A system for tracking difference limen for frequency (ΔF) is described. The system uses a function generator to frequency modulate a BFO. The amplitude of the modulating voltage is controlled by a recording attenuator. Intuitive spectral analysis of the modulated tone proves to be insufficient and correction factors are given to allow determination of ΔFfrom de calibration data. When real rather than intuitive spectrum is used for calibration of the system, ΔFdoes not increase with modulation rate from 1 to 16 Hz. Consideration of the spectra produced by various waveforms indicates that the discrepancy between de calibration and spectral width is minimal for a trapezoidal modulating waveform.