Another important issue in interferometer is increasing use of electronic methods with high accuracy to measure directly the phase difference appeared in the interference pattern. Since electronic sections of interferometer can result in the increase of nonlinearity error [
8-
16], it is necessary to develop high-accuracy methods to detect phase. Null method has been considered as one of initial techniques. A small modulation optical path difference was introduced in Null method [
17,
18]. Another method first defined by Peck and Obetz was electronically fringe counting [
19]. However, in this paper, we utilized super-heterodyne interferometry. In this method, a small displacement appears in the one of the interferometer beams [
20,
21] that was detected with the heterodyne method. In the super-heterodyne method first, two signals respectively received from base and measurement paths were amplified and converted into lower frequencies by double-balanced mixing. Finally, the phase difference, which is corresponded to the displacement, was measured by phase meter section.