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How is an optical chopper eliminating drift? Or in more concrete words: Why is a chopped signal eliminating drift (DC -> AC)?

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    $\begingroup$ In what context? Normally one would use the pulsed signal in conjunction with a lock-in amplifier to isolate whatever signal (change in experiment by the light) is being measured. $\endgroup$
    – Jon Custer
    Commented Aug 4, 2020 at 18:00

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A chopper by itself does not eliminate drift, a chopper and a highpass (or bandpass) filter eliminate drift. The point of "chopping" is to place the information bearing signal from being near dc having low frequency content to be at a convenient so-called carrier frequency (for a chopper the "carrier frequency" is the rate of spinning times black lines) that can be amplified, filtered (bandpass) and then demodulated/detected.

It is quite difficult to make an amplifier, or a general receiver chain, to work accurately at very low frequencies. The reason for this is that all instrumentation drift is itself low frequency and are caused by many external problems: temperature, aging, humidity, bias, etc. When the information is moved to a high frequency then these problems are suppressed by the bandpass filter as the drift is not "chopped" and stays at/near dc while the signal is around now the carrier.

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    $\begingroup$ This is a good answer. While not necessary, a chopper can also be used along with a lock-in amplifier $\endgroup$
    – uhoh
    Commented Aug 5, 2020 at 2:17
  • $\begingroup$ Thank you! I wasn't aware of that highpass filter. I think I got it, at least conceptually but I'm afraid I'd need some visualized signal drawings where I can see the frequencies being separated to get it in detail. I also wonder about the "chopping frequency". There must be a certain frequency that is needed, or? $\endgroup$
    – Ben
    Commented Aug 5, 2020 at 7:11
  • $\begingroup$ Maybe I'm not that smart but.. why isn't a highpass filter not doing the job alone? $\endgroup$
    – Ben
    Commented Aug 5, 2020 at 7:47
  • $\begingroup$ I'm googling myself dead and can't find any source that provides basically more than 3-5 sentences.. can you advice me a good literature or something else? Good means easy to understand :) $\endgroup$
    – Ben
    Commented Aug 5, 2020 at 8:21
  • $\begingroup$ Usually a highpass filter is "just" a capacitive coupling (capacitor in series) that removes dc before driving the input gate (or base or grid) of the receiver amplifier but before you would detect the signal you want to minimize the high frequency noise too, hence a low pass filter (series inductor/shunt capacitance or something like that) is used before the rectifying. The highpass/lowpass combination is a bandpass filter. $\endgroup$
    – hyportnex
    Commented Aug 5, 2020 at 9:42

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