Constant Fraction Discriminator 2128N

Description: The Model 2128N replaces the Model 2128. It is a fully dc-coupled constant fraction discriminator with a dynamic range of up to 1000: 1. Three operating modes provide optimum time resolution for many detector types and applications. For example:  

  • CFT-Constant fraction mode for fast detectors
  • CFRR-Slow rise time reject mode for Ge detectors
  • LET-Leading edge mode for single photon counting 

The Model 2128N has a dc-coupled 50 ohm input which accepts negative pulses. The constant fraction composite signal is formed by the sum of a direct, attenuated signal path and a delayed. unattenuated path. The delay time is user selected by cable delay. Optimum selection of this external delay provides full compensation for timing distortions due to both amplitude- and rise time variations in the input signal. Four simultaneous, independent output signals are provided. The two positive outputs are adjustable in width, the width duration sets the internal dead time required to suppress spurious outputs due to input signal anomalies. The two negative outputs are fixed-width pulses keyed to the start of the dead time period. The front panel ADJUST control and INSPECT output permit the user to trim the time walk characteristics of the experimental setup for optimum timing resolution. A novel frontpanel LED indicates count rate by color change. 


  • 100 MHz Count Rate
  • 1000: 1 Dynamic Range
  • Three Operating Modes
  • Walk < 30 ps typically for 100: 1 Range
  • Simultaneous Dual Positive and Negative Outputs
  • Multicolor Count Rate Indicator
  • DC coupling
  • Easy to setup and use


Inputs input: Accepts - 5 mV to - 3.5 V linear pulses: width:> 1ns, Zin= 50 Ohms, dc coupled; front panel BNC connector. 

Delay: 2 front panel BNC connectors accept 50 Ohm delay cable to form the internal constant fraction signal. 

Inspect: Displays signal of zero crossing discriminator for use in trimming time walk. 

Neg Outputs: Two independent negative current outputs, each providing -32 mA into 50 ohms; rise time < 3 ns, pulse width 5 ns nominal, dc coupled. 

Pos Outputs: Two independent positive voltage outputs providing 2 V (minimum) into 50 ohms, rise time < 10 ns, width adjustable by adjacent width trimming potentiometer, which also determines internal dead time.

Front panel 10-turn locking dial potentiometer to set acceptance threshold for input pulses: range - 5 mV to - 1V. 

Adjust: Front panel trimpot to compensate walk of the internal zero crossing discriminator. 

Leading edge width: width of the leading edge signal is internally set by the trimming pot on the bard to 20 ns.

CFRR-CFT-LET: Front panel three position switch to select constant fraction with slow rise time reject (CFRR), basic constant fraction timing (CFT), or leading edge timing (LET) modes of operation. 

Output width: Front panel 22-turn screwdriver adjustable potentiometer to set width of slow positive output pulse, which is equal to the internal dead time of the discriminator - max. setting: 1.5 micro sec.


Dynamic Range: 1000 : 1

CF Mode Walk: <±50ps (typically ±30ps) for -30mV to -3V range with <2 nsec rise time. 

Counting Rate: to 100 MHz, limited by dead time (OUTPUT WIDTH setting). 

Pulse Pair Resolution: <10 ns, or as limited by dead time. 

Threshold Stability: Better than ±0.02 %/°C (± 200 ppm/°C) 

Threshold Linearity: ±0.25% Integral

Temperature Range: 0 to +50°C

Typical Cable Lenghts: (RG-58) For Plastic, Nal and Si (S.B.) detectors: 0.5 to 1.0 m For Planar Germanium detectors: 1.0 to 2.0 m For Coaxial Ge: 2.0 to 4.0 m

Typical Power Requirements: Standard version +6 V  150 mA, - 6 V  450 mA

Pysical size: Single width NIM module 3,43 X 22,12 cm (1.35 X 8.71 inches) per TID-20893 (rev.) 

Net weight: 0.7 kg (2.0 lbs.) 

Shipping weight: 2.2 kg (4.9 lbs.) 

can be modified to accept positive pulses
- ±12 V available on special request. 
- on request 220 V / 110 V (ac) independent from a NIM bin
- Output width for negative output can be modified
- threshold setting supplied externally via rear panel connector
- signal shaping using small fractions and RC shaping by modifying f and capacitors