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Minimising small cell manufacturing cost

Posted: 09 Dec 2014 ?? ?Print Version ?Bookmark and Share

Keywords:small cell? Multi-DUT testing? PXI? base station? RF verification?

The test executive software itself 每 which controls the test flow 每 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.

 Small cells

Figure 5: Simplified setup for testing multiple small cell base stations in parallel.

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 每 a cellular base station 每 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.

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