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An analysis of timing jitter from leading-edge discriminators (LED's) and constant-fraction discriminators (CFD's) is presented. The jitter is calculated for the cases of random white noise, coherent noise (e.g., digital switching noise or ac power-line hum), and mixed (random and coherent) noise for both discriminator architectures. A general jitter equation valid for all of these conditions is derived. It is shown that the discriminator bandwidth for minimum jitter is strongly dependent on the amount of coherent noise. This effect is shown to be more pronounced for the LED. Even though off-line walk adjustments are possible for many timing measurements, the CFD is shown to provide a major advantage by acting as a correlated double sampler that removes much of the coherent noise expected in large, multichannel collider detector systems.