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Grating Detector Circuit with DC Threshold Store

IP.com Disclosure Number: IPCOM000089289D
Original Publication Date: 1977-Oct-01
Included in the Prior Art Database: 2005-Mar-04
Document File: 2 page(s) / 50K

Publishing Venue

IBM

Related People

West, DL: AUTHOR

Abstract

Automatic threshold tracking in order to compensate for light source aging, circuit parameter drifts, etc., is easily accomplished for AC signals, but is difficult to accomplish in situations where the mechanism comes to rest. The illustrated circuit for storing a DC voltage, while not being new, is unique in its simplicity and low cost.

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Grating Detector Circuit with DC Threshold Store

Automatic threshold tracking in order to compensate for light source aging, circuit parameter drifts, etc., is easily accomplished for AC signals, but is difficult to accomplish in situations where the mechanism comes to rest. The illustrated circuit for storing a DC voltage, while not being new, is unique in its simplicity and low cost.

Simplistically, the circuit is an analog-to-digital, digital-to-analog converter with a positive and negative peak detector. Fig. 1 illustrates a typical signal developed at the collector of a phototransistor which, for example, is receiving light through a shifting grating from a light source. Aging of the light source, contamination, circuit drift, etc., may cause the peak-to-peak value or the average of the peak-to-peak (VTH) to shift throughout the life of the machine. In any detection scheme where the signal may fluctuate from DC up to a maximum frequency of the particular system (several thousand cycles per second) and the signal is converted to a digital output, the threshold of switching from a zero to a one and back to a zero state becomes very important. The ideal condition is to be able to sense and store the threshold voltage for some predetermined period.

Fig. 2 illustrates a typical system circuit design for storing in a simple and yet inexpensive manner the DC threshold to compensate for the aging, etc. With relative motion between, for example, the grating (not shown) and the detector phototransistor 1, an AC signal similar to that shown in Fig. 1 will be generated by the phototransistor 1 and pass through the isolation amplif...