Constant fraction discriminator

Typical input signals for CFDs are pulses from plastic scintillation counters, such as those used for lifetime measurement in positron annihilation experiments.

The scintillator pulses have identical rise times that are much longer than the desired temporal resolution.

The time-to-digital converter needs fast rising edges with normed height.

The plastic scintillation counter delivers fast rising edge with varying heights.

Practically, it is difficult to achieve a high dynamic range for the variable-gain amplifier, and analog computers have problems with the inverse value.

– 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.

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).