  A lock-in amplifier is a versatile instrument frequently used in physics
research. However, many students struggle with the basic operating principles
of a lock-in amplifier which can lead to a variety of difficulties. To improve
students' understanding, we have been developing and evaluating a
research-based tutorial which makes use of a computer simulation of a lock-in
amplifier. The tutorial is based on a field-tested approach in which students
realize their difficulties after predicting the outcome of simulated
experiments involving a lock-in amplifier and check their predictions using the
simulated lock-in amplifier. Then, the tutorial guides and helps students
develop a coherent understanding of the basics of a lock-in amplifier. The
tutorial development involved interviews with physics faculty members and
graduate students and iteration of many versions of the tutorial with
professors and graduate students. The student difficulties and the development
and assessment of the research-based tutorial are discussed.
