Thursday, January 1, 2009

s/h to the mouchous


random signals...

Zener Type Avalanche Noise Generation:

"Matthew J Bennion,, pointed out that noise sources are important as random number generators, for situations where you really do need truly random numbers (as opposed to the pseudorandom number generators implemented in software). An example where this is necessary might be encryption and decryption"

"I agree with what you have on Noise Diodes, but I think you overlook the main use: measurement of noise figure. To characterize, for example, a receiver, one adds noise from a noise generator until the observed noise just doubles (or changes by some other convenient factor). Shannon's theorem says that noise always adds (Murphy's law says the same thing). Since the noise generator would typically make a relatively high level of noise that would be attenuated down by precicesly calibrated attenuators, one can know how much noise was required to equal the intrinsic noise of the receiver, and thus infer its performance."

"This white noise generator is based on the avalanche noise generated by a zener breakdown phenomenon. It is created when a PN junction is operated in the reverse breakdown mode. The avalanche noise is very similar to shot noise, but much more intense and has a flat frequency spectrum (white). The magnitude of the noise is difficult to predict due to its dependence on the materials."

Shot Noise Generation:

"Shot noise exists because phenomena such as light and electrical current consist of the movement of discrete, quantized 'packets'. Imagine light coming out of a laser pointer and hitting a wall. That light comes in small packets, or photons. When the spot is bright enough to see, there are many billions of light photons that hit the wall per second. Now, imagine turning down the laser brightness until the laser is almost off. Then, only a few photons hit the wall every second. But the fundamental physical processes that govern light emission say that these photons are emitted from the laser at random times. So if the average number of photons that hit the wall each second is 5, some seconds when you measure you will find 2 photons, and some 10 photons. These fluctuations are shot noise."

The full picture/recent developements and changes in thinking...

The Poisson Distribution:


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