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).
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).
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