Planar coupling scheme for ultra low noise DC SQUIDs

We have devised and tested a planar coupling scheme in which the technology used to produce ultra low noise tunnel junction dc SQUIDs is employed to achieve tight coupling between the SQUID loop and and the input coil. In our scheme the planar, ungroundplaned, inductive loop of the dc SQUID acts as the wide single turn primary of a thin-film transformer. The input coil consists of a multiple-turn secondary in the form of a spiral stripline fabricated directly above the primary. The two tunnel junctions are located at the outside edge of the SQUID loop. A low inductance stripline structure connects the junctions to the region of high current flow in the SQUID loop. We have evaluated this coupling scheme experimentally in SQUIDs with 10-turn, 19-turn, 50-turn, and 100-turn input coils. All have mutual inductances per turn of approximately 80 pH, in good agreement with numerical calculations. Detailed measurements on the 50-turn SQUID gave a mutual inductance M of 3.8 nH, an input coil inductance Liof 190 nH, a SQUID self inductance of L of 89 pH, and a coupling constant k²= M²/LiL of 0.86. The 100- turn version is estimated to have similar coupling performance with an input coil inductance of approximately 0.8 μH.