Improved components for 2SB THz receivers with enhanced sideband rejection performance

Licentiate thesis, 2013

The thesis presents work done in improving the sideband rejection ratio for 2SB cryogenic radio astronomy receivers in general, but more specifically, for the future Onsala 20 m 3-mm receiver system and the ALMA Band 5. In order to attain better than 15 dB sideband rejection ratio over the entire bandwidth (approximately 26% RF fractional bandwidth and one octave the IF bandwidth), it is necessary to improve both RF and IF hybrid assembly to optimum performance, i.e. making the hybrids with minimum achievable amplitude and phase imbalance. For this reasons, a 4-8 GHz IF assembly comprising a compact 90° hybrid chip, two bias-T circuits and two transmission line circuits was designed and characterised. The compact size of the hybrid chip allows it to be integrated into virtually any sideband separating (2SB)mixer operating at cryogenic temperatures and is furthermore especially advantageous for multi-pixel 2SB receivers or low noise balanced amplifier layouts. The hybrid was fabricated using thin-film technology on Si-substrates and uses air bridges to inter-connect the fingers of the coupler. The hybrid assembly that integrates the bias-T circuit using a MMIC-like approach, where capacitors are integrated on-chip and fabricated using thin-film technology and micro-fabrication. Characterization of the hybrid assembly was performed at a cryogenic temperature of 4 K and sophisticated calibration and fixture de-embedding procedures was used. The measurement result shows that the IF hybrid assembly has state of the art performance, with amplitude and phase imbalance better than 0.5 dB and ± 2° respectively at cryogenic temperatures. In order to allow complete characterization of the ALMA Band 5 cartridge a highly versatile measurement setup has been built. The measurement setup includes all necessary hardware and achieves automatic measurements for the system noise and sideband rejection with inbuilt optimization procedures. The measurement setup additionally comprises measurements of the receiver saturation, phase and amplitude stability, as well as optical beam characterization. The performance of the IF hybrid was verified in ALMA Band 5 receiver cartridge using the ALMA Band 5 measurement setup, demonstrating an average sideband rejection ratio of better than 15 dB, i.e. an improvement of 3-9 dB. To further improve the sideband rejection ratio, a novel design for a RF 90° waveguide hybrid was proposed (163-211 GHz). This suggested hybrid employs the multiple branch waveguide design. The specific feature of the proposed hybrid design is the use of controllable ripple in the operational frequency band for achieving better overall amplitude imbalance. This general design approach design could be used for waveguide hybrids at any frequency. Experimental verification of the hybrid shows nearly excellent agreement with the simulations yielding measured amplitude imbalance of 0.15 dB and phase imbalance of ± 2.7° (cf. Fig.5) over most of the band. Such a waveguide hybrid is expected to further improve the sideband rejection ratio.