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Signal to Noise Scanning System

IP.com Disclosure Number: IPCOM000094050D
Original Publication Date: 1966-May-01
Included in the Prior Art Database: 2005-Mar-06
Document File: 2 page(s) / 25K

Publishing Venue

IBM

Related People

Kippenhan, BW: AUTHOR [+2]

Abstract

The performance of scanning systems utilizing photomultiplier tube detectors can be enhanced over a wide operating range by controlling the signal-to-noise ratio of the system. Noise contributions to the signal stem from three basic sources, namely, dark current noise, shot or random noise, and thermal noise. In those systems using photomultiplier tubes as detectors, only the second noise source is significant.

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Signal to Noise Scanning System

The performance of scanning systems utilizing photomultiplier tube detectors can be enhanced over a wide operating range by controlling the signal-to-noise ratio of the system. Noise contributions to the signal stem from three basic sources, namely, dark current noise, shot or random noise, and thermal noise. In those systems using photomultiplier tubes as detectors, only the second noise source is significant.

Shot noise is random in nature and has a minimum RMS value proportional to the square root of the signal at the time of conversion from light to electrical signal. The RMS value has an amplitude proportional to the square root of the bandwidth of the electronic circuits. The light brightness from the CRT is limited by the tube characteristics. Therefore, a slow scan velocity requires a lower intensity or brightness. The signal-to-noise ratio SNR is K square root brightness/bandwidth.

Thus, as the intensity decreases, the SNR is also degraded. The ratio can be maintained constant, or near optimum, by maintaining maximum intensity for a given velocity and decreasing the bandwidth a corresponding amount. Such is made possible due to the reduction of the information rate at lower scan velocities.

In the circuit a signal proportional to velocity controls the band width of the output circuit as well as the beam intensity. Both the intensity and bandwidth are varied in proportion to the scan velocity to maintain a constant signal-t...