锁相放大电路(英文版),针对交流小信号采集。 Synchronous detectors can extract small signals, buried in the noise floor, to measure properties such as very small resistance, light absorption or reflection against bright backgrounds, or strain in the presence of high noise levels. In many systems, the noise increases as the frequency approaches zero. For example, op amps have 1/f noise and light measurement is subject to noise from changing ambient light conditions. Moving the measurement away from the low-frequency noise increases the signal-to-noise ratio, allowing weaker signals to be detected. For example, modulating a light source at a few kilohertz facilitates measurement of reflected light that would otherwise be buried in the noise. Figure 1 shows how modulation enables recovery of a signal that was originally below the noise floor.