Introduction: RTL-SDR Direct Sampling Mod
Many dongles are not able to make use of frequencies below 30Mhz however it is possible to modify some devices to do so using a method call Direct Sampling. In direct sampling we apply a signal directly to the dongles 'brain' effectively bypassing the filtering electronics.
Be warned however that this mod also bypasses the protection circuitry, making it possible to fry your dongle.
Step 1: Open It Up!
Start by dissembling the dongle, with mine this was a matter of removing a few screws and the thermal pads to access the printed circuit board. Make note of how the how the dongle was assembled and keep thermal pads away from dust and dirt.
Step 2: Modify
This mod requires that the board has exposed 'Q' or 'I' pads available on the PCB, look over the board at all unpopulated solder pads. Once located solder a small wire to the pad, ideally using enamelled single core copper wire however insulated types will work too, do not use bare wires as they may cause shorts with other components on the board.
Chase the wire back to the SMA connectors center pin, carefully plan the wires route over the board so that it avoids interfering with other components. Finally cut and solder the wires to the SMA connector and using back multimeter make sure no shorts exist between center conductor and ground pins.
Step 3: Software Configuration
Configuration will differ between the various software packages however almost all offer direct sampling.
For SDR-sharp simply click on the cogs icon, then select 'Sampling Mode / Quadrature sampling' and from the drop-down menu select 'Direct Sampling - Q branch'