  The Transition-Edge Sensors (TES) is a mature, high-resolution x-ray
spectrometer technology that provides a much higher efficiency than dispersive
spectrometers such as gratings and crystal spectrometers. As larger arrays are
developed, time-division multiplexing schemes operating at MHz frequencies are
being replaced by microwave SQUID multiplexers using frequency-division
multiplexing at GHz frequencies. However, the multiplexing factor achievable
with microwave SQUIDs is limited by the high slew rate on the leading edge of
x-ray pulses. In this paper, we propose a new multiplexing scheme for
high-slew-rate TES x-ray calorimeters: the spread-spectrum SQUID multiplexer,
which has the potential to enable higher multiplexing factors, especially in
applications with lower photon arrival rates.
