A Programmable Cryogenic Waveguide Calibration Load With Exceptional Temporal Response and Linearity

We have developed a programmable, fast switching, accurate, and miniaturized calibration load for use in millimeter and submillimeter low-noise amplifier characterization, and Earth/planetary science radiometers. The proposed solution uses a thermally conductive waveguide vane attenuator with low thermal mass, integrated heater, and silicon thermometer. In the present design, we utilize a 125- $\mu$ m-thick z-cut crystal quartz vane due to its low dielectric constant (relative to silicon), high cryogenic thermal conductivity, chemical robustness, and small thermal contraction. To provide adequate attenuation, the bottom side of the quartz fabrication wafer has an nm thick resistive Ti metal layer deposited. On the top of the quartz wafer, a pattern of Au is deposited to allow adhesion of the heater resistor, thermometer, and internal heat strap. The z-cut quartz vane is mounted on three low thermally conductive Torlon posts, centered on the maximum E-field, and positioned across the waveguide. With this approach the quartz vane, protruding all the way into the waveguide, approximates a blackbody with a physical temperature T . The design uniqueness lies in the choice of cryogenically suitable materials coupled with detailed thermal analyses and proper miniaturization. When operated in a proportional $-$ integral $-$ derivative loop, these properties combine to facilitate a programmable calibration load with a switching speed of $\lesssim$ 10 s. It will be shown that the W-band design operates overmoded to $\sim$ 230 GHz and that the concept is in principle scalable to terahertz frequencies.