The Transition-Edge Sensor (TES) is an extremely sensitive device which is used to measure the energy of individual X-ray photons. For astronomical spectrometry applications, SRON develops a Frequency Domain Multiplexing (FDM) read-out system for kilopixel arrays of such TESs. Each TES is voltage biased at a specific frequency in the range 1 to 5 MHz. Isolation between the individual pixels is obtained through very narrow-band (high-Q) lithographic LC resonators. To prevent energy resolution degradation due to intermodulation line noise, the bias frequencies are distributed on a regular grid. The requirements on the accuracy of the LC resonance frequency are very high. The deviation of the resonance frequencies due to production tolerances is significant with respect to the bandwidth, and a controller is necessary to compensate for the LC series impedance. We present two such controllers: a simple orthogonal proportional-integrating (PI) controller and a more complex impedance estimator. Both controllers operate in baseband and try to make the TES current in-phase with the bias voltage, effectively operating as phase-locked loops (PLL). They allow off-LC-resonance operation of the TES pixels, while preserving TES thermal response and energy resolution. Extensive experimental results -- published in a companion paper recently -- with the proposed methods, show that these controllers allow the preservation of single pixel energy resolution in multiplexed operation.
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