A real-time mixed signal (analog and digital) demodulator for interferometric fiber optic sensors operating at high signal levels was designed, implemented, and tested. The inputs to this fiber optic sensor demodulating instrument are a phase modulated optical signal and an electrical carrier reference signal. This high performance demodulator has an intrinsic 84 dB linear dynamic range that is switch selectable over a 193 dB range, a demodulated signal output amplitude-frequency product of 500,000 rad (DOT) Hz (-3 dB), and an absolute accuracy to better than 0.4%. The group delay from the sensor input is 4.8 microsecond(s) to the demodulated analog output, and 2.3 microsecond(s) to the digital demodulated output. One of the unique simplifying design attributes of this arctangent type demodulator eliminates the need for jump detector/counter circuitry. The details of this design, the tests performed, and the test results are examined.© (1994) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.