David has written an article on measuring SDR noise figure:
One number I wanted to know was the Noise Figure (NF) of the SDR. Mark has a bunch of SDRs so we got on a roll and checked out the NF on all of them.
Now there are a lot of people playing with SDRs, but very few tutorials on HowTo engineer radio systems. For example, how to answer questions like “can I send this many bit/s over X km with SDR hardware Y?” So I thought it might be useful to explain how we measured Noise Figure (NF) and present the results for a bunch of SDRs.
More details at Rowetel site.