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Why not just use two ADCs (say GPIB from digital multimeters) and use them to collect the data. Afterward, you can just emulate the LIA by empirically determining the phase offset between the signal and reference signal?

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most non-linear filtering techniques like this (which admittedly I'd never heard of before) were incredibly convenient before digital circuits could keep up - it's designed to be analogue and a digital filter can be designed to have the same bandwidth (with the LIA technique or other) but faces the normal analogue vs digital trade-offs. Why do you say 2 ADC's? –  user3125280 Jul 3 at 4:12

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The main function of a lock-in amplifier is to determine the amplitude of a narrow band signal buried in wideband noise (SNR's can be -60 dB or lower). It does this my multiplying the input signal by a reference sine wave at the same frequency as the input signal. The output of such a multiplier includes a DC term that is proportional to the amplitude of the input signal. Being DC, it can be low pass filtered as much as desired, the trade off being longer response time. However, using time constants of many seconds provides an effective bandwidth of a fraction of a Hertz. This very low bandwidth effectively filters out most of the input noise which is assumed to be spread over a wide band. Some of the functions of a lock-in amplifier can be done using digital techniques but 2 ADC's are not enough.

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I guess I don't see why I can't take the input from two ADCs into an analysis program such as matlab for some amount of time (say I expect to need 100 s time constants on the 6 dB/octave filter, then just collect for 500 s), take the product of the two signals (with a phase shift I will empirically determine) then apply a low pass filter. I must be missing something that can't be done digitally. –  sciencenewbie Jul 3 at 2:23
    
I think you can do it digitally, which is why nobody buys lock-ins anymore. What you describe is basically a very long optimal filter. Will you need low pass if you do an optimal filter correlation? –  C. Towne Springer Jul 3 at 3:33

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