Impedance-matched low-pass stripline filters

TL;DR

This paper presents the design and construction of impedance-matched low-pass stripline filters with broad stopbands and adjustable passbands, suitable for cryogenic microwave applications in quantum and superconducting device measurements.

Contribution

The authors develop simple, compact, and adjustable low-pass filters using dissipative dielectric materials for cryogenic environments, with high attenuation and low reflection over a broad frequency range.

Findings

Filters achieve >100 dB attenuation at high frequencies.

Passband extends from dc to 0.3-0.8 GHz, adjustable by length.

Return loss exceeds 10 dB up to 40 GHz.

Abstract

We have constructed several impedance-matched low-pass filters using a stripline geometry with a dissipative dielectric. The filters are compact, simple to construct, and operate in cryogenic environments. The dissipative dielectric consists of magnetically-loaded silicone or epoxy, which are commercially available under the trade name Eccosorb. For a stripline length of 32 mm, the filters have a passband that extends from dc to a 3 dB bandwidth between 0.3 and 0.8 GHz. The 3 dB bandwidth can be adjusted beyond this range by changing the filter length. An extremely broad stopband at higher frequencies, with attenuation exceeding 100 dB, is achieved along with a return loss greater than 10 dB measured up to 40 GHz. This combination of high attenuation and low reflected power across a broad stopband ensures that spurious or unwanted signals outside the passband do not reach or return to the device under test. This type of filter has applications in microwave frequency measurements of sensitive non-linear devices such as superconducting heterodyne mixers, quantum tunneling devices, and quantum computing elements.