The actual measurement is always accompanied by the unwanted noise and interference. It affects the measurement accuracy and sensitivity, especially when the noise power is bigger than the signal power. That needs to use weak signal detection apparatus and equipment to recover or detect the original signal. The famous intrument of weak signals is lock--in amplifier（LIA）. This paper introduced the priciple of IdA,than weak signals and those signals added different noise were tested. It proved that LIA has a good function in the dection of weak signals.%对锁定放大器的功能原理进行了介绍，并用锁定放大器对微弱信号及加了不同噪声的微弱信号分别进行了测试，实验证明锁定放大器具有良好的检测微弱信号的功能．
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