  We report on the realization of a high sensitivity RF noise measurement
scheme to study small current fluctuations of mesoscopic systems at milliKelvin
temperatures. The setup relies on the combination of an interferometric ampli-
fication scheme and a quarter-wave impedance transformer, allowing the mea-
surement of noise power spectral densities with GHz bandwith up to five orders
of magnitude below the amplifier noise floor. We simultaneously measure the
high frequency conductance of the sample by derivating a portion of the signal
to a microwave homodyne detection. We describe the principle of the setup, as
well as its implementation and calibration. Finally, we show that our setup
allows to fully characterize a subnanosecond on-demand single electron source.
More generally, its sensitivity and bandwith make it suitable for applications
manipulating single charges at GHz frequencies.
