From "Understanding and Repairing CB Radios For the Professional Technician" by Lou Franklin:
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Single-Conversion VS. Dual-Conversion
Figure 4-2 shows block diagrams of the two basic CB receiver types: the single-conversion and double- or dual-conversion superheterodyne. ("Heterodyne" is another term for mixing.) The "superhet" has been around virtually unchanged since the early 1930s, which tells you something about its usefulness. Its most important advantage is that the bulk of the gain and selectivity takes placein a single narrow-bandwidth IF (or two, in the case of the dual-conversion) regardless of the actual input frequency band. This allows simpler and cheaper circuits, since gain blocks are most effective when designed for a single frequency.
Techs have argued for years about the relative merits of each, but in my opinion there's only one real determining factor: cost. The dual-conversion circuit is more complicated, requires more parts, and is therefore more expensive. The main advantage of single-conversion is that fewer stages are needed to produce a given signal-to-noise ratio, since each stage adds more noise along the way. The reason AM/SSB single-conversion types are found at all in today's crowded bands is because improved manufacturing techniques allow more selective crystal IF filters to be used. But you get what you pay for and to provie it, compare the receiver performance of chassis like the Cobra 140/142GTL or 146GTL/PC244 (single-conversion) to the Cobra 148/2000GTL (dual-conversion) chassis on AM.
As shown, the dual-conversion circuit requres a second Local Oscillator (L.O.) and a second mixer stage. The IF frequencies are usually 10.695 MHz and 455 KHz, univeral standards for which parts are readily available. Some popular chassis use a 7.8 MHz or 11.275 MHz IF in a single-conversion circuit, or as the high IF of a dual-conversion circuit with 455 KHz as the 2nd AM IF. There are a few unusual CB IFs, like the 4.3 MHz used in some Johnsons. The dual-conversion type has superior image rejection.
Incidentally, the reason for using the same IF path on AM and SSB in most single-conversion receivers is because only
one of the two AM sidebands will be detected. Each sideband has exactly the same voice intelligence. Since SSB uses a very narrow IF filter, one AM sideband of up to 2.5 KHz width can pass through it, but not both. Another "economy" move... (end quote)