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

This article 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 YBa₂Cu₃O_7-x deposited 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 to 10.6 GHz with high out-of-band isolation, a single local oscillator resonance at 11.54 GHz and an intermediate frequency 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 (R_n) scatter observed in HTS JJs. By implementing the proposed design methodology, a stable percentage bandwidth and S₁₁ ≤ −10 dB is maintained between R_n = 5 to 10 Ω, which coincides with the fabrication variance for He-FIB JJs. The HTS mixer is manufactured, and its DC and high-frequency behaviors are characterized at 6, 10, 20, 30, and 40 K. The measured R_n is 9.77 Ω, which significantly deviated from the predicted value of 5 Ω, emphasizing 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.