Impedance-matched coplanar-waveguide metal-powder low-pass filters for cryogenic applications.

We developed impedance-matched metal-powder low-pass filters based on coplanar waveguide design and characterized them at room temperature and 77.4 K. The coplanar waveguide metal-powder (CPW-MP) filters have a return loss better than 9.8 dB at frequencies up to 10 GHz at 77.4 K. We find that the filter attenuation per length scales linearly with frequency from 29 to 220 dB/m within the 1-5 GHz frequency range at 77.4 K, achieving the total attenuation above 100 dB for the 1 m-long CPW-MP filter at frequencies above 2 GHz. The CPW-MP filter integrated with the multipole LC filter demonstrates the low cut-off frequency, the attenuation above 100 dB in the stop band, and the high roll-off of 590 dB per decade. The proposed CPW-MP filter is easy to fabricate, has very good thermal-shock resistance, is flexible for integration with other types of low-pass filters, delivers the same performance as classical cryogenic filters, is more compact compared to 50 Ω coax cables with the same type of lossy dielectric, and provides ample opportunities for further miniaturization.