Lock-in Amplifier

What is a Lock-In Amplifier?

A lock-in amplifier is a type of amplifier used to extract quiet signals out of noisy data. High quality lock-in amplifiers can extract signals up to a million times quieter than the surrounding noise. The output of a lock-in amplifier is a DC signal showing the strength of the signal to be extracted.

How Does a Lock-In Amplifier Work?

Conceptually, a lock-in amplifier works by exploiting the orthogonality of sinusoidal functions. Use of a lock-in amplifier requires a clear reference signal at the frequency of the signal to be extracted. This reference signal is multiplied by the noisy input signal and the product is integrated over a set time. When sinusoidal functions are multiplied together and integrated over a significant amount of time, the result will be zero unless the two sinusoidal functions have the same frequency. This is the before-mentioned orthogonality of sinusoidal functions. For the output of a lock-in, this means that the contributions of all signals not at the reference frequency will be attenuated very close to zero. The output is a DC signal showing the strength of the original input signal at the reference frequency.

From a circuits standpoint, a lock-in amplifier consists of a homodyne detector followed by an adjustable low pass filter. Traditional lock-in amplifiers used analog frequency mixers and RC filters for the demodulation, but modern devices are typically digital and use fast digital signal processing. The out of phase component of the signal that has the same frequency as the reference signal is also attenuated (sine functions are orthogonal to cosine functions of the same frequency), making lock-in amplifiers phase sensitive detectors. Sine and cosine demodulation is usually performed simultaneously (dual phase demodulation).

Useful Links

Informational PDF from Stanford Research Systems Paper on Phase Sensitive Detection