Constant fraction discriminator
Encyclopedia
Maxima and minima
In mathematics, the maximum and minimum of a function, known collectively as extrema , are the largest and smallest value that the function takes at a point either within a given neighborhood or on the function domain in its entirety .More generally, the...
of a pulse by finding the zero of its slope
Slope
In mathematics, the slope or gradient of a line describes its steepness, incline, or grade. A higher slope value indicates a steeper incline....
.
Some signals do not have a sharp maximum, but short rise times
.Typical input signals for CFDs are pulses from plastic scintillation counter
Scintillation counter
A scintillation counter measures ionizing radiation. The sensor, called a scintillator, consists of a transparent crystal, usually phosphor, plastic , or organic liquid that fluoresces when struck by ionizing radiation. A sensitive photomultiplier tube measures the light from the crystal...
s, such as those used for lifetime measurement in positron annihilation
Electron-positron annihilation
Electron–positron annihilation occurs when an electron and a positron collide. The result of the collision is the annihilation of the electron and positron, and the creation of gamma ray photons or, at higher energies, other particles:...
experiments. The scintillator pulses have identical rise times that are much longer than the desired temporal resolution. This forbids simple threshold triggering, which causes a dependence of the trigger time on the signal's peak height, an effect called time walk (see diagram). Identical rise times and peak shapes permit triggering not on a fixed threshold but on a constant fraction of the total peak height, yielding trigger times independent from peak heights.
From another point of view
A time to digital converterTime to digital converter
In electronic instrumentation and signal processing, a time to digital converter is a device for converting a signal of sporadic pulses into a digital representation of their time indices. In other words, a TDC outputs the time of arrival for each incoming pulse...
assigns timestamps. The time to digital converter
Time to digital converter
In electronic instrumentation and signal processing, a time to digital converter is a device for converting a signal of sporadic pulses into a digital representation of their time indices. In other words, a TDC outputs the time of arrival for each incoming pulse...
needs fast rising edges with normed height.
The plastic scintillation counter
Scintillation counter
A scintillation counter measures ionizing radiation. The sensor, called a scintillator, consists of a transparent crystal, usually phosphor, plastic , or organic liquid that fluoresces when struck by ionizing radiation. A sensitive photomultiplier tube measures the light from the crystal...
delivers fast rising edge with varying heights.
Theoretically, the signal could be split into two parts. One part would be delayed and the other low pass filtered, inverted and then used in a variable gain amplifier
Amplifier
Generally, an amplifier or simply amp, is a device for increasing the power of a signal.In popular use, the term usually describes an electronic amplifier, in which the input "signal" is usually a voltage or a current. In audio applications, amplifiers drive the loudspeakers used in PA systems to...
to amplify the original signal to the desired height. Practically, it is difficult to achieve a high dynamic range for the variable gain amplifier and analog computer
Analog computer
An analog computer is a form of computer that uses the continuously-changeable aspects of physical phenomena such as electrical, mechanical, or hydraulic quantities to model the problem being solved...
s have problems with the inverse value.
Principle of operation
The incoming signal is split into three components.One component is delayed by a time
, with 
- it may be multiplied by a small factor to put emphasis on the leading edge of the pulse -
and connected to the noninverting input of a comparator
Comparator
In electronics, a comparator is a device that compares two voltages or currents and switches its output to indicate which is larger. They are commonly used in devices such as Analog-to-digital converters .- Input voltage range :...
.
One component is connected to the inverting input of this comparator.
One component is connected to the noninverting input of another comparator.
A threshold value is connected to the inverting input of the other comparator.
The output of both comparators is fed through an AND gate
AND gate
The AND gate is a basic digital logic gate that implements logical conjunction - it behaves according to the truth table to the right. A HIGH output results only if both the inputs to the AND gate are HIGH . If neither or only one input to the AND gate is HIGH, a LOW output results...
.
Note from this description that a discriminator without that constant fraction would just be a comparator.
Therefore the word discriminator
Foster-Seeley discriminator
The Foster-Seeley discriminator is a common type of FM detector circuit, invented in 1936 by Dudley E. Foster and Stuart William Seeley. The circuit was envisioned for automatic frequency control of receivers, but also found application in demodulating an FM signal.It uses a tuned RF transformer...
is used for something different
(namely for an FM-demodulator).
Note also that often the logic levels are shifted from -15 V < low < 0 < high < 15 V delivered by the comparator to 0 V < low < 1.5 V < high < 3.3 V needed by CMOS logic.
Applications
If the discriminator triggers a sampler with a following comparator this is called a single channel analyzer (SCA).If an Analog-to-digital converter
Analog-to-digital converter
An analog-to-digital converter is a device that converts a continuous quantity to a discrete time digital representation. An ADC may also provide an isolated measurement...
is used, this is called a multi channel analyzer (MCA).