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 CFD - Constant fraction discriminator


<p>CFD V 2.0</p>

CFD V 2.0

Constant Fraction Discriminator (CFD) is a perfectly suitable solution for the low-jitter digitization of analogue signals in systems that use microchannelplates or channeltrons as electron multiplier devices.

Elettra Sincrotrone Trieste's CFD exploits a fast crossover configuration, which is somehow sensitive to variations in the width of the pulses, but is more robust against noise if compared with other kinds of configurations (e.g. zero crossing CFDs).

The delay-line (usually equivalent to a 4ns time delay) is obtained with a transmission line of proper length.

The amplitude of the charge pulses produced by an MCP stack shows a wide dynamic range and there is evidence that pulse height distributions strongly depend on the voltage applied to the MCPs and on the effective rate of events.
CFDs, rather than simple threshold discriminators, are required in timing measurements because the arrival time of a particle cannot be measured with accuracy if the process is influenced by signal amplitude variations. Signal width (FWHM), instead, is almost constant and mainly depends on the electron multiplying system.
The CFD are optimized by default for pulses with a nearly Gaussian shape, full width at half maximum (FWHM) of about 4 ns, amplitude between 0.5V and 2V, rise and fall times around 2.8 ns. In such conditions, a measured jitter lower than 20 ps and a walk error lower than 40 ps are achieved.