To test a range of Multi-Channel Analyzer functions, we have designed a random pulse generator that utilizes a charge control concept. We provide controlled voltage transitions at average rates from 10 Hz to 1 MHz. These voltage transitions may be coupled to a charge sensitive amplifier system via the standard calibration capacitor to induce impulses of charge, thereby closely approximating the signal from a detector. Between pulses, the voltage decays in an exponential fashion not unlike any tail pulse generator. However, the voltage transitions (or step) is independent of the decay, thus recurring steps will ride up the tail of the decay, eliminating the step amplitude dependence upon decay time (Td) found in other tail pulse generators. This last fact permits repetition rates far in excess of the 1/8 Td limitation commonly incurred.
More on Applications for the DB-2
The primary use of this pulse generator is to simulate actual operating conditions without requiring a live source and detector combination. Such parameters as frequency response, linearity, and discrimination levels may easily be measured without the inconvenience of dim oscilloscope display or long accumulation times. Proper operation of baseline restorer circuits may be quickly verified. Scalers and ratemeters may be checked for satisfactory pulse recognition under random pulse conditions.
The negligible amplitude shift with frequency of the pulser makes the standard frequency test using a live source and a low rate precision pulse generator unnecessary.
Although most test applications will find the pulser connected to the test input of a charge sensitive preamplifier, it is possible to simulate the preamp itself with the pulse generator. The pulser is connected directly to the main amplifier and the preamp decay time constant is matched by proper selection of the pulser fall time. Set up of a system containing an inaccessible preamp can then be accomplished with ease.
For accurate simulation of detector pulse shapes, the rise time control should be adjusted to match 2.2 times the detector decay time constant. For example, if a pulse shape analyzer working with CsI-NaI phoswich is to be tested, the pulse generator rise time should be set to 0.5 µsec rise time for the NaI signal, and 2 µsec for the CsI signal. Intermediate signals are best obtained by mixing the outputs from two synchronized generators, 2 µsec rise time. By varying the amplitude ratio of the two generators, intermediate values of rise time are generated.
Solid state and plastic detectors have decay constants far shorter than the adjustment range of this generator. However, the shaping time constants used in virtually all systems are greater than the 100 nsec minimum rise time. The ballistic deficit formula predicts the reduction in amplitude, B. D., for a shaping system containing identical time constants for all shaping.
B.D. = (4)
where n = the number of integrations with time, constant =RC, and tr is the rise time of the preamp output. The preamp output rise time may be calculated from:
where tp is the pulser rise time and ti is the rise time of the preamp in response to a unit step of zero rise time. The ballistic deficit for a preamplifier with a ti of 10 nsec used with a shaping amplifier6 with 1 µsec time constants would be only 0.02% when used with this pulse generator. Therefore, the ballistic deficit caused by this pulser may be ignored for most applications.
The external reference allows remote programming of the amplitude of the pulser, and the external trigger permits control of the output pulse rate. The latter provision is especially convenient if the average random rate needs to be controlled and an external random clock is unavailable. By placing the pulser in the random mode, a periodic waveform at the external trigger input will control the average random rate.