The Model 128A Lock-ln Amplifier enables the accurate measurement of signals contaminated by broad-band noise, power line pickup, frequency drift, or other sources of interference. It does this by means of an extremely narrow band detector which has the center of its passband locked to the frequency of the signal to be measured. Because of the frequency lock and narrow bandwidth, large improvements in signal-to-noise ratio can be achieved, allowing the signal of interest to be accurately measured, even in situations where it is completely masked by noise.