For the characterization of noise and stability of any measuring instrument a new and very practical method is introduced. It follows the principles of the "Allan variance" well estabilished for the characterization of the stability of frequency standards. The plot of the Allen variance versus integration time enables one to determine the different types of noise power spectra from the output of any instrument. In particular, the best range of the integration time for optimum use of the system can accurately be evaluated. Theoretical considerations and experimental results with components of the 3-m radiotelescope of Univ. of Cologne are presented.