Analysis and improvement of the return loss performance in HTS filter subsystem

In this article, both an 8-pole High Temperature Superconductor (HTS) filter and a cryogenic low noise amplifier (LNA) with high performance are fabricated. The HTS filter subsystem is realized by installing the HTS filter in front of the cryogenic LNA. Although the HTS filter has been fine tuned specially, the return loss performance of the HTS filter subsystem is often deteriorated due to the mismatch with the LNA. In order to improve the return loss performance of the HTS filter subsystem, we discuss and analyze the phase-matching effect between the HTS filter and the LNA. The article also proposes that tuning of the cryogenic LNA's phase by adjusting its drain current is a helpful and relatively easy method to improve the HTS filter subsystem's return loss performance in cryogenic environments.