Ultra-Wideband Chip Attenuator for Precise Noise Measurements at Cryogenic Temperatures

A 20-dB chip attenuator designed for cryogenic noise measurements from dc up to 40 GHz is presented. The chip is based on the use of temperature-stable tantalum-nitride thin-film resistors, a high thermal conductivity substrate such as crystal quartz (z-cut), and a suitable design that avoids inner conductor thermal heating, which is an important limiting factor for the precision of cryogenic noise measurements. A high-accuracy temperature sensor installed inside the attenuator module provides precise temperature characterization close to the chip location. The high thermal conductivity of the chip substrate in the designed attenuator assures a negligible temperature gradient between the resistive elements in the chip and the sensor, thus improving the measurement accuracy. The attenuator also shows an excellent electrical performance with insertion losses of 19.9 dB ± 0.65 dB and return losses better than 20.6 dB in the whole frequency range at 296 K. The insertion loss change when cooled to 15 K is less than 0.25 dB, which demonstrates its temperature stability.