Constant fraction discriminator

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.Some signals do not have a sharp maximum, but short rise times t r {displaystyle t_{r}} .A time to digital converter assigns timestamps. The time to digital converter needs fast rising edges with normed height.The plastic scintillation counter 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 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 computers have problems with the inverse value.The incoming signal is split into three components.One component is delayed by a time t d {displaystyle t_{d}} , with 0 ≪ t d ≤ t r {displaystyle 0ll t_{d}leq t_{r}} - 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.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.If the discriminator triggers a sampler with a following comparator this is called a single channel analyzer (SCA).If an Analog-to-digital converter is used, this is called a multi channel analyzer (MCA).