TABLE OF CONTENTS The direct conversion, or synchrodyne, receiver was invented in the late 1920s, but only with the advent of modern semiconductor technology has it come into its own as a real possibility for good-performance receivers. Although most designs are intended for novices, and lack certain features of high-grade superheterodyne receivers, the modern direct-conversion receiver (DCR) is capable of very decent performance. A case can be made for the assertion that the modern DCR is capable of performing as good as many midgrade ham and SWL communications receivers. Although that assertion might seem very bold, experience bears it out. Although no one, least of all me, would represent the DCR as capable of the best possible performance, modern DCR designs are no longer in the hobbyist curiosity category. In this chapter, you will find the basic theory of operation and some of the actual designs tried on the workbench.
The DCR is similar to the superheterodyne in underlying concept: The receiver radio frequency (RF) signal is translated in frequency by nonlinear mixing with a local oscillator (LO) signal ( heterodyning ). Figure 6-1 shows the basic block diagram for the front end of both types of receiver. The mixer is a nonlinear element that combines the two signals, F RF and F LO. The output of the mixer contains a number of different frequencies that obey the relationship:
| (6-1) |  |
where<i class="emphasis"> F</i><subo</sub> is the output frequency<i...
