This lecture covers the principles and applications of phase-locked loops (PLL) and lock-in amplifiers in electrical metrology. It explains the operation of PLL, including phase detectors, voltage-controlled oscillators, and locked conditions. Additionally, it delves into the concept of lock-in amplifiers, which extract signals from noisy environments. The lecture also discusses digital sampling oscilloscopes and spectrum analyzers, highlighting their roles in analyzing signals in the time and frequency domains.