• Overview

 
• Signal Multiplication

 
• Low Pass Filter Equations

 
• Phase Sensitive Detector I: Schematic Diagram

 
• Phase Sensitive Detector II: Output

 
• Phase Sensitive Detector III: Frequency Response

 
• Features of a Lock-In Amplifier

 
• Switching Type Lock-In Amplifiers

 
• A Simple Demonstration

 
• Links and References

Any periodic signal can be constructed from a sum sinusoidal waves. The constituent elements of the PSD are linear circuits (a linear differential equation governs their behavior). So, we can analyze the response of the PSD to a sinusoidal input signal, and build up any other signal we need from there.

Once we have learned to predict the output from a PSD in response to sinusoidal input and reference signals, we can examine its output across the frequency spectrum. The PSD heavily attenuates any signal that does not match the reference frequency. This is where the operation of a lock-in amplifier becomes clear. If we modulate our input signal using a reference frequency in a quiet area of the frequency spectrum, the input signal will be able to pass through the PSD unhampered, while noise is attenuated away.

After we get a handle on the internals of the lock-in amplifier, I will demonstrate its use through a simple experimental setup for an optics lab. In this experiment, a laser beam will be passed through two nearly crossed linear polarizers (thus attenuating the signal) and then the signal will be retrieved with the lock in amplifier.