Lets say you had a receiver front end, that consisted of a 30 dB LNA with a 2 dB noise figure, that fed a mixer with a 6 dB conversion loss. The desired operating frequency is 10 GHz, the LO frequency is 9 GHz, so the IF frequency is at 1 GHz.
You go out into the lab and make a noise figure measurement. You expect the system noise figure to be a little higher than 2 dB, but instead you are surprised to see the noise figure meter reading 5.3 dB. What is up? What do you have to do to get the system down to a 2.2 dB noise figure?