Phase-Locked Loop (PLL) for coherent demodulation
Problem: Coherent demodulation requires local carrier synchronized to transmitted carrier in both frequency and phase.
Phase Detector: Multiplies input with VCO output. DC component proportional to phase difference.
Loop Filter: Averages phase error, removes high-frequency components.
VCO: Voltage-Controlled Oscillator adjusts frequency based on filtered error.
Lock: When phase error stays small, PLL is "locked" - carrier is recovered!
Costas Loop: Special PLL that works with suppressed-carrier signals (BPSK, QPSK).