  Scaling of multi-pixel cryogenic detectors for imaging becomes increasingly
difficult with size due to complexity of readout circuitry and cryogenic
constraints (thermal load from wiring). We propose and demonstrate a new
readout scheme based on a highly stable RF oscillator composed of a
superconducting tunnel junction which exhibits relaxation oscillations. The
oscillation frequency is almost linear with the analog bias signal over a wide
operation range. The frequency signals from different detectors can be combined
into one single readout line. The current noise of an optimized circuit is
about 5 pA/sqrt(Hz), which is comparable to standard SQUID amplifiers. We show
experimental data from `stand-alone' operation as well as response to
microcalorimeter X-ray signals.
