CONSTANT-FRACTION CIRCUITS FOR PICOSECOND PHOTON TIMING WITH MICROCHANNEL PLATE PHOTOMULTIPLIERS

Since single-photon pulses of photomultiplier tubes (PMTs) have statistically fluctuating amplitude, constant-fraction trigger circuits (CFTs) are normally employed for accurate timing. However, with the subnanosecond signals of microchannel-plate types (MCPs) nonideal CFT behavior is observed. A residual amplitude-dependent time walk sets the ultimate resolution in time-correlated photon counting (TCPC). We present a quantitative analysis of the problem and discuss published results. We assess the effect of pulse shaping filters and derive criteria for selecting the fast preamplifier. For MCP models with 40-ps intrinsic resolution, we show that by changing the pulse delay and attenuation ratio in the available CF]rs, with minor circuit modifications, the residual time walk is strongly reduced and the detector performance can be fully exploited. For faster MCP types, with 20-ps resolution or better, the improvement is also remarkable, but we show that there is margin for further improvement and we point out criteria for designing new CFTs.