Optical Receiver Circuits

The overall task of the optical receiver is to extract the information that has been placed on the modulated light carrier by the distant transmitter and restores the information to its original form. The typical through-the-air communications receiver can be broken down into five separate sections. These are: light collector (lens), light detecto
Optical Receiver Circuits - schematic

r (PIN), current to voltage converter, signal amplifier and pulse discriminator. There may also be additional circuits depending on the kind of the signal being received. As an example, a receiver that is extracting voice information will need a frequency to voltage converter and an audio amplifier to reproduce the original voice signal. Computer data receivers will also need some decoding circuits that would configure the transmitted serial data bits into 8 bit words. However, this section will concentrate on the circuits needed for processing voice information. Volume II of this book will contain additional circuits for digital data receivers. As discussed in the section on light detectors, the silicon PIN photodiode is the recommended detector for most all through-the-air communications. Such a detector works best when reversed biased. In the reversed biased mode it becomes a diode that leaks current in response to the light striking it. The current is directly proportional to the incident light power level (light intensity). When detecting light at its peak spectrum response wavelength of 900 nanometers, the silicon PIN photodiode will leak about 0. 5 micro amps of current for each microwatt of light striking it. This relationship is independent to the size of the detector. The PIN photodiode size should be chosen based on the required frequency response and the desired acceptance angle with the lens being used. Large...

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