NIST researchers develop high-temperature superconducting microwave power limiter.

Superconducting electronics are being explored for many high-performance communications applications that require very low insertion loss, or unsurpassed sensitivity or dynamic range. However, such high-performance superconducting components are vulnerable to damage if exposed to high-power transient signals, such as lightening strikes or unintentional radar illumination and thus need to be protected by an input microwave power limiter. Conventional power limiters would increase the insertion loss or significantly reduce the dynamic range of the system, thereby compromising the advantages of using superconducting components.

In order to address these issues, researchers have designed, fabricated, and tested a microwave power limiter based on high-temperature superconductor thin-film technology. In order to be compatible with the highest performance superconducting digital circuits, researchers have designed a limiter for minimum insertion loss and maximum possible bandwidth. The power limiter takes the form of a 50 [ohm] coplanar wave-guide (CPW) transmission line fabricated from a YB[a.sub.2]C[u.sub.3][O.sub.7-[delta]] film grown on a sapphire substrate. The CPW transmission line 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 signal limiter displays very low insertion loss and extremely wide bandwidth in the nonlimiting state with constant impedance over the entire microwave range, making it essentially "transparent" to the system. When the input power exceeds the designed limit, the device switches into the normal state, effectively blocking the signal above the critical value. The device provides only the amount of attenuation that is needed and continues to pass a portion of the incident signal during the overpower event.

One of the key parameters of a microwave power limiter is its switching time. In order to protect sensitive components from high-power signals, the limiter must turn on in a sufficiently short period of time. The researchers have demonstrated that the superconducting limiter switches from the non-limiting to the limiting state in less than a nanosecond, with no detectable leakage through the device on this time scale. The superconducting limiter is now being evaluated for use in a variety of high performance communication systems.

CONTACT: James Booth, [phone]; [e-mail].