Although automotive radar systems operating at 77 and 79 GHz may seem like the upper limits of microwave engineering technology, a growing number of researchers are looking to tap into terahertz radio technologies. One group, led by scientists from the California Institute of Technology, GGB Industries, and Northrop Grumman Corp., recently reported on two-port on-wafer vector network analyzer (VNA) measurements from 325 to 508 GHz. The measurements were made with the help of prototype WR2.2 coplanar waveguide probes from GGB Industries and WR2.2 frequency extenders from Oleson Microwave Laboratories. The VNA calibration was performed using a through-reflect-line (TRL) technique.

In the test set, the high-frequency probes and frequency extenders were linked with a model 8510C VNA from Agilent Technologies, with a nominal coaxial top frequency of 50 GHz. The S-parameters of the WR2.2 probes were determined by means of a calibration process performed with the WinCal software tool from Cascade Microtech.

The calibrated test system was used to perform on-wafer measurements on amplifiers fabricated in Northrop Grumman's sub-50-nm indium-phosphide (InP) high-electron-mobility-transistor (HEMT) process. One of these devices was a single-stage amplifier capable of operation through 508 GHz. Another circuit was an amplifier that exhibited small-signal gain of 6.2 dB at 401 GHz. In both cases, the measurements were close to the results predicted by simulations. See "On-Wafer S-Parameter Measurements in the 325-508-GHz Band," IEEE Transactions on Terahertz Science and Technology, Vol. 2, No. 2, March 2012, p. 186.