Design, Characterization, and Calibration of Low-Noise Terahertz Receivers

Doctoral thesis, 2011

This thesis is focused on the field of instrumentation for radio-astronomy and atmospheric science, and specifically deals with problems such as optical design and its verification, horn antennas and receiver optics’ components, different aspects of measurement systems in order to characterize low-noise THz receivers, and its calibration techniques. Ultra low-noise Terahertz receivers, used in radio-astronomy, are typically coupled to the telescope system through relay optics that provides guiding of the signal to the receiver frontend with minimal loss. Any misalignments, beam truncation, and distortion in the optical system will introduce losses and degrade the overall noise performance of the system. It is therefore critical that the optical design could be verified through measurements. In this thesis the author presents a vector beam measurement system to create possibilities to accurately characterize the optical performance of multi-band receivers over a very wide frequency range. The developed beam measurement system employs a novel circuitry allowing the use of a single reference source, different frequency harmonics of which allow to generate the required RF and LO signals yielding the desired IF, while obtaining perfect phase-coherence and initial phase-noise cancellation. An advantage of such wideband measurement system is that it could be used in different projects, nearly independent on frequency. Implementation of the measurement system, covering 163 GHz to 500 GHz, for the APEX receiver - Swedish Heterodyne Facility Instrument (SHeFI) - and the ALMA Band 5 receiver cartridge is described. Apart from optical characterization, noise-, sideband rejection, amplitude- and phase stability, and gain saturation measurements of the receivers are the measurements performed within the very same measurement setup. The measurement setup includes all necessary hardware and achieves largely automatic measurements with in-built optimization procedures. Another part of the thesis describes the author’s work on design of horn antennas. The feed horn is one of the most critical components in an optical system since it has direct impact on the total system performance. Two horn designs are presented in this thesis, one profiled corrugated feed horn, where particle swarm optimization (PSO) was successfully employed in the design procedure. The PSO drastically reduces the computational time and provides a robust optimization in terms of finding global optimum. A second horn design, implemented within PHOCUS, the Swedish sounding rocket project, is used for two water vapour radiometers, 183 GHZ and 557 GHZ. These antennas are based on the Potter horn and designed to provide FWHM of 5° without any additional focusing elements. Apart from the antenna design, two different calibration systems for both the 183 GHz and the 557 GHz radiometers on the PHOCUS sounding rocket have been suggested, designed, verified and implemented. The calibration systems shall meet challenging requirements associated with the rocket platform such as acceleration, shock, vibrations, limited space, and short flight time.