Pulse timing constant fraction

Figure 7. Slow inactivation of Na channels is potentiated by STX. The graph shows the time required for the recovery of Na channels to an activatable state after a long (1 sec, +50 mV) inactivating depolarization. When tested by a brief test pulse, control currents (A) recovered in a fast (r = 233 msec) phase. Addition of STX (q, 2 nM, which approximately halved the currents with no inactivating pulse) approximately doubled the fraction of currents recovering in the slow phase and also increased the time constant of slow recovery. The fast recovery rate was unaffected. (Reproduced with permission from Ref. 47. Copyright 1986 The New York Academy of Sciences).

Time-Correlated Single-Photon Counting. For the application of TCSPC in the picosecond time domain, lasers with pulses whose half-widths are 20 ps or less are used. For better time resolution, the combination of a microchan-nel plate photomultiplier tube (MCP-PMT) and a fast constant fraction discriminator (CFD) are used instead of a conventional photomultiplier tube (PMT). A TCSPC system with a time response as short as 40 ps has at its core a Nd YLF (neodymium yttrium lithium fluoride) laser generating 70-ps, 1053-nm pulses at... 

Fig. 6.4 Temporal behavior of the normalized signal S when a glucose pulse is applied, increasing the glucose from 5 mM to 10 mM and lasting for 60 min. The PI control features are clearly seen, starting with a rapid proportional response, followed by a slow integral control. Increasing the time constant k-, responsible for the integral control, results in a faster saturation of the response. The smallest and maximal value of the active fraction is Xq = 0.2 and Xmax = 0-8 respectively.

Fig. 5. The schematic diagram of the pulsing system and the ion beam pulse radiolysis system with an optical emission spectroscopy. PMT denotes photomultiplier tube HV, high voltage supply CFD, constant fraction discriminator TAC, time to amplitude converter and PH A, pulse height analyzer. From Ref. 36...

TCSPC is Ulnstrated in Fig. 3a. In addition to a mode-locked laser for pnlsed excitation and a detector with high time resolntion (nsnally a micro-channel plate photomultiplier tube capable of time-resolution of 20-30 ps), the required instrumentation inclndes constant-fraction discriminators to generate electrical pnlses triggered by fluorescence photons and by the reference (the excitation pulse), a time-to-amphtude converter or other device to measnre the time lag between reference and flnorescence connts, and a multichannel scaler to accumnlate... 

The zero-crossing method reduces the errors due to jitter and walk by picking the time from the zero crossing of a bipolar pulse (Fig. 10.22). Ideally, all the pulses cross the zero at the same point, and the system is walk free. In practice, there is some walk because the position of zero crossing depends on pulse risetime. The dependence on pulse risetime is particularly important for Ge detectors because the pulses produced by Ge detectors exhibit considerable variations in their time characteristics. To reduce the uncertainties still present with the zero-crossing method, the constant-fraction method has been developed specifically for Ge detectors. 

D. A. Gedcke, W.J. McDonald, Design of the Constant Fraction of Pulse Height Trigger for Optimum Time Resolution, Nucl. Instrum. Meth. 58, 253-260 (1968)... 

Another important feature of TCSPC is the use of the rising edge of the photoelectron pulse for timing. This allows phototubes with nanosecond pulse widths to provide subnanosecond resolution. This is possible because the rising edge of the single photon pulses are usually steeper than one would expect from the time response of the PMT. Also, the use of a constant fraction discriminator provides improved time resolution by removing the variability due to the amplitude of each pulse. 

Another important component of a TCSPC system is a device called a constant fraction discriminator (CFD), whose primary function is to compensate for amplitude variations in the triggering pulses and yield reliable timing. 

CONSTANT FRACTION TIMING A method of timing discrimination that uses a constant fraction of the peak amplitude for each input pulse. 

Fig. 3.26. Comparison between the measured and calculated real-time dynamics in 39,39K2 at moderate laser pulse intensities, adapted from [42, 351]. The fluctuations in the experimental pump< probe signal are mainly caused by instabilities of the molecular beam during the experiment. The ratio of the modulation amplitude to the overall averaged ion intensity amounts to 0.6 (expriment) and 1.9 (theory). The difference is due to the fact that, in contrast to the theoretical data, the experimental ion signal contains a constant fraction of ions which originate from the interaction of a single pump or probe pulse with the molecules (taken from [328])...

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