High-Tc superconducting (HTS) mixers have the merits of high sensitivity, low noise, low LO power requirement as well as cheaper cryogens or small cryocooler. In the past few years, the research group at CSIRO has developed a series of Josephson junction (JJ) monolithic microwave integrated circuit (MMIC) mixers at microwave, millimeter-wave and even Terahertz frequencies. On the other hand, harmonic mixers play an important role in mixers, for they can avoid using expensive LO sources at high frequencies, separating the RF and LO signals and enhancing the isolation between them compared with conventional mixers. Current reported HTS MMIC harmonic mixers are all based on one single Josephson junction, which would limit the performance of the mixers. In the contrast, sub-harmonic mixers use the even harmonics of LO signals in the mixing process and have better noise and conversion loss performance, making them more favourable in communication systems.
In this work, a new type of HTS sub-harmonic mixer is designed using a pair of anti-parallel Josephson junctions. Although many sub-harmonic mixer designs have been presented using anti-parallel diodes pair in semiconductor technologies, there is no such design been demonstrated using superconducting technologies. One reason is that the Josephson junction is a reciprocal network, which would bring difficulty in providing biasing currents with opposite directions for two parallelly-placed Josephson junctions. To overcome this problem, a capacitor is placed between two JJs to isolate the DC signal. In this case, the biasing current passing through the two JJs has the same amplitude but opposite directions, which could help to realize the mixing of the RF signal and the sub-harmonics of the LO signal. A 24 GHz sub-harmonic mixer is modelled using a MgO substrate and simulated in a full-wave electromagnetic simulator. HTS filters and matching circuits are integrated monolithically with an HTS YBa2Cu3O7-x (YBCO) step-edge Josephson junction on the single MgO chip. Results show that when the temperature is 40 K (which is attainable using a single stage cryocooler), the proposed design can realize a positive value of conversion gain with 1GHz bandwidth at the IF port. More technical details and experimental results would be presented in the ISS2023 conference.
[1] J. Du et al., “HTS step-edge Josephson junction terahertz harmonic mixer,” Supercond. Sci. Technol., vol. 30, no. 2, Nov. 2016, Art. no. 024002.
[2] T. Zhang, J. Du, and Y. J. Guo, “An 8–10-GHz low-loss image-reject HTS mixer based on cascaded Josephson junctions,” IEEE Microw. Wireless Compon. Lett., vol. 31, no. 8, pp. 945–948, Aug. 2021.