TABLE OF CONTENTS Current radio receiver designs are, in general, inherently narrowband and can only achieve general (or broadband) coverage by the switching or alteration of narrowband elements. Certain designs, such as those used in many scanning receivers, do not attempt to overcome some of the fundamental receiver problems, such as blocking and image rejection, but rely on the user being able to eliminate interference by positioning of the set, or some other mechanism such as the use of a directional antenna. Where this is not possible, the user must tolerate the problem and the restriction in frequency usage whichensues, as the price of achieving wideband coverage.
The aim of the ideas presented in this section is to propose systems and techniques for the elimination of many or all of the fundamental problems which prevent the truly universal radio receiver from becoming a reality. The ideas presented are not fully developed solutions, currently in production, but more a series of proposals as to how some of these fundamental issues might be addressed.
There are three basic problems which need to be solved:
The diplexer filter required in a full-duplex transceiver must be specifically and carefully designed for its intended band of operation. This filter is usually either a helical component or formed from a dielectric (such as ceramic) and hence is almost impossible to tune in any sensible fashion over a reasonable range of frequencies. A multiple-band transceiver would therefore require a number of diplexer filters and this would very quickly...
