A system is disclosed for detecting the presence of a received data signal in which a pair of band-pass filters is employed to separate out two pilot tones from the received signal. A mixer is employed to generate a difference frequency which drives a phaselocked oscillator system. The phase-locked oscillator system provides an output signal having the same frequency and phase as the average value of the difference signal. The phase relationship between the difference signal and the output from the phase-locked oscillator system is compared to provide a signal for enabling a data decoding circuit.