  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 detector's 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.
