TABLE OF CONTENTS There are a number of functions that are performed in specialized receivers that, dependent upon their particular use, are not present in all receivers. We have already addressed AGC, which is used in almost all AM receivers and some FM receivers. In this chapter, we address a number of other circuits. The first three-noise limiting and blanking, squelch, and AFC-have been used in many receivers. Diversity reception systems have been used extensively to counteract multipath fading. Adaptive processing and quality monitoring have begun to appear in many modern receivers, especially those that require frequency changes to optimize performance because of propagation or interference conditions.
The wholesale move to DSP-based radios has brought with it a host of additional ancillary devices and circuits, many of which have more to do with computers than with radios. Such components include:
Microprocessors, including specialized devices such as DSPs and ASICs
Control elements and control surfaces, including keyboards and displays
Software elements, including BIOS and applications code
The ultimate manifestation of the digital radio is the "receiver on a chip" approach. Examples of such designs, their benefits and drawbacks, are addressed in Chapter 10.
While the standard measurement of receiver sensitivity is performed with additive gaussian noise, pulse interference is often the limiting noise for communications receivers at frequencies through the lower portions of the UHF band. This unpleasant interference may be generated by many different sources. To provide a better understanding of this problem,...
