Constant-Fraction Discriminators

Of course, a simple leading edge discriminator cannot be used to trigger on such pulses. The amplitude jitter would introduce a timing jitter of the order of the pulse rise time (Fig. 4.1, left). In practice the timing jitter is even larger because any discriminator has an intrinsic delay that depends on the signal slope speed and the amount of overdrive. 

The new CFDs use the differential inputs of the comparators to trigger at the baseline transition between the difference of the input pulse and a delayed pulse (Fig. 4.1, right). 

Theoretically the temporal position of the baseline-eross point is independent of the pulse amplitude. In praetiee there is a residual jitter due to the dependenee of the intrinsic delay of the diseriminator on the slope speed and the amount of overdrive. The residual jitter ean be minimised by selecting a trigger threshold slightly different from zero. 

In practice, direct triggering at a signal level of exaetly zero is impossible. Ultrafast discriminators tend to oscillate at zero input voltage, and the smallest noise amplitudes outside a PMT pulse cause the discriminator to trigger. Therefore, practical implementations of the principle use a combination of a leading edge discriminator and a zero eross trigger. The principle is shown in Fig. 4.2. 

The comparator of the leading edge discriminator, Cl, is connected directly to the input and responds when the input voltage exceeds a defined threshold. 

---

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... 

Figure 8 Layout of the gated ion-counting detection system. CFD, constant-fraction discriminator. (From Ref. 27.)...

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...

Fig. 31. Data aqmsition system used for research on polymers at DORIS. The start/stop signals emanating from a linear position sensitive detector is digitized by time-digital converter. The patterns are stored in a 64 kbyte memory. The time information is furnished by a time frame generator. The output of a thermocouple etc. is stored in the calibration channel unit. Data evaluation is possible by a PDF 11/24 computer. (C Constant Fraction Discriminator A Amplifier)...

CFD = Constant Fraction Discriminator TDC = Time to Digital Converter... 

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... 

Fig. 4.2 Constant fraction discriminator using zero cross triggering...

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. 

Figure 4.17. Constant fraction discrimination in TCSPC. Tbp Timing error due to puUe bei variations using leading-edge discrimination. Bottom Operation of a constant fraction discriminator.

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. 

FIGURE 4.5 Pulses processed by (a) a low-level discriminator, and (b) a constant-fraction discriminator. 

---