A self-attenuating superconducting transmission line for use as a microwave power limiter

We have designed, fabricated, and tested microwave power limiter based on high-temperature superconductor thin-film technology. The power limiter takes the form of a 50 /spl Omega/ coplanar waveguide transmission line that is reversibly driven from the low-loss superconducting state to the high-loss normal state when the microwave currents within the device exceed a critical value. When operated at 70 K, the power limiter displays very low insertion loss in the nonlimiting state (<0.1 dB at 40 GHz), and extremely wide bandwidth (>40 GHz), with constant impedance over the entire microwave range. The maximum power transmitted by the device can be engineered by varying the transmission-line dimensions, and can be further tuned once the device has been fabricated by varying the operating temperature. In the over-power state the device continues to pass a portion of the incident signal, although with reduced linearity. Switching times for the power limiter are estimated to be on the order of microseconds or less, based on both pulsed rf measurements and on measurements using a periodic amplitude-modulated drive signal. We expect this device to be useful for protecting high-performance receiver circuits from over-power conditions without limiting the dynamic range or bandwidth of receiver systems.