  We present a detailed analysis of the fundamental noise sources in
superconducting transition-edge sensors (TESs), ac voltage biased at MHz
frequencies and treated as superconducting weak links. We have studied the
noise in the resistive transition as a function of bath temperature of several
detectors with different normal resistances and geometries. We show that the
excess noise, typically observed in the TES electrical bandwidth, can be
explained by the equilibrium Johnson noise of the quasiparticles generated
within the weak link. The fluctuations at the Josephson frequency and higher
harmonics contribute significantly to the measured voltage noise at the
detector bandwidth through the nonlinear response of the weak link with a
sinusoidal current-phase relation.
