The Enhancement of Sensitivity in Cryogenically Integrated Radio Receivers

Abstract

In radio astronomy applications, cryogenic radio receivers are predominantly used to ensure higher sensitivity in the observation of astronomical phenomena. One of the main components of these types of radio receivers is Low Noise Amplifiers (LNAs), which play a significant role by possessing high gain and minimizing the noise as much as possible. The work presented in this thesis is divided into two main parts: In the first part, a highly sensitive cryogenic radio receiver for a 5.3-meter radio telescope dish is modeled, fabricated, and cryogenically measured. This radio telescope serves as the reference antenna for calibrating a large reflector antenna, using the holographic technique. System Parameters and related noise temperature. The noise temperature of the system, considering the zenith elevation, is 66.25 K, and the G/Tsys is measured ∼ 42 dB/K. Further, the quality of polarization conversion using a 90-degree hybrid is measured, and the results are presented. This research project was a collaboration with POAM Electronics company under the Innovative Training Networks (ITN) ACO project. In the second part, we explore further improvements in mitigating the noise in High Electron Mobility Transistors (HEMTs). HEMT-based amplifiers are widely used devices, offering ultra-low noise amplification with a noise performance of 6 to 10 times the quantum limit in radio astronomy. As the physical temperature of the HEMT decreases to cryogenic levels, its noise also decreases linearly until reaching a noise plateau around a physical temperature of ∼ 20 K, potentially due to the self-heating in the HEMT channel. Recent experiments that involved immersing Low Noise Amplifiers (LNAs) in liquid helium failed to mitigate the self-heating issue, and showed no reduction in noise figure. Given that these devices are passivated with SiN, we hypothesize that this layer introduces a thermal boundary (Kapitza) resistance between the passivated surface and the liquid cryogen, hindering the dissipation of phonons from the active channel. To verify this assumption, the author conducts experimental investigations to explore the impact of passivation layer thickness on Kapitza resistance between a passivated quartz substrate and liquid cryogens (nitrogen and helium). Our observations reveal that depositing SiN results in an increased Kapitza resistance compared to an unpassivated sample, introducing an additional barrier to the cooling of semiconductor devices in liquid helium. Further, to mitigate the effect of Kapitza resistance primarily caused by acoustic impedance mismatching, we applied an anti-reflective coating approach commonly used in optics to optimize the Kapitza resistance. The experimental results verified that employing this method may be able to reduce the thermal boundary resistance between a passivated sample and liquid helium and consequently may mitigate the selfheating issue in HEMTs.

Date of Award	1 Aug 2024
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Mark Mcculloch (Supervisor) & Lucio Piccirillo (Supervisor)