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Please forgive me if I ask too many questions at once or I am too much of a rookie. I want to understand the working-principle of lock-in amplifiers (LIAs).

I understood the calculation that yields the outputs for X,Y,R and Φ. I am only interested in R (the root-mean-square amplitude of my original signal that was buried in noise).

But then I got bamboozled: In all these explanations on LIAs, it seems to be assumed that R is constant??? But R is not constant, my original signal can vary over time! When it is said that LIAs only measure AC and are not capable yet of DC signals, I am sure they refer to the modulation, not the original signal. But then I thought, that probably you would just average over a certain time period and this would give me a trade-off between signal response and noise cancellation. --> Is this train of thought accurate ?

Also, where do I set the time span over which I want to average? Is this the bandwidth of the low-pass filter?

(Maybe too off-topic: Can someone tell me what DC offsets are in the context of LIA ?)

I would be really happy for responses and I hope I don't get downvoted for asking non-expert questions...

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    $\begingroup$ Can you explain X, Y, and R? $\endgroup$ – Mahendra Gunawardena Nov 3 '20 at 11:25
  • $\begingroup$ X denotes the in-phase component, Y the quadrature component, R is the root-mean-square amplitude of the original signal and you get those variables from multiplying the original signal as a complex number with the reference signal as a complex signal. The real component will be X, the imaginary component will be Y, and the magnitude will be R. zhinst.com/others/en/resources/principles-of-lock-in-detection $\endgroup$ – AskingBecauseIHaveQuestions Nov 3 '20 at 11:56
  • $\begingroup$ thank you for the link. I suggest updating the post with relevant information from the link. Also include a link in the question body. $\endgroup$ – Mahendra Gunawardena Nov 3 '20 at 21:36
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    $\begingroup$ But I am the one here who needs help, from people who understand lock-in amplification. $\endgroup$ – AskingBecauseIHaveQuestions Nov 4 '20 at 16:39
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    $\begingroup$ The community members volunteer their free time to help. All the community ask is to be respectful of everyone's time and provide the relevant and meaningful information concisely. $\endgroup$ – Mahendra Gunawardena Nov 4 '20 at 23:54

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