The Discriminator or Single-Channel Analyzer SCA

The SCA is used to eliminate the electronic noise and, in general, to reject unwanted pulses. When a pulse is amplified, the electronic noise that is always present in a circuit is also amplified. If one attempts to count all the pulses present, the counting rate may be exceedingly high. But electronic noise is a nuisance and it should not be counted. 

In some cases, one may want to count only pulses above a certain height, 

There are two dials on the front panel of the unit. One is marked E, for energr, or LLD, for lower-level dial the other is marked AE or ULD/AE, for upper-level dial/AE. There is also a two-position switch with INT (integral) and DIFF (differential) positions. In the INT position, only the E dial operates, and the unit functions as a discriminator. In the DIFF position, both E and AE operate, and the unit is then a single-channel analyzer. 

In some other commercial models, instead of INT and DIFF positions, the instrument has special connectors for the desired output. 

The single-channel analyzer (switch position DIFF). Both E and AE dials operate. Only pulses with heights between E and E + AE are counted (Fig. 

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