Two Port Scattering Parameters Measurements and De-Embedding in Cryostat from 300 K down to 20 K

Several methods of de-embedding scattering parameters (S-parameters) at cryogenic temperatures (CT) are compared. The optimal one that was selected is 12-term error short, open, load and thru (SOLT) method as the 1-step, 2-step and TRL de-embedding procedures exhibit errors. Short, open, load and thru structures were designed and fabricated on PCB. Each structure was placed at the reference de-embedding plane and designed to be as close to an ideal standard as possible. Analysis of short, open and thru S-parameters shows a decrease of transmission line losses with decreasing temperature. Symmetrical line termination for load structure is accomplished by using two parallel 100 Ω resistors. Impact of SMD resistor type on the characteristics of load structure is analyzed. Thin film device with the lowest tolerances and stable characteristics over temperature range from 300 K down to 20 K is found as the optimum choice. The SOLT de-embedding method is demonstrated on measured S-parameters in a frequency range from 3 MHz to 3 GHz of commercially available device, Infineon’s BFP420 bipolar transistor in temperature range from 300 K down to 20 K.