A Novel Low-Temperature Superconductor Power Limiter

This paper presents a novel concept for realizing an RF power limiter for protecting superconductor digital receivers. A lumped-element niobium (Nb)-based filter is used as a protection circuit. It consists of lumped-element resonators formed using spiral inductors and metal-insulator-metal capacitors integrated on a multilayer Nb process. The circuit operates as a filter at low-power levels and as a reflector at high-power levels. The lumped-element filter circuit is studied in detail to explain the performance of the filter at high-power levels. It is concluded that some of the lumped-element inductors switch from being inductors when operating at low-power levels to being capacitors when operating at high-power levels. When the lumped-element inductors switch to capacitors, the filter circuit that consists of LC resonators switches to a circuit that consists of capacitors, causing the input power to be reflected back. Both the theoretical and experimental results are presented to verify this phenomenon. In addition to applications in RF power limiters, the concept can be employed to realize transmit/receive (T/R) switches in order to isolate the (T/R) circuit from the receive circuit.