A wide bandwith, high-ratio, planar transmission-line transformer for use at cryogenic temperatures

Transmission-line transformers are passive devices which can offer considerably higher gain-bandwidth products than their lumped circuit counterparts. Transmission-line transformers have been built and operated at liquid He temperatures using twisted-pair line wound on torroidal cores. We propose, however, constructing such a wideband transformer in a planar (deposited or etched) geometry. The way in which this transformer operates, and some design considerations, are presented. One of the many circuit forms that this type of transformer may assume was selected and macroscopic models of it were built and tested at room temperature. Based on their performance, successful operation of the postulated transmission-line transformer is predicted. Specifically, impedance transformation ratios up to 256 to 1, rise times better than 1 ns, and bandwidth ratios in excess of 10⁴can be expected.