A Feasibility Examination Using Microwave Stripline Resonators for Low-Temperature Sensing

A stripline transmission-line-based temperature sensor is designed and fabricated for examination of use in low-temperature environments. A custom-built chassis encloses the stripline, shielding the low-temperature sensor from environmental disturbances such as frost and condensation, which would otherwise disrupt temperature measurement. The base resonator geometry is a 6.05-mm ring, modified with a split and transmission lines (TLs) extended inward to operate at frequencies between 910 and 927 MHz over temperatures from <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$- 70~^{\circ }$ </tex-math></inline-formula>C to <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$10~^{\circ }$ </tex-math></inline-formula>C. The one-port sensor exploits the thermal coefficient of dielectric constant (TCD) of −459 ppm/°C of a commercial dielectric substrate. As temperature changes, so does the dielectric constant of the substrate, correlating to a shift in resonant frequency. A series of thermal cycling and stability experiments demonstrate repeatable and stable resonant frequency and temperature measurements. Several independent temperature step experiments between <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$- 70~^{\circ }$ </tex-math></inline-formula>C and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$10~^{\circ }$ </tex-math></inline-formula>C are performed, resulting in a repeatable linear sensitivity of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\approx ~199$ </tex-math></inline-formula> kHz/°C. The results confirm the feasibility of using microwave (MW) stripline resonant sensors on commercial substrates for low-temperature sensing.