Monte Carlo time domain noise simulation in nuclear electronics

Since radiation detectors and associated electronics operate on pulses, time domain or transient simulations are used but neglect electronic noise. Traditional noise analysis yields the rms magnitude of noise fluctuations but does not show how noise affects time-variant or nonlinear circuits. These effects are not well described by either noise-free time domain analyses or noise analyses, which yield only the rms magnitude of the noise. We have developed software that produces a time domain simulation, which represents electronic noise as a randomly varying waveform. The waveform is simulated as a superposition of random noise impulses into the preamplifier, with the appropriate noise weighting functions and with amplitude and frequency derived from the electrical parameters of the circuitry. The result has an accurate rms amplitude and frequency spectrum, including white series noise, white parallel noise, and series 1/f noise terms. The software combines this random electronic noise, time variant processes such as variations in detector risetime or pulse pile-up, and models both time-invariant and time-variant circuitry. It provides a physically intuitive and quantitative connection between noise sources, noise weighting functions, and a noisy waveform. This software was been used in designing the latest generation of Amptek's digital processor but is generally applicable for many different systems.