Lock-in amplifiers are used to detect and measure very small AC signals, all the way down to a few nanovolts. Accurate measurements may be made even when the small signal is obscured by noise sources many thousands of times larger. Lock-in amplifiers use a technique known as phase-sensitive detection to single out the component of the signal at a specific reference frequency and phase. Noise signals, at frequencies other than the reference frequency, are rejected and do not affect the measurement.
Why Use a Lock-In?
Let's consider an example. Suppose the signal is a 10 nV sine wave at 10 kHz. Clearly some amplification is required to bring the signal above the noise. A good low-noise amplifier will have about 5 nV/√Hz of input noise. If the amplifier bandwidth is 100 kHz and the gain is 1000, we can expect our output to be 10 µV of signal (10 nV × 1000) and 1.6 mV of broadband noise (5 nV/√Hz × √100 kHz × 1000). We won't have much luck measuring the output signal unless we single out the frequency of interest.
If we follow the amplifier with a band pass filter with a Q=100 (a VERY good filter) centred at 10 kHz, any signal in a 100 Hz bandwidth will be detected (10 kHz/Q). The noise in the filter pass band will be 50 µV (5 nV/√Hz × √100 Hz × 1000), and the signal will still be 10 µV. The output noise is much greater than the signal, and an accurate measurement cannot be made. Further gain will not help the signal-to-noise problem.
Now try following the amplifier with a phase-sensitive detector (PSD). The PSD can detect the signal at 10 kHz with a bandwidth as narrow as 0.01 Hz! In this case, the noise in the detection bandwidth will be 0.5 µV (5 nV/√Hz × √.01 Hz × 1000), while the signal is still 10 µV. The signal-to-noise ratio is now 20, and an accurate measurement of the signal is possible.
The rest of this article describes the detailed operation of a lock-in amplifier together with various design and performance considerations plus practical advice to achieve the best performance in a number of different measurement scenarios. This information will help you get the best out of the Stanford Research Systems (SRS) family of lock-in amplifiers (SR850, SR844, SR830, SR810, SR530, SR510, SR124). Please download below:
About Lock-in Amplifiers_Application Note