New frequency-modulation readout based on relaxation oscillations

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.