A heterodyne interferometer and a data acquiring algorithm have been developed to measure the movement of a mirror in one dimension, as well as its rotation around two axis. The interferometer uses spatially separated beams to reduce periodic optical non-linearities, furthermore the optical set-up was designed for low drift, few number of optical elements and easy adjustment. The FPGA-based signal processing is based on an undersampling technique with the aim to minimise the calculation effort. The working principles of the interferometer and the electronics are described and their remaining non-linearities are investigated. Finally, the z-position, the tip and tilt angle of a planar stage are measured with the described system as an example of use.