Stefan Groißmeier 2017-01-10 17:37:
That is why I'm searching a way to correlate the signals of the sdrs without hardware modification.
Just my 2 cents:
Intriguing idea, BUT (sorry...):
For corrleation you need a common time base, that is a common clock source. That is not the case for multiple RTL-SDR dongles. Moreover you need a stable and known phase relation (runtimes of the clock signal) for your dongles.
The USB bus does not provide the clock for the dongles, so this is no way to synchronize them
The key word here would be MIMO - Multiple In Multiple Out. That is what WLAN 801.11n does, in one device.
I do not know if any of the currently availabe dongles have a clock input (for a common source) or a clock output (to act as clock master)
73 de Patrick OE6PSE