Complex filter configuration for lock-in amplifiers.

Lock-in amplifiers are the workhorse of experimental physics and material research for detection of low level AC signals in the presence of high levels of noise and interfering signals. Many measurements require the mixture of DC and AC signals, for instance differential conductance and AC susceptibility. Traditional lock-in amplifiers employ a simple IIR filter which can have long measurement times to produce acceptable noise reduction. Using measurements on model systems, we will demonstrate that a combination of digital filters of a variety of types, high pass, low pass, infinite impulse response and finite impulse response can produce improved noise reduction and measurement time over the traditional IIR filters. We will present step response, equivalent noise bandwidth and autocorrelation times for a n pole IIR filter (n <=4) followed by a filter response of any finite length. Finally, we will present criteria for statistically independent samples for the various filter configurations.