Superconducting lithographed resonators have a broad range of current and potential applications in the multiplexed readout of cryogenic detectors. Here, we focus on LC bandpass filters with resonances in the 1-5 MHz range used in the transition edge sensor (TES) bolometer readout of the Simons Array cosmic microwave background (CMB) experiment. In this readout scheme, each detector signal amplitude-modulates a sinusoidal carrier tone at the resonance frequency of the detectors accompanying LC filter. Many modulated signals are transmitted over the same wire pair, and quadrature demodulation recovers the complex detector signal. We observe a noise in the resonant frequencies of the LC filters, which presents primarily as a current-dependent noise in the quadrature component after demodulation. This noise has a rich phenomenology, bearing many similarities to that of two-level system (TLS) noise observed in similar resonators in the GHz regime. These similarities suggest a common physical origin, thereby offering a new regime in which the underlying physics might be probed. We further describe an observed non-orthogonality between this noise and the detector responsivities, and present laboratory measurements that bound the resulting sensitivity penalty expected in the Simons Array. From these results, we do not anticipate this noise to appreciably affect the overall Simons Array sensitivity, nor do we expect it to limit future implementations.
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