True software instrument set to revolutionize RF test: Page 3 of 4

August 15, 2012 // By Jean-Pierre Joosting
Moore’s law has been a driving force in electronics since the first integrated circuit. However, test and measurement has not benefitted to the same degree — largely dominated by vendor-specific test boxes and modules. Software-based instruments exist today but such instruments are still designed with a view to the vendor-specific test paradigm and remain programmable in a limited sense. Four key factors define the effect of Moore’s law: smaller size, faster operation, lower power consumption and higher integration with increased functionality. This process is illustrated by the evolution of the first wireless handset through to today’s smartphones.
to test standards such as 256 QAM 802.11ac with an error vector magnitude (EVM) of better than -45 dB (0.5%) at 5.8 GHz. Compared to the current industry ‘gold standard’ this represents an improvement of 3 db in EVM, along with an improvement in measurement speed by 20 fold.

Testing power amplifiers

To accurately calibrate a PA, a power-level servo feedback loop is used to determine the final gain. Power-level servoing captures the current output power with an analyzer and controls the generator power level until desired power is achieved, which can be a time-consuming process. In simplest terms, it uses a proportional control loop to swing back and forth in power levels until the output power-level converges with the desired power. A VST is ideal for power-level servoing because the process can be implemented directly on the user-programmable FPGA, resulting in a much faster convergence on the desired output power value (Figure 2).

Figure 2: Power-level servoing with a VST results in much faster convergence on the desired output power. Click image to enlarge.

The software instrument approach implemented in the VST reduces the test time of typically 5 seconds per measurement in this case to around 5 ms. This represents an improvement of three orders of magnitude by just moving the instrument into an FPGA (Figure 3).

Figure 3: Screenshot showing the performance improvement achieved using a VST versus traditional test setup. Click image to enlarge.

2x2 MIMO channel emulation

In this test setup, engineers can now program fading models used to simulate air interference, reflections, moving users, and other naturally occurring phenomenon that can hamper an RF signal in a physical radio environment into the FPGA to implements a real-time radio channel emulator.

Figure 4 shows a 2x2 MIMO radio channel emulator implemented using two VSTs in LabVIEW. Settings for the fading models are shown on the left and in the center of the screen. The resulting

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