Sub Project 8

Constant Fraction Discriminator

The Constant Fraction Discriminator (CFD) is the level one trigger in the VERITAS telescope array. The output from each Photo Multiplier Tube (PMT) is routed directly in to the input of the CFD. There are 500 PMTs per telescope which each constitute a pixel in the camera. The CFD will “trigger” (produce an output pulse) if the voltage of the output from the PMT crosses the threshold set by the telescope operator.

The constant fraction part of the CFD is a circuit that causes the output trigger to occur at the same after the same amount of time from the beginning of the event regardless of amplitude or shape of the input pulse. This is done by using a zero crossing trigger. The incoming pulse is split in two, one is inverted and attenuated by some fraction f. Which has been determined to be most efficient at 0.3. The resulting pulses are added to create a zero crossing discriminator.

There are four programmable features on every CFD. They are threshold, output width, rate feed back and trigger mode.

1. Threshold: As mentioned above this setting determines how large an input pulse must be to produce an event or trigger. A digital to analog converter (DAC) is used to program this setting. A binary sequence representing an integer value is streamed to the CFD and the DAC converts it in to a voltage.

2. Output Width: The output of the CFD is a standard ECL pulse. The width of this pulse is determined by the setting given by the operator. Just like the threshold setting a DAC is used to vary the amount of voltage driven across the capacitor whose discharge time determines the width of the pulse.

3. Rate Feed Back: The Rate Feed Back (RFB) circuit offsets input pulse to the zero crossing discriminator through the use of a frequency to voltage translator. This will automatically adjust the offset when the noise level changes. The strength of this circuit is controlled by changing the resistance in a programmable potentiometer.

4. Trigger Mode: The CFD can operate in two modes. Simple trigger and zero crossing discriminator.

CFD Specifications

Input 8-16 mV/pe (to be determined based on level of electronics noise)
Bandwidh c500 MHz analog
Slew Rate < 1 ns over 1:1000 dynamic range.
Electronics Noise Dispersion at Input < 1 pe
Output ECL
Output Pulse Width Range 5-25 ns (programmable, 12-bit)
Output Pulse Width Dispersion <1 ns
Trigger Modes CFD/Threshold (selectable)
Threshold Range +1.0 - -2.5 (programmable, 12-bits)
Time Jitter** < 1-2 ns (pending discussion and approval)
Time Jitter/Trigger Efficiency Rate Feedback (programmable 7-bits)
RFB Status Output 0-5 V (analog signal, update rate slower than 2 Hz)
Program Interface Rate < 3 MHz

Overview of Level 1 Trigger Project

Veritas L1 Trigger -- Constant Fraction Descriminator (Jun 2001)

CFD & fADC (Aug 3-6, 2001)

Graphical Test Data

Overview of CFD

The following documentation details the testing process of the CFD

CFD Test Definitions

CFD Database Table Definitions

CFD Testing Manual

CFD Testing How-To

ADR2000 Control Board Manual