New frequency modulation readout based on relaxation oscillations

Scaling of multi-pixel cryogenic detectors for imaging is rendered increasingly difficult with size by the 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 comprising a superconducting tunnel junction which exhibits relaxation oscillations. The oscillation frequency is almost linear to the analog bias signal over a wide operation range. The frequency signals from different detectors can be combined to one single readout line and easily detected outside the cryostat. The current noise of an optimized circuit is about 5 pA/sqrt(Hz), comparable to standard SQUID amplifiers. We show experimental data from 'stand-alone' operation as well as response to microcalorimeter signals due to X-ray events