Multi-DUT PXI approach reduces small cell manufacturing cost

Technology News | November 23, 2014

By eeNews Europe

Due to the growth in small cell deployments, base stations are becoming an increasingly high-volume product. As a result, challenges such as reducing the cost of test and maximizing test throughput are becoming increasingly important in device manufacturing. Multi-DUT testing can provide great cost and throughput benefits. This article talks about the major aspects of setting up multi-DUT test systems and also discusses the advantages of the PXI modular platform to implement such systems.

Small cells – high-volume production demands low test cost

As the adoption and usage of smart phones continues to rise, mobile network operators see the demand for data rate and coverage growing – exponentially (Figure 1). Cisco’s Visual Networking Index estimates that from 2013 to 2018 there will be an 11-fold increase in global mobile data traffic. A view commonly held in the industry is that by 2020 operators must improve mobile network capacity 1000-fold, compared to the beginning of the decade to meet market demands.

Figure 1: Mobile data traffic increases exponentially. Click image to enlarge.

Smalls cells are a critical technology in addressing the challenge of delivering higher network capacity. Their coverage area is much smaller than that of traditional “macro cells” (couple of 100 meters versus several kilometers). Small cell base stations operate at lower transmission power, have a smaller size and cost less than macro base stations. However, mobile operators will install – and vendors must manufacture – them in much larger numbers than their bigger counterparts (Figure 2).

Figure 2: Small cells can provide cost-effective coverage and vast capacity in dense urban areas and indoor environments. Click image to enlarge.

A number of engineering challenges arise from testing small cell base stations in a high-volume manufacturing environment – and one of the largest concerns is the cost of test.

A big factor in lowering test cost is to increase test throughput. Increase in throughput improves overall production output – and one of the most basic ways to improve throughput is to use high-speed test equipment. Another important factor test engineers should consider is how much their test equipment is utilized.

Utilize your equipment

One of the most important first steps in optimizing a manufacturing line is to better utilize test equipment. In typical manufacturing test of traditional base stations, expensive components such as spectrum analyzers, vector signal analyzers and generators frequently sit idle for long periods of the time. One of the first steps to maximizing test equipment utilization is to use advanced test executives such as TestStand to implement optimized test procedures that allow pipelined, quasi-parallel testing of multiple devices. To fully appreciate the importance of this approach, let us first have a look at the steps taken in typical manufacturing test and then consider specific aspects of multi-DUT testing in more detail.

Anatomy of a typical manufacturing test

Each produced base station goes through a series of manufacturing test steps that characterize and verify the device (Figure 3). First, the test stand operator loads the unit into a fixture, where the device boots, and may perform a self-test or load firmware. The next step is to calibrate the RF frontends. Then, typical RF transmitter and receiver tests verify items such as delivered output power, frequency error and linearity against specified limits to produce a pass or fail verdict.

Figure 3: Phases in small cell base station manufacturing tests and typical relative duration.

The majority of test plans for base stations will execute each measurement sequentially – performing the entire suite of test steps on a single device before moving on to test the next unit. The problem with this approach is that RF test equipment is used only a fraction of the time to do calibration as well as transmitter and receiver tests. Depending on the complexity of the product and the test plan, RF measurements typically take between 30 and 50 percent of the time; the other 50 to 70 percent the RF equipment remains idle (Figure 3).

For large base station equipment that is capable of spanning macro cells, poor test equipment utilization might be acceptable: Such products are made in moderate volumes and typically sell for a lot more than a small cell base station. Small cell base stations more resemble a consumer-grade product in that they are made in larger quantities and come at a lower price than macro equipment. Multi-DUT testing is a cost-efficient approach used by handset manufacturers for some time now. The same technique holds substantial benefits for small cell testing as well.

Multi-DUT testing

Multi-DUT testing (also known as “multi-site”) is an advanced test procedure that has the explicit goal of maximizing the utilization of the more valuable test assets such as RF vector signal analyzers. Implementing a multi-DUT test plan requires test engineers to reorganize their test sets and procedures such that they can pipeline the DUTs through the individual test phases (Figure 4). From these modifications, they can reap tremendous benefits in utilization of the instrument and, ultimately, in the cost of test.

Figure 4: Parallel testing and auto-scheduling reduce test time and, thus, increase test throughput. Click image to enlarge.

There are several questions one may ask in this process: How many devices should be tested in parallel? How must a test be structured for efficient pipelining of the individual phases? What about software and processing requirements? What is a good switching concept to route all the relevant signals from the multiple DUTs to the test equipment? Let us address each of these questions in a bit more detail.

How many DUTs?

Choosing the ideal number of devices to test in parallel is depends upon the time allocation of the current test plan. As an example, consider a traditional configuration for testing a single device at a time and where the RF instrument is active half of the time. Then, ideally, one can use this instrument during its idle time to test another device. This way, things like the first DUT booting can happen whilst the second device is measured and vice versa. Typical setups handle between two and ten devices. The exact number and how much more test throughput one can achieve may only be evident after careful inspection of the test procedures. For example, one should identify bottlenecks in the setup and software in terms of data bandwidth and the possibility of parallelizing individual test steps. Let us consider this next.

Writing and running multi-DUT tests

In order to efficiently pipeline a manufacturing test plan, one must carefully examine dependencies between individual steps that may use the same instrument. For example, the test procedure may use a vector signal analyzer for calibration as well as for transmitter testing. Then, if the test executive software performs transmitter tests on the first DUT, it must delay calibration procedures for a second DUT until the tests on the first device conclude. Modern test executives such as TestStand can handle such dependencies but rely on test designers to modularize their test code appropriately. The basic structure should be along the broad general test phases identified earlier: DUT loading and start-up, calibration, and RF verification. Within each phase, the designer should break up the code further to identify all instrument calls. Some examples are the steps of signal generation, capturing RF data on an analyzer instrument and subsequent processing on a computation unit. Structured accordingly, the test code consists of several sections that can run in parallel and several sections for which execution – that is, access to the measurement instrument – must be scheduled in sequential order.

The test executive software itself – which controls the test flow – is another important element of optimizing test time. For multi-DUT testing, one will likely require the possibility for multi-threaded sequence execution. This is useful in instances where the actual acquisition of measurement data by the instrument is much faster than the subsequent analysis on a processing node. In such cases, the analysis phases of multiple tests, if run subsequently one-by-one, would limit the test throughput and cause the instrument to run idle unnecessarily. Parallelization of these processing phases avoids this bottleneck because data acquisition can proceed while the analysis of previously taken samples still runs on. Multi-threading allows test designers to write simple code for individual test steps such as measurement acquisition and subsequent analysis, and allows the test executive to handle parallel processing of multiple measurements. Naturally, parallelization also requires multi-core computing resources like those available with state-of-the-art PXI controllers.

Auto-scheduling is another feature of advanced test executives. The software itself changes the order of execution of self-contained tests or test steps to maximize instrument utilization and throughput (Figure 4).

The widely adopted NI TestStand has all these powerful features. As off-the-shelf test management software, it allows test designers to easily update test sequences and enables them to re-use test code for future DUTs.

Multi-DUT switching

Another key element in optimizing test systems is the switching of signals from parallel DUTs to the instrumentation. In a manufacturing test environment, so-called fixtures are used to quickly and securely connect all of the relevant interfaces of a DUT with the test circuitry. In the small cell context, a typical fixture would provide multiple RF ports for cellular, WiFi, and GPS technology, and Ethernet and DC connectors would control and power the DUT. Such equipment comes from specialized vendors and may require substantial customization to fit individual base station designs.

In a multi-DUT configuration, test engineers must add signal switching components to their equipment to successively connect one of the multiple DUTs to the instrument (Figure 5). In addition to the RF signals whose parameters are to be measured, there is typically a frequency reference to be shared and trigger signals to be propagated from the DUTs to the test set. Now, let us go into more detail on these switching requirements.

Figure 5: Simplified setup for testing multiple small cell base stations in parallel. Click image to enlarge.

When testing RF ports, one must consider that the nature of testing both transmit and receive signals often requires the use of bi-directional switching. When selecting switches for these signals, engineers should ensure adequate isolation between the signal paths to prevent any interference from impacting measurement results. To that end, apart from a good isolation value itself, the possibility to programmatically terminate the switched ports is extremely helpful. Test engineers will also look for a low voltage standing wave ratio (VSWR) value because this determines the final measurement accuracy through the amount of reflection the switch introduces.

Of course, the switching components must meet all of these requirements within the required frequency range; for a small cell, this includes the 3GPP operating bands for cellular standards, may also encompass 2.4 and 5 GHz WiFi, and perhaps even GPS/Galileo/GLONASS frequencies around 1.6 GHz.

Note that a true RF signal switch simply connects one of its input ports to one of its output ports. This topology allows engineers to take measurements on a single device at a time only. By contrast, products like a combiner/splitter can be used to feed the test signal simultaneously to multiple DUTs to verify their receivers in parallel. This is a common technique used in handset testing. However, generally, this is not possible for base station testing where the DUT – a cellular base station – dictates the timing of when it transmits and expects to receive signals, just as it would in a real cell. In that case, test engineers cannot reproduce the framework to make all the base stations under test align their frame timings. Consequently, it is typically not possible to use a single signal generator to test the receivers of multiple base stations in parallel.

Because base stations expect to receive signals at a time they dictate, one must ensure tight synchronization between the DUT and the signal generator for base station receiver tests. Base station designers can simplify manufacturing tests by providing an output port and a corresponding trigger signal to indicate the start of a frame or similar temporal structure. Then, the signal generator aligns its transmission timing with the DUT’s trigger without any need for a time-consuming and error-prone synchronization procedure between the base station and the test equipment. A multi-DUT base station test set should provide input ports for the trigger lines of all DUTs and/or switch between them.

Multi-DUT with PXI

Setting up a multi-DUT test set is relatively straightforward to do with a modular platform such as PXI. “Modularity” in this context refers to a user-defined selection of components where each component – called modular instrument – has a specific purpose. With modular instrumentation, engineers can match test capabilities to their needs much better than with a “one-size-fits-all” traditional instrument.

PXI/PXI Express is one of the most common platforms for modular instrumentation. Since its invention in 1997, PXI has developed into the most prevalent modular instrumentation platform, with over 1,500 modules available from more than 70 vendors. Using PCI bus technology, PXI offers high bandwidth and low latency data transfer to and from instruments, helping to improve test speed (Figure 6).

Figure 6: NI PXI modular instruments and extensive RF measurement software including 3GPP, WiFi and GPS testing lower the cost of test for small cell base stations.

Example modular instruments include RF signal generators, spectrum analyzers, and – relevant to multi-DUT testing – switches. PXI switches and combiners achieve high analog quality and high density – and are relatively inexpensive compared to the cost of the actual measurement instrument.

One of the most significant benefits of PXI multi-DUT testing is the measurement speed allowed by the PXI platform. Here, the combination of a high-speed data bus (PCI express) and highly capable signal processing technologies (multi-core processors) enables PXI instruments to perform most measurements three to ten times faster than traditional instruments.

Conclusion

Driven by demands for improved network capacity, small cell deployments are becoming increasingly popular with network operators. Consequently, small cell base station vendors are required to increase their manufacturing test throughput and to lower their cost of test. Higher test throughput and lower test cost require new test approaches in the wireless infrastructure industry. Multi-DUT testing helps engineers to improve test equipment utilization and, in turn, throughput and cost.

PXI offers many benefits for small cell testing and its modular architecture lends itself well to the multi-DUT approach. TestStand is a powerful test executive that provides advanced features such multi-threading and auto-scheduling to make productive use of the hardware capabilities.

Setting up a multi-DUT test stand is not for free – it takes some additional hardware, perhaps software upgrades, and test designers must exercise more care in writing their tests – but the speed and throughput benefits far outweigh the slight increase in upfront effort.

Thomas Deckert is a Senior Systems Engineer at National Instruments, www.ni.com.