The Need For Large Signal Measurements at mmW and Sub-THz Frequencies
Performing device characterization measurements at millimeter-wave and sub-THz frequencies can be challenging for several reasons. First, it’s difficult to achieve accurate and repeatable power-control at the DUT reference plane when using waveguide extenders between 110 GHz and 1.1 THz, as the extenders typically output a fixed power. However, controlling the power delivered to the input of the DUT is critical in setting the device’s operating conditions and characterizing its performance over power, such as the large-signal characteristic gain compression. Second, load pull, which means changing the load impedance presented to the DUT to an arbitrary non-50Ω value, is difficult as passive load pull impedance tuners are typically unavailable above 110 GHz, and even then, are limited in their ability to present high mismatches at the DUT reference plane.
Load pull measurements are necessary for transistor designers to properly characterize and model the high-speed behaviors of their devices. For circuit designers, load pull measurements are used to determine the ideal matching conditions and optimize performance, at powers where every fraction of a dB is important. Load pull can also be used to test systems, such as new mm-wave radars, where both contacted and over-the-air performance testing is needed.