  Metallic magnetic calorimeters (MMCs) are cryogenic detectors that offer an
excellent energy resolution, a signal rise time of <100 ns, a high dynamic
range and almost optimal linearity. MMCs are of high interest for many
experiments. One of them, the ECHo experiment, requires the utilization of
large MMC detector arrays. The readout of such MMC arrays is a challenging
task, which can be tackled using software-defined radios (SDRs). Though SDR is
a well-known approach in communications engineering, a dedicated implementation
for frequency division multiplexed readout of MMCs is new and one of the
technological key elements of the ECHo project. ECHo will be the first
experiment to use microwave SQUID multiplexed MMC detectors and therefore
pioneering the hardware, firmware and software development in this domain. This
paper presents the detailed concepts and current status of the development.
