An AGC system for RTL-SDR "wideband" receivers
operating in "Direct" (Q-branch) mode.
"But the Dongle already has an AGC!"
One advantage of using a dongle with an upconverter - a device that would, say, converter 0-30 MHz to the range of 125-155 MHz - is that it then places these signals within the range where the R820T chip can operate - and this chip does have RF filtering and a sort of AGC - at least by way of being able to have its gain adjusted by software.
Aside from the frequency drift issues related to this frequency up-convertion mentioned elsewhere, the problem with this is that the R820T chip really isn't that "strong" in terms of its ability to handle widely disparate signal levels. While the RTL2832 chip does have an AGC, the gain of both chips in the signal path must be carefully controlled to maximize performance. Unfortunately, the precise nature of how these all work together isn't well documented and the general consensus seems to be that at HF, it doesn't work all that well.
While the AGC can work, we decided to avoid combining the somewhat marginal performance of the R820T signal path and the unknown nature of the AGC operation with the already-marginal 8 bits of A/D conversion.