Introduction
1
Introduction
1.1 General description
The model 2128N is a fully dc-coupled constant fraction discriminator with a dynamic range of up to 1000:1.
Selectable fraction and 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 (e.g. germanium detectors)
LET
leading edge mode for single photon counting
(more features are given in section 6)
The model
2128N
accepts negative polarity pulses to its 50Ω terminated dc-coupled input. For positive
polarity input pulses see section 6.
The constant fraction composite signal is formed by the sum of a direct, attenuated (fraction f) signal path
and a delayed, unattenuated path. The delay time is selectable according to the propagation delay of a
(external) 50Ω BNC cable. Careful selection of fraction and delay cable provides full compensation of timing
distortions due to both amplitude and rise time variations in the input signal. Output signals are generated
whenever the the input signal exceeds the selected threshold set by a front panel 10 – turn dial
potentiometer.
Four simultaneous, independent output signals are provided. The two positive NIM voltage outputs are
adjustable in width; the width duration sets the internal dead time as required to suppress spurious outputs
due to input signal anomalies. The two independent negative NIM current outputs are derived as fixed width
pulses (see also section 3 and 4).
The wide dynamic range of the model 2128N permits its use in many timing applications without the need of
fast pulse amplifiers.
1.2 Setup information
1.2.1
Selection of modes
Front panel mode switch
CFT
constant fraction discriminator
CFRR
constant fraction discriminator with slow rise time reject
LET
leading edge trigger
1.2.2
Fraction
The factory set fraction is f = 0.4. If a different fraction is required, the resistors R
1
and R
2
( printed board ) have to be modified as follows:
Z
0
= 50
R
1
= Z
0
· f
R
2
= Z
0
/ f
6
F
ComTec GmbH