This chapter has 2 purposes. First, it provides some detailed figures and background information on electronics, which will aid the reader in understanding other chapters of this book. Second, and more importantly, it provides for the 1st time in electronic monitoring literature, a rigorous, stage-by-stage description and comparison of signal processing (i.e., secondary) circuits of the 7 AC monitor designs most commonly used for studies published from 1964 to 1994. Effects of these circuits on both the emf and R components of aphid waveforms are considered. Like the more theoretical comparisons of chapter 3, our comparisons assume that these 7 monitor designs have been slightly modified to use a 10⁹ Ω input impedance (rather than their designed 10⁶ Ω impedance). This allowed us to assume that all emf signals would be present at the beginning of signal processing, so that we could then determine the subsequent effect of each stage on emf. These comparisons are based on recently acquired schematics, some of which previously have never been available, for all 7 monitor designs. We also include simulations of frequency response characteristics of each circuit stage in each monitor design, which provide for the 1st time a realistic assessment of the degree of passage of specific R or emf component frequencies, through each stage of each monitor. It is generally concluded that all AC monitor designs are equally functional at detecting relatively low-frequency, R component waveforms, the function for which they were all originally designed. However, for detection of more detailed waveforms composed of both R and emf components, each AC monitor design will function very differently. Therefore, there is no such thing as a single AC system; each actual monitor design must be individually considered. Such information is extremely valuable for the design of future AC monitors which will detect both R and emf components.