An instrument for measuring the first and second derivatives of l-V characteristics of tunnel diodes and point contact diodes as a function of D.C. bias was designed. The principle of operation is based on the harmonic analysis of the current. The circuit involved is rather simple. As an example for the application of this instrument, plots of first and second derivatives of a tunnel diode and a point contact diode were shown. From the given results, it has been pointed out that tunnel diodes may be used in low level detectors.