Even for simple counting applications, it is essential to discriminate between the low-amplitude noise and the higher-amplitude signals. The lower level threshold in any of the timing discriminators is intended for that purpose. To make a crude adjustment of that threshold, turn off the source of molecular ions, and lower the threshold until the rate of discriminator output pulses increases abruptly. Then raise the threshold until that high counting rate barely disappears. For this adjustment, the output logic pulse from the discriminator can be counted using a Model MCS-PCI-P, a ratemeter, a counter/timer, or in a time digitizer. If an oscilloscope is available, it can also be used to monitor the output counting rate.

For the counting application depicted in the diagram below, a separate timing discriminator is not needed, because the required noise discriminator function is built into the MCS-PCI-P input. The Model 9353-P Time Digitizer also has a built-in noise discriminator for the Start and Stop inputs, so that in cases where the minimum pulse-pair resolving time is more important than avoiding timing slewing with pulse amplitude ("walk"), the external timing discriminator can also be deleted.

For applications where the inherent variation in flight times for the lightest molecule is not large compared to the rise time of the detector pulse, a timing discriminator that minimizes "walk" is essential for achieving the best time or mass resolution. Walk can be simply explained with reference to the diagram below, which depicts two signals having the same rise time, but differing amplitudes. The arrival time of the pulses is determined by when they cross the fixed discriminator threshold on the leading edge of the pulse. Clearly, the pulse with the larger voltage amplitude crosses the threshold earlier than the pulse with the smaller amplitude. Historically, this difference in crossing time has been called "walk", because the arrival-time marker walks to later times as the pulse amplitude is decreased. Walk will contribute to the time resolution by an amount roughly equal to the rise time of the pulses. Because the pulse-height variations are random, the walk will combine with other sources of timing uncertainty, such that the final uncertainty is the square root of the sum of the squares of all individual contributions to the timing uncertainty.

Two types of timing discriminators are available to virtually eliminate the walk contribution to the timing spectrum. The first is the Constant-Fraction Timing Discriminator as incorporated in the Model 935-P. In this circuit the original pulse is delayed, inverted and added to an attenuated copy of the prompt pulses, as shown in the diagram below.

For pulses with rise times <1 ns and widths <1 ns (FWHM), a more manageable technique creates a bipolar pulse with a zero-crossing that occurs after the maximum amplitude of the pulse, as shown in the diagram below.

Bipolar pulse shapes employed in the pico-TIMING discriminators for
fast microchannel plate detectors. The arrival time is derived from the zero-crossing point.

This approach is used in the Model 9307-P pico-TIMING Discriminator and the Model 9327-P 1 GHz Amplifier and Timing Discriminator. Because all pulses cross through zero at the same time, independent of pulse amplitude, this method also eliminates the walk, and is ideal for the very narrow signals produced by the fastest microchannel plate detectors.

To compare the major features of the various timing discriminators, click here.