Investigation of an electronically tuned 100 GHz superconducting phase shifter

Turnable superconducting millimeter-wave phase shifters are discussed. The tuning mechanism is based on inductance modulation of NbN microstrips by excess quasiparticle injection. When measured by a DC method, using the interference patterns and resonances of specially designed DC SQUIDs, the inductance of NbN microstrips can be varied by up to 300%. Based on these results, superconducting microstrip interferometers to operate as low-power 100 GHz phase shifters were designed and fabricated. The 100 GHz signal is coupled into and out of the interferometers by finline antennas. Amplitude modulation of the output is used as the criterion for phase shift in one of the interferometer branches. Thin superconducting films (thinner than the penetration depth) and dielectric layers are needed to achieve tunability of the inductance. The conflicting requirements of tunability and efficient signal coupling are quantified in the interferometer testing, and the range of parameter optimization is bracketed. The elements of this required optimization are presented.