The Model 935-P Quad 200-MHz Constant-Fraction Discriminator incorporates four separate and independently adjustable timing discriminators in a single-width NIM module. Except where indicated otherwise, the descriptions and specifications apply to each of the four channels in the module.
The ability of the Model 935-P to provide constant-fraction timing on fast, negative-polarity signals as narrow as 1 ns (FWHM) makes it ideal for use with microchannel plates, fast photomultiplier tubes, fast scintillators, and fast silicon detectors. The exceptionally low walk delivered by the Model 935-P is vital in achieving the excellent time resolution inherent in these fast detectors over a wide dynamic range of pulse amplitudes. The Model 935-P can also be used with scintillators such as Nal(TI) which have long decay times. To prevent multiple triggering on the long decay times, the width of the blocking output can be adjusted up to 1 µs in duration. The Model 935-P uses the constant-fraction timing technique to select a timing point on each input pulse that is independent of pulse amplitude. When properly adjusted, the generation of the output logic pulse corresponds to the point on the leading edge of the input pulse where the input pulse has risen to 20% of its maximum amplitude. To achieve this constant-fraction triggering, the input pulse is inverted and delayed. The delay time is selected by an external delay cable (DLY) to be equal to the time taken for the input pulse to rise from 20% of maximum amplitude to maximum amplitude. Simultaneously, the prompt input signal is attenuated to 20% of its original amplitude. This attenuated signal is added to the delayed and inverted signal to form a bipolar signal with a zero crossing. The zero crossing occurs at the time when the inverted and delayed input signal has risen to 20% of its maximum amplitude. The zero-crossing discriminator in the Model 935-P detects this point and generates the corresponding timing output pulse.
"Walk" is the systematic error in detecting the time for the 20% fraction as a function of input pulse amplitude. Minimizing walk is important when a wide range of pulse amplitudes must be used, because walk contributes to the time resolution. The Model 935-P uses a transformer technique for constant-fraction shaping to achieve the exceptionally wide bandwidth essential for processing input signals with subnanosecond rise times. As shown in Fig. 1, this results in a walk guaranteed <±50 ps and typically <±25 ps over a 100: 1 dynamic range of input pulse amplitudes. The patented shaping technique also provides a zero-crossing monitor output that facilitates quick and accurate walk adjustment, because it displays the full input signal amplitude range.
The extremely short pulses from microchannel plate multipliers and ultra-fast photomultiplier tubes require very short constant-fraction shaping delays. To accommodate these detectors, the Model 935-P incorporates a selectable compensation for the inherent internal delay.
The Model 935-P includes a number of controls that considerably broaden its utility. The threshold discriminator is useful fo rejecting low-level noise. A front-panel test point permits precise measurement of its setting in the range from -20 to -1000 mV. Each channel provides three bridged timing outputs. These are standard, fast negative NIM outputs. The outputs can be selected to have either updating or blocking characteristics. The updating mode is useful for reducing dead time in overlap coincidence experiments. The blocking mode simultaneously minimizes multiple triggering and dead time on scintillators with long decay times. The output pulse width is adjustable from <4 ns to >200 ns in the updating mode, and from <5 ns to >1 µs in the blocking mode. The pulse-air resolution is <5 ns at minimum pulse width in the updating mode.
Switches on the printed circuit board allow selection of which channels will respond to the front-panel fast-veto input. Additional fast gating capability is provided by individual gate inputs for each channel on the rear panel. The mode of these separate gate inputs can be individually selected to be either coincidence or anticoincidence via DIP switches on the printed circuit board. Each channel can also be programmed to ignore or respond to the slow bin gate signal on pin 36 of the power connector for NIM incorporating that signal.