C-BASS

Abstract:

Several technologies are now being considered for modulating the polarization in various B-mode instruments, including rotating quasioptical half-wave plates in front of the focal plane array, rotating waveguide half-wave plates and Faraday rotators. It is not at all clear that any of these techniques is feasible without heavy penalty in cost or performance. A potentially much more efficient method is to use a pseudo-correlation polarimeter in conjunction with a planar circuit phase switch.We investigate three different devices for use as mm-wave switches, SIS tunnel junctions, capacitively coupled superconducting nanostrips and RF MEMS. The SIS tunnel junction switches operate by switching between two different bias voltages, while the nanostrip switch operates by changing the impedance of a resonant circuit by driving the nanostrip from the superconducting to normal state. In each case the RF signal sees two substantially different complex impedance states, hence could be switched from one transmission line branch to another. In MEMS this is achieved by mechanical movement of one plate of a parallel plate capacitor system. Although RF MEMS have been reported at high microwave and low mm-wave frequencies, in this work we have investigated cryogenic MEMS for operation at high mm-wave frequencies (225 GHz) using superconducting transmission lines.We present and compare designs and simulations of the performance of phase switches based on all three switching C, technologies, as well as preliminary experimental results for each of the switches. Finally we also present designs of phase shift circuits that translates the on/off switching into phase modulation.