FMDemodulation

FMDemodulation, commonly written as FM demodulation, is the process of recovering the original modulating signal from a frequency-modulated carrier. In FM, information is encoded in the instantaneous frequency of the carrier rather than its amplitude, so a demodulator must translate frequency variations into amplitude variations representing the modulating signal.

The demodulator typically follows the instantaneous frequency or phase of the input signal. For a narrowband model, f_i(t) ≈ f_c + k_f m(t), so demodulation yields m(t) ≈ (f_i(t) − f_c)/k_f. In practice the carrier is converted to a baseband or audio signal through a detector or feedback loop.

Common approaches include the Foster-Seeley discriminator, which uses a transformer-coupled slope detector followed by limiting and rectification to produce a voltage proportional to frequency deviation; the ratio detector, similar but with improved center-frequency stability; phase-locked loop (PLL) based demodulators that lock a VCO to the input and derive the demodulated voltage from the control signal; quadrature detectors that extract frequency information from the phase difference between in-phase and quadrature components; and simpler zero-crossing or differentiator-based slope detectors that are fast but sensitive to amplitude fluctuations.

In broadcast practice, FM demodulation is followed by de-emphasis to restore the original spectral tilt and reduce high-frequency noise, with standard time constants of 50 or 75 microseconds depending on region.

Applications include FM broadcasting, aviation and land-mobile radio, and data communications. The method provides good resistance to amplitude noise and multipath, but demodulator linearity and stability are important, and wideband FM requires careful design to maintain fidelity and adequate dynamic range.