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Fast Frequency Shift Keying (FFSK) is a phase coherent binary FSK modulation technique with a deviation ratio of 0.5 and an ideal error rate performance equivalent to that of phase-shift keyed (PSK) modulation. Coherent reception of FFSK and PSK systems is degraded by imperfect receiver reference signals. The various forms of PSK modulation-binary (BPSK), conventional quadriphase (QPSK) and offset quadriphase (OK-QPSK)-as well as FFSK, exhibit the same ideal error rate performance but are not equally sensitive to imperfect carier phase reference. It is shown that while FFSK is similar in structure to OK-QPSK, its sensitivity to reference phase error is less. FFSK is shown to require 2.5 dB less receiver phase reference SNR for a specified detection loss than OK-QPSK.