Introduction to Receiver Design
The basic structure of an optical receiver (figure 5.1) is similar to that of a direct detection r.f. receiver: a low-noise preamplifier, the front-end, feeds further amplification stages, the post-amplifier, before filtering. An important point to note is that the pre- and post-amplifiers are usually non-saturating. (If the amplifiers did saturate, charge storage in the transistors would tend to limit the maximum detected data-rate.) Because of this, we can use the same pre- and post-amplifier combination to detect analogue or digital signals. The difference between the two receivers arises from the way they process the signals after amplification. As digital optical communications systems are quite common, most of the work presented is devoted to a performance analysis of digital receivers. However, analogue systems are used to transmit composite video and signals from optical fibre sensors, and so we will consider analogue receiver performance towards the end of this chapter.
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