Design, fabrication and characterization of wideband waveguide terminations
Typ
Examensarbete för masterexamen
Master's Thesis
Master's Thesis
Program
Wireless, photonics and space engineering (MPWPS), MSc
Publicerad
2023
Författare
Flosason, Karl Birkir
Modellbyggare
Tidskriftstitel
ISSN
Volymtitel
Utgivare
Sammanfattning
This thesis reports on two novel designs for high performance, wideband waveguide
terminations for terahertz frequencies, demonstrated here in the range 210-370 GHz
corresponding to ALMA bands 6-7.
With advancing technology in the design and fabrication of high sensitivity
radioastronomy receivers, each component and aspect of the system needs to be
pushed to the limits when it comes to performance and bandwidth. The highest
sensitivity and state-of-the-art receivers today for radioastronomy use 2SB mixer
topologies, which require terminations to terminate the fourth port of a waveguide
RF hybrid coupler. Reflections from that termination will both increase the level
of noise generally but also risk introducing spectral line images or ghosts from the
heterodyne downmixing process, which may cause confusion of spectral lines when
the tools are used for spectroscopy. Minimizing the reflections from that waveguide
termination is a step in improving over-all performance, and taking full advantage
of increased performance of the 2SB topology.
The goal of the project was to design, fabricate and measure two distinct and
novel termination designs, fabricated with well established methods, and materials
with known and reliable behaviours in cryogenic operation, and achieve the best
possible performance over the widest possible bandwidth. One design based on an
E-probe on quartz substrate, showing in simulations S11 < -26 dB over 270-370 GHz,
and another design based on a tapered finline structure showing in simulations S11
< -37.5 dB over 220-390 GHz. Both designs use TiN resistive thin-film to dissipate
the RF energy in ohmic losses.
To validate and verify the design and fabrication process, the performance
was then measured using a VNA and the required frequency extensions. Measuring
reflection accurately at such low levels and at these frequencies is challenging, and
the measurements are likely to underestimate the real performance. Uncertainties
in the measurements are described and their scale evaluated both by measurements
and simulation.
Beskrivning
Ämne/nyckelord
Terahertz, waveguide termination, wideband, rf load, waveguide load, VNA, microwave metrology, reflection coefficient