Design of a constant fraction discriminator for the vfat3 frontend asic of the cms gem detector. The model 7029a is a fully dc coupled discriminator which can be simultaneously used as a normal constant fraction discriminator bld part and as a differential constant fraction discriminator sca part with upper and lower level settings that allow precise selection of energy ranges from which the timing signal is to be derived. However in our case, the use of cfd is not expected to o. Two different values for the fraction in a constant fraction discriminator showing how the fraction depends on the delay delay cable plus internal delay and how time walk is virtually eliminated for signals of different amplitudes. However our work demands that all four requirements be met. Constant fraction discriminators cfds use a constant fraction or percentage of the input pulse to precisely determine the timing of the output pulse leading edge relative to the input signal. Te 1 for tne te 2b in this special case of variable counting rate, rt and rt can be interpreted as the probability per unit time of observing an event at. Model 7029a constant fraction differential discriminator. The expected timing performance obtained from an ideal coaxial geli detector using a triple constant fraction discriminator is calculated. Constant fraction discriminator involving automatic gain. An evaluation of the triple constant fraction discriminator. Measurements of timing resolution of ultrafast silicon detectors with. A multichannel integrated circuit for use in low and.
The constant fraction discriminators cfd8c, cfd7x, cfd4c, cfd1c and cfd1x 11. The cfd timing discriminator modules the roentdek cfd constant fraction discriminator units can be used for improving the timing response of electronic. The conventional constant fraction discriminator meets the first two of t. The principle operation is based on detecting the zerocrossing of the bipolar pulse obtained by subtracting a fraction of the input unipolar signal to its delayed copy fig. Implementation of the led is relatively easy, because the led generates the trigger signal at a speci.
However, a practical cfd produces some time walk because of the dependency of the response time. For that reason the constant fraction discriminator was created. Therefore we developed a constant fraction technique dcf. Constant fraction discriminator jeremy smith jeter hall david kieda vladimir vassiliev. Constant fraction discriminator in pulsed timeofflight. This manual describes the latest product series cfd8c4c1c and cfd1x7x. In this way a signal that has a wide time variance using a leading edge discriminator can be narrowed. Creation of a variable threshold tracking the signal always at a certain fraction of its amplitude out in d in the output bipolar signal has a zero crossing time t 0 depending only on network parameters t d, f t d 1. In this way a signal that has a wide time variance using a leading edge discriminator can be narrowed signi. An additional benefit of the constantfraction discriminator is that it essentially eliminates amplitudedependent time walk for signals having consistent rise times. A constant fraction discriminator cfd is a widely used timepickoff circuit because it ideally produces no time walk for signals with equal rise times.
Some signals do not have a sharp maximum, but short rise times. Obtaining results that are competitive but not state of the art, we also investigated the factors limiting our. The timing jitter depends on the slope at the zerocrossing depends on choice of f and td. The roentdek cfd constant fraction discriminator units can be used for. If the pulse shape remains invariant, then the cfd provides the best possible timing. A threshold is set to a fraction f amp of this amplitude, and the signal timing is determined by the crossing point. The model 2128 is a fully dccoupled constant fraction discriminator with a dynamic range of up to. Pdf the constant fraction discriminator in pulsed timeofflight. The design, simulation and printedcircuitboard pcb performance of cfd circuit were shown. Pdf development of a monolithic constant fraction discriminator. It must be ordered with a factory build in fixed delay in order to offer maximum performance. Time resolution studies using digital constant fraction.
Brown university authentication for webbased services. Design of a constant fraction discriminator for the. Timing measurements constant fraction timing basic principle. This circuit uses an onchip, distributed, rc delayline to realize the constant fraction shaping. A constant fraction discriminator was developed with a bipolar applicationspecific ic asic technology from nec without a delay. Page 6 of 26 the constant fraction discriminators cfd8c, cfd7x, cfd4c, cfd1c and cfd1x 11. The timing leg consists of a nowlin pseudoconstant fraction discriminator cfd composed of a leadingedge and a zerocrossing discriminator. It is called the differentiation constant fraction discriminator. Overview the lecroy model 3420 is a 16input constant fraction discriminator in a singlewidth camac module.
Figure 2 illustrates the process used to produce this trigger at a. The delayline is constructed from a narrow, 500mu serpentine layer of polysilicon above a wide, grounded, second layer of polysilicon. Model 7029a differential constant fraction discriminator. A constant fraction discriminator cfd is an electronic signal processing device, designed to. However, while leds with very low trigger thresholds yield good timing. Noise contribution to timing with fraction discriminators. Triple constantfraction discriminator sciencedirect. The ortec model 584 constantfraction discriminator allows good time resolution to be obtained from all commonly used detectors such as hpge, silicon chargedparticle, fast plastic, naitl, and photomultiplier tubes. Constant fraction discrimination is a technique to provide amplitudeindependent information about arrival time of an event. Abbaneo et al 2016 jinst 11 c01023 view the article online for updates and enhancements.
For this purpose the amplitude amp of the digitized pulse is determined using a digital algorithm for example parabolic interpolation around the sample m. The input range is optimized for typical detection tasks from pulse amplifiers of secondary. The are standalone modules for 19 cfd8c7x racks one height unit and come with an external mains adapter for 100250 v ac 5060 hz. Development of a monolithic constant fraction discriminator. The operation of the bipolar conversion and fast discriminator block is.
A comparison of a commercial constant fraction discriminator cfd with. Cftconstant fraction mode for fast detectors cfrrslow rise time reject mode for ge detectors. These logic pulses are passed to an ortec 567 tac gated by an ortec. A monolithic, cmos, constant fraction discriminator cfd was fabricated in the orbit semiconductor, 1. Performance of nondelayline constant fraction discriminator timing circuits. Fast constant fraction differential discriminator for positron lifetime spectroscopy 103 incertitude, an amplifier stage of cascode type q2q5 and the emitter follower q3 is inserted between bipolar conversion and fd. The suitability of several onchip constant fraction discriminator cfd shaping methods for use in the multichannel phenix lead scintillator detector has been investigated. The time resolution, which is an important parameter for the use of the gem. The nim cfd module was exclusively developed to operate on high repetition rates an short pulses below 10 ns fwhm. What is the theoretical time precision achievable using a dcfd.
Timing measurements another view of constant fraction discriminators the constant fraction discriminator can be analyzed as a pulse shaper, comprising the delay attenuator subtraction driving a trigger that responds to the zero crossing. When the constant fraction discriminators received two input pulses that were close together, 19ns apart, the constant fraction discriminator failed to provide an output in certain cases. Discriminators generate precise logic pulses in response to input signals exceeding a given threshold. The output from each constant fraction discriminator was connected to a tof clock and an hp, a900 computer for the data acquisition. The ortec model 584 constant fraction discriminator allows good time resolution to be obtained from all commonly used detectors such as hpge, silicon chargedparticle, fast plastic, naitl, and photomultiplier tubes. Apr, 2012 constant fraction discriminator cfd is one of theoretic method which can locate timing point at same fraction of echo pulse in pulsed timeofflight tof laser rangefinding. These logic pulses are passed to an ortec 567 tac gated by an ortec 414a fast coincidence module. Principle constant fraction discriminator cfd discriminator input bandwidth 4 ghz time resolution fwhm rms, electr. A highperformance, lowcost, leading edge discriminator. Fraction cf mode or constant fraction with slow risetime reject sr. Circuit diagram of the triple constantfraction discriminator.
Discriminator allows good time resolution to be obtained from all commonly used detectors such as hpge, silicon. In this paper, the theory of cfd method was analyzed in reality condition. The second method requires use of a cable delay, so it is difficult to achieve stability and high density integration. The zerocrossing discriminator has its offsets dynamically nulled. The constant fraction discriminator alleviates this problem by using a constant fraction, f, of the input pulse to precisely determine the timing of the output pulse relative to the input signal. This manual is intended to provide instruction regarding the setup and operation of the lecroy model 3420 fraction discriminator. Cft constant fraction mode for fast detectors cfrrslow rise time reject mode for ge detectors. Constant fraction discriminator involving automatic gain control to reduce time walk abstract. The constant fraction discriminators cfd8c, cfd7x, cfd4c.
The choice among one or another realization of constant fraction discriminator cfd 1 or double fraction discriminator dfd 2 has to be done. Three operating modes provide optimum time resolution for many detector types and applications. Fasttiming discriminator introduction ortec where time t1 is distinguished from t only for the purpose of integration over the time interval, and utnete is a unit step function defined by utnete 0 for tne. Positron annihilation lifetime spectroscopy pals report.
Constant fraction discriminator cfd is one of theoretic method which can locate timing point at same fraction of echo pulse in pulsed. A constant fraction discriminator cfdworks by looking at the whole signal and emits the logic pulse when the input signal reaches a certain fraction of the peak value. Constant fraction discriminator wikipedia republished wiki 2. The cylindrical fission chamber was inserted 90 cm deep into the 75t lead pile in a hole whose axis is 60 cm from the bottom and rear face of the lead cube and was surrounded by machined lead blocks so that. However, a practical cfd produces some time walk because of the dependency of the response time in an involved comparator on the input signal size. Integrated constantfraction discriminator shaping techniques. There are two main types of discriminators, the leading edge discriminator and the constant fraction discriminator. Three cfd circuits utilizing a distributed rc delayline, a lumpedelement rc delay line and the nowlin shaping method have been realized in a standard 1.
Design of a constant fraction discriminator for the vfat3. The results are compared with experimental data and methods of obtaining optimum performance discussed. The first method is very common, but with a penalty on time walk. A constant fraction discriminator cfd is an electronic signal processing device, designed to mimic the mathematical operation of finding a maximum of a pulse by finding the zero of its slope. The signals from a given detector will typically have identical pulse shapes and a cfd utilizes this fact in order to provide a constant trigger point regardless of the signal amplitude. The phillips scientific model 715 constant fraction discriminators that we used did not behave exactly as expected.
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