Thermal blocking filters find wide use in cryogenic applications ranging from quantum computing to ultra-low-noise detectors. They can be used to provide the environmental isolation between cooled devices and the warmer temperature supporting bias and readout circuitry. In particular, they are effective in rejecting thermal radiation, limiting radio frequency interference, and providing a convenient means of heat sinking or realizing a vacuum feedthrough for signal lines.
In a microwave instrumentation setting, a well-defined characteristic impedance, typically approximating a matched boundary condition, is desirable. From a radiometric perspective, such filter structures limit the available power by modifying the transmitted response and effectively reducing the photon density of states in the Planck distribution to a single dimension. The resulting thermal emission is controlled through appropriate definition of the filter’s temperature.
A variety of thermal blocking filter construction techniques and designs have been explored. In the devices’ 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 inter-stage isolation and spurious transmission resonances, limiting the filter’s total shunt capacitance, and achieving adequate control over circuit parameters as a function of temperature. Dissipative solutions based on loosely distributed microwave structures can be used to achieve a broadband lowpass transmission response. More recent efforts have strived to retain these desirable properties while providing a well defined impedance match. Specific examples include coaxial lines and strip-line powder filters. These designs are largely empirical and do not enable prediction of response prior to fabrication.
In this work, simple matched filter designs based on easily realized absorptive dielectric transmission lines are improved upon. The filter’s response is calculable, repeatable under cryogenic cycling, and is capable of providing an intrinsically broadband matched impedance termination. The device can be realized from readily available commercial materials, and is intended as an inexpensive and high-performance thermal (low-pass) blocking filter for general laboratory use. The absorptive thermal (lowpass) blocking filters for cryogenic microwave applications feature filter input characteristic impedance designed to match 50 Ω, and its transmission 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. Design considerations and equations are provided that enable this approach to be scaled and modified for use in other applications.