A 16-channel flex circuit for cryogenic microwave signal transmission

Heterodyne focal plane arrays used in the terahertz (THz) regime currently require a discrete set of rigid coaxial cables for the transmission of individual intermediate frequency (IF) signals. Consequently, the size of an array is limited to ∼10s of pixels due to limited physical space and the complexity of assembly. In order to achieve an array with ∼1000 pixels or greater, new interconnections must be developed capable of carrying multiple IF signals on a single carrier which is flexible, robust to noise, and terminated with a high density RF connector. As an intermediate step to the development of a ∼1000 pixel heterodyne focal plane array, the Kilopixel Array Pathfinder Project (KAPPa) has developed a 16 channel IF flex circuit. Initially, design simulations were performed to evaluate various means of high-frequency (1∼10 GHz) signal transmission, including microstrip, stripline and coplanar waveguides. The method allowing for the closest signal spacing and greatest resistance to radio frequency interference (RFI) was determined to be stripline. Designs were considered where stripline transitioned to microstrip in order to terminate the signal. As microstrip transmission lines are sensitive to RFI, a design featuring just stripline was evaluated. In both the stripline-to-microstrip and stripline-only designs, a three-layer copper-coated polyimide substrate was used. Signal transitions were accomplished by a signal carrying âœhota via passing through a series of three conductive pads, similar to work by Leib et al. (2010). The transition design essentially mimics a coaxial line, where the radial distance between the pads and the ground plane is optimized in order to achieve desired impedances. In simulation, 50 Ohm impedances were achieved throughout, with crosstalk and return loss limited to-30dB. Terminations are made via an array of Corning Gilbert G3PO blind mate connectors, which are small enough to match the 6mm pixel pitch of the KAPPa focal plane unit. In addition, circuits with SMA terminations were designed to enable straightforward testing with a vector network analyzer (VNA). Initial designs use 1/2 oz. (18 microns thickness) copper conductors. In the KAPPa application, the copper conductor is still suitable for cryogenic applications because of the very small cross section presented by the copper conductor. The stripline design allows the interconnect to be clamped securely for heat sinking with a copper clamp at 10K and 60K. Heat load to the 4K stage is limited to 10 mW if the circuit is heat sunk at 10K 150mm from the 4K focal plane. Future designs could be implemented with phosphor bronze as the conductor to further limit heat load at the expense of added loss.