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.
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.