Design and Performance of a 10-GHz HTS He-FIB JJ Mixer on LSAT Substrate

Abstract

This paper presents a compact, single-ended high-temperature superconducting (HTS) helium-focused ion beam (He-FIB) Josephson junction (JJ) mixer. The planar circuit is fabricated from a thin film layer of YBa2Cu3O7-xdeposited on a 10 mm × 10 mm Lanthanum Strontium Aluminium Tantalate (LSAT) substrate. A comprehensive six-step microstrip design methodology is outlined, detailing the JJ's physical geometries, high frequency and DC grounding, impedance matching, and filtering networks. The complete microstrip topology is designed to achieve an approximate radio frequency (RF) input passband between 9.9 GHz to 10.6 GHz with high out-of-band isolation, a single local oscillator (LO) resonance at 11.54 GHz and an intermediate frequency (IF) lowpass cut-off at 2.3 GHz. The proposed RF impedance matching network is uniquely designed to mitigate the significant performance limitation associated with normal resistances ($\rm{R}_\rm{n}$) scatter observed in HTS JJs. By implementing the proposed design methodology, a stable percentage bandwidth and$\rm{S}_{11}$$\leq$-10 dB is maintained between${\mathit{R}}_\rm{n} =$5$\Omega$to 10$\Omega$, which coincides with the fabrication variance for He-FIB JJs. The HTS mixer is manufactured and its DC and high frequency behaviours are characterised at 6 K, 10 K, 20 K, 30 K, and 40 K. The measured${\mathit{R}}_\rm{n}$is 9.77$\Omega$, which significantly deviated from the predicted value of 5$\Omega$, emphasising the importance of the adaptability and versatility of the implemented impedance matching network. Exposed to the lowest measured temperature, the HTS mixer exhibited a maximum conversion gain of -4.93 dB which demonstrates the potential capabilities in He-FIB induced barrier JJs on LSAT substrates as frequency mixers.