TABLE OF CONTENTS Modern communications receivers are almost invariably of superheterodyne design. Depending on the application, there may be one, two, or occasionally three frequency conversions. The circuit in which a frequency conversion is performed is usually referred to as a mixer, although the term converter is also used. In older literature, the first or second detector is often used to designate the first or second mixer. The demodulator circuit in this case is usually referred to as the nth detector, where n is one more than the number of frequency conversions. In the mixer circuit, the RF signal and an LO signal are acted upon by the nonlinear properties of a device or devices to produce a third frequency, referred to as an IF, or the nth IF when there is more than one mixing process. The IF is selected by a filter from among the various frequencies generated, and higher-order products may produce various spurious responses, as described in an earlier chapter.
Because of the large number of signals received by the antenna, it is customary to use preselection filtering to limit the potential candidates for producing spurious responses. The narrower the preselection filter, the better the performance of the receiver in regard to spurious responses and other IM products. However, narrow filters tend to have relatively large losses that increase the NF of the receiver, and for receivers designed for covering a wide range of frequencies, the preselector filters must be either tunable or...
