6.4.9 FM Threshold Effect

In conventional FM receivers, a threshold effect occurs when the input carrier-to-noise ratio (𝐶/𝑁) falls to approximately 10–13 dB. Above this threshold, the FM demodulator provides the expected improvement in signal-to-noise ratio. Below it, however, the output S/N deteriorates abruptly, and impulsive noise artifacts (crackling or sputtering effects) appear in the demodulated signal.

This behavior arises from the nonlinear nature of FM demodulation. When the input C/N falls below the threshold, random noise voltage occasionally becomes comparable to or exceeds the signal voltage. If the instantaneous noise component is approximately 180° out of phase with the signal, small phase changes in either component can produce large phase fluctuations in their vector sum. Since an FM demodulator responds to instantaneous frequency (the time derivative of phase), these rapid phase variations are interpreted as sharp frequency excursions, appearing as noise spikes at the output.

In terrestrial communication systems, the threshold effect imposes a practical constraint: transmitter power and link distance must be managed so that C/N remains above the threshold. For many years this limitation made satellite broadcasting of analog FM television signals demanding in terms of satellite EIRP and earth station antenna size, as sufficient C/N had to be maintained to avoid abrupt picture degradation.

The problem was largely mitigated by about 1980–1985 through the introduction of threshold-extension demodulators, together with improvements in satellite transmitter power and earth station low-noise amplifiers. These techniques reduced the effective FM threshold by several decibels (typically 3–6 dB), enabling reliable television transmission over satellite links at significantly lower carrier powers.

The issue became largely irrelevant in the 1990s when analog FM television was replaced by digital modulation schemes (initially QPSK and later DVB-S). Although digital systems exhibit their own “cliff effect,” this behavior is fundamentally different from the classical FM threshold phenomenon.