  White noise measurement can provide very useful information in addition to
normal transport measurements. For example thermal noise measurement can be
used at sub Kelvin temperature to determine the absolute electron temperature
without applying any heating current. And shot noise measurements helped to
understand the properties of nano and mesoscopic normal metal/superconductor
structures. But at low temperature and for relatively small resistance it is
difficult to measure the sample's noise magnitude because the background
thermal noise can be much larger and usually there are other pick-up noises.
Cross correlation technique is one way to solve this problem. This article
describes an improved cross correlation algorithm that averages in both
frequency and time domain, and the realization of a simple instrument set-up
with PC and sound card. With this set-up it is shown even with much larger
background noise and pickup noises, 100pV/$\sqrt{Hz}$ white noise level can be
easily measured in seconds. Compared to the normally used cross-correlation
methods, it is several orders of magnitude faster.
