Superconducting LC Filter Circuits for Frequency Division Multiplexed Readout of TES Detectors

Inductor-Capacitor (LC) filter circuits form an essential component for the readout chain of TES detectors, in the case of frequency division multiplexing. They serve the functions of blocking wide band noise from adjacent pixels and separation of bias frequencies. A key requirement is a high quality factor Q (narrow band resonance) to guarantee voltage bias of the TES detectors, implying a full superconducting circuit, low dielectric loss capacitors and low magnetic loss inductors. Typically, Q should be larger than 500 * f [MHz] * L [mu H]. Here f is the bias frequency and L the inductor value, which is coupled to the detector speed for stable electro thermal feedback response. In our case, Q must be in the range of 1000 to 10000. The astronomical applications for which these filters are being developed are the micro calorimeter read-out of the XMS instrument on the IXO-mission and far infrared bolometers for the SAFARI instrument on the Japanese mission SPICA. For the latter mission the goal is to readout 160 TES pixels with one SQUID in the frequency range 1 to 2 MHz. We describe fabrication procedures for fully superconducting circuits, resulting in a maximum observed Q of 12000 for a-Si based capacitors. We also report on efforts to minimize the required surface area for the filters and to improve the predictability of the center frequency of the filters. We show measurements on filters in a prototype 16 channel FDM readout setup.