Abstract: A time domain reflectometer includes a combined a pulse generator for generating a pulse which is fed to an object under test such as a optical fiber and a sampler for sampling a reflection signal such as a back scattering signal. The pulse generator is controlled by analog delay means and the sampler is controlled by digital delay means. It, therefore, is possible to sample the small back scattering signal and to measure a very long fiber without influences of undesirable signals.

Abstract: In an amplifier including an AC amplifier, its operation level shifts according to input waveforms. When a positive input signal is, for example, inputted, the circuit feeds out a synthesized signal to the amplifier in which signal a positive and negative portions are symmetry and have a time difference. The circuit compensates an operating level of the amplifier so that it amplifies a small signal to keep always a zero level of the signal being constant in spite of various signal waveforms and amplitudes whereby a linearity of the amplifier is improved and fluctuations of a dynamic range thereof are eliminated.