Superconducting 4–8-GHz Hybrid Assembly for 2SB Cryogenic THz Receivers

Abstract

We present here the design and characterization of an intermediate frequency (IF) assembly comprising a compact 90° hybrid chip (coupled line coupler-Lange coupler-coupled line coupler), two bias-T circuits for biasing the Superconductor-Insulator-Superconductor (SIS) mixers, and two transmission line circuits. Specifically, the miniaturized 3-section hybrid chip fabricated using thin-film technology utilizes superconducting Niobium (Nb) transmission lines, air bridges to connect the fingers of the Lange coupler (middle section), and is complemented with two bias-T circuits with integrated MIM capacitors. The assembly was designed to ensure amplitude and phase imbalances better than 0.6 dB and ±2° respectively. Experimental verification of the assembly at 4 K shows good agreement between the measurements and simulations with amplitude imbalance of 0.5 dB and maximum phase imbalance of ±2°. The ALMA band 5 (163-211 GHz) receiver will include such assembly. The receiver tests shows sideband rejection ratio better than 15 dB over the entire RF band, i.e. a systematic improvement of 3-9 dB as compared to the previously reported results.

Design of Wideband Waveguide Hybrid With Ultra-Low Amplitude Imbalance

“…Furthermore, the matrix calculations reveals the relationship between the bandwidth and the amplitude imbalance; specifically, the wider the bandwidth the greater the amplitude imbalance at the center frequency. This property limits the usefulness of the hybrid in wideband applications, for example in sideband separating receivers [8].…”

“…This is an undesired feature in many applications, especially in receiver systems that employ 2SB layout . Instead, a waveguide structure that allows for controllable ripples in the operating band, similar to substrate based multi-section hybrids [1], [6]- [8] is desirable. This issue has partially been addressed in [13] and implemented in ALMA B5 production receiver.…”

Dependence of the scatter of the electrical properties on local non-uniformities of the tunnel barrier in Nb/Al-AlOx/Nb junctions

“…Since the measurements are performed at cryogenic temperatures and the induced temperature gradient alters cable characteristics, a dedicated calibration procedure 17,20 was applied in these measurements.…”

Direct Measurement of Superconducting Tunnel Junction Capacitance

“…Once the position of the DUT's response in time is known, we can benefit from the time-domain processing techniques in the PNA [17]. Afterwards, the time gating procedure [18] is used to isolate the reflected signal at the DUT plane from other parasitic reflections both for the short-circuit reference and the SIS junction in a similar fashion as was performed in [19], [20]. Because of the gating span (1 ns), the result still contains response from the circuitry beyond calibration plane A in Fig.…”