which can be configured for multiport measurements. It does not require any external switching.

The PXI platform provides the VNA with greater flexibility and scalability, allowing it be easily configured to measure DUTs with different numbers of ports (e.g., 2, 4, 6, or more, up to 32). The use of the PXI platform also helps test engineers realise space and cost savings, while also delivering the high performance and speed needed to ensure fast and accurate measurements. 

As an example of a VNA that can be configured for multiport measurements consider the left-most block diagram in Figure 1. The PXIe VNA shown is a full 2-port VNA that fits into one PXI slot. It can be configured as a multiport VNA with multiple PXI modules installed in the same chassis. Jumper cables are used to interconnect the modules in the chassis to synchronise the LO, reference, and trigger signals for multiport measurements. Assuming each VNA test port has independent reference and test receivers for the multiport measurements, the S-parameter data for all measurement paths can be captured simultaneously.

Using a multiport test solution such as this provides test engineers with a number of critical benefits. For example, the ability to simultaneously capture data with multiple receivers, rather than through sequential measurements, enables faster measurement speed and throughput. The cost-of-test is also reduced because test engineers can simultaneously characterise many devices using just a single PXI chassis. As there is no attenuation between the DUT and measurement receivers in this configuration, the multiport measurements are both highly accurate and stable. In fact, with no loss due to the use of external switches, the solution delivers higher performance than a standalone VNA. 

One area where this performance edge is apparent is the test system’s dynamic range. The multiport VNA realises a much wider dynamic range in the higher frequency region than its switch-based counterpart. That means that when using the multiport test system, a wider Intermediate Frequency (IF) bandwidth (IFBW) can be selected to achieve the same trace noise. If the dynamic range is increased by 20dB, for example, then a 100-times wider IFBW can be selected and 100-times faster measurement speed can be achieved, with the same trace noise, to get the same measurement result.

Multiplying throughput with a multi-site capability

Another key advantage the PXI-based VNA solution has is the ability to easily make multi-site or parallel measurements; that is, to simultaneously test multiple DUTs using the same test station. Multi-site operation of a PXI VNA provides simultaneous sweeps of different measurement paths (e.g., high band and low band) of multiport components. Unlike sequential measurements by a switch-based solution, the PXI-based VNA can test multiple or different devices simultaneously (Figure 2). As an example, DUTs with 2-, 4- or more ports can be tested at the same time with PXI modules identified under a single PXI controller. Simultaneous measurements of different devices or different measurement paths of a single multiport component can be run with optimised stimulus setup (e.g., frequency, power level, IFBW, number of points, etc.) to increase the overall measurement throughput for production testing. 

By simultaneously sweeping inside multiport DUT, throughput is doubled and the cost of manufacturing test reduced, along with the capital expense from test stations. Designers also gain greater flexibility than is possible with conventional solutions.

Old versus new

While it’s easy enough to list the key advantages the PXI multiport VNA solution has, the proof lies in the details of how each solution performs those measurements. As an example, consider that a 2-port VNA with switch matrix needs to make a forward and reverse sweep from both test ports for accurate measurements with full 2-port calibration. If a 4-port VNA with switch matrix is used, multiport error correction is applied by making a series of four sequential measurement sweeps from each test port. The multiport VNA, however, captures data with multiple receivers, one for each test port. As a result, measurements can be completed much faster and with significantly less sweeps. 

Another way of looking at this is that to measure an 8-port device using either a 2- or 4-port VNA with external switch it would require 56 and 24 sweeps, respectively, to obtain all S-parameters. Using a multiport (8-port) VNA, the same measurement would require only 8 sweeps. The difference is even more dramatic as the number of ports on the DUT increases. To obtain all the S-parameters for a 24-port device, for example, it would take 552 sweeps with the 2-port VNA, 264 sweeps with the 4-port VNA and a mere 24 sweeps with the multiport (24-port) VNA (Figure 3).

Now consider temperature stability. When solid-state switches are used in the switch matrix of a switch-based test system, the system becomes subject to temperature variation in the environment. To correct for the drift errors caused by the switches, frequency calibrations must be performed. The same is not true for a multiport VNA test system. It does not rely on external switches and therefore, the need for periodic recalibration on multiple test ports is reduced or in some cases, eliminated altogether. The result is a dramatic reduction in overall measurement time.

As more ports are added to components, the need for fast and accurate multiport network analysis will only become more acute. Fortunately, a multiport VNA solution based on a PXI platform provides the performance, speed, multi-site measurement functionality, and flexibility today’s engineers need to quickly and reliably test multiport components. Such capabilities are critical to realising improved measurement throughput in high-volume production testing.

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