Innovators at NASA's Goddard Space Flight Center have designed, fabricated, and characterized absorptive thermal blocking filters for cryogenic microwave applications. The device allows direct integration of the high-frequency signal and microwave readout, and mitigates spurious resonances in the circuit response. This leads to improved electrical performance and a reduction in the required circuit area. The transmission line filter's input characteristic impedance is designed to match 50 ohms and its response has been validated from 0 to 50 GHz. The observed return loss in the 0 to 20 GHz design band is greater than 20 dB and shows graceful degradation with frequency. The filter's response is calculable, repeatable under cryogenic cycling, and is capable of providing an intrinsically broadband matched impedance termination.
The prior art offers a variety of thermal blocking filter construction techniques and designs. In the device's most basic form, a large shunt capacitor forms a single-pole low-pass filter. More generally, multiple low-pass lumped element stages can be combined in series to produce compact and broadband non-dissipative filter structures. The challenges presented by these implementations include controlling interstage isolation and spurious transmission resonances, limiting the filter's total shunt capacitance, and achieving adequate control over circuit parameters as a function of temperature.
In this work, simple matched filter designs based on easily realized absorptive dielectric transmission lines are improved upon to create absorptive thermal blocking filters for cryogenic microwave applications. This technology is generally applicable in the realm of high-frequency circuits, is scalable, and can be modified for use in other applications. It can be used in wide ranges of applications. Its intended use is as an element of a superconducting planar spectrometer for astrophysics applications at NASA's Goddard Space Flight Center. More generally, the meta-material choke could find use in any microwave or sub-millimeter circuit application where high isolation is required between differing frequencies for signal, readout, bias, and so on.