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Describes a true constant fraction trigger circuit for picosecond photon-timing with ultrafast microchannel plate photomultipliers. Operation principle of the circuit; Instrument structure and key features; Circuit description; Reduction of residual amplitude dependent time walk.

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 CFTs, 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. [ABSTRACT FROM AUTHOR]

Ultrafast microchannel-plate photomultipliers offer picosecond resolution in time-correlated photon counting, provided all additional causes of timing jitter be minimized. Here we analyze the contribution of the electronic circuit noise, mainly dependent on the fast preamplifier and no more negligible at picosecond level. Criteria are derived for minimizing this contribution by proper design or selection of the amplifier. We highlight that it does not make sense to select amplifiers with bandwidths of 3 GHz or more, looking mainly to the rise time of the fast microchannel-plate pulse and paying minor attention to the noise. We demonstrate that (i) the noise spectral amplitude has chief importance, (ii) the bandwidth providing minimum jitter is around 1 GHz, and (iii) satisfactory results are obtained with lower bandwidths, in some cases down to about 500 MHz. [ABSTRACT FROM AUTHOR]

Avalanche photodiodes biased above the breakdown voltage are an interesting alternative to photomultiplier tubes in time-correlated single-photon counting. The characteristics and performance of a commercially available device (RCA C30921S) have been investigated. The time resolution is found to improve as the excess bias above the breakdown voltage is increased. Full width at half-maximum values down to 400 ps have been measured with the detector cooled at - 40°C, and down to 460 ps at room temperature. The best results were obtained with an active quenching circuit, suitable for operation with excess bias voltage up to 40 V; at room temperature, fast gated operation was used for attaining optimum performance. Experimental data on the statistical behavior of the avalanche current pulses in these devices are reported and discussed. [ABSTRACT FROM AUTHOR]