Can someone ELI5 lock in amplifiers?

Is the signal input in this diagram assuming that the signal is already modulated with the carrier frequency?> Is the signal input in this diagram assuming that the signal is already modulated with the carrier frequency?

No.

what's the purpose of the bandpass and lowpass filters?

  • Bandpass: to pass only the signal in the band of interest, remove everything else
  • lowpass: to pass only the difference of the 2 frequencies (A-B) and remove the sum (A + B)

why can't this just be done in software

It can.

requiring a specific piece of very expensive equipment?

It will require expensive, fast and accurate A/D converter, and a DSP, so it's not that cheap to do it in software either.

is this about right?

About.

why not just take one detector input

A detector will remove the frequency component in the input signal, so it will not help you.

You need a faithful reproduction of the input.

No.

what's the purpose of the bandpass and lowpass filters?

if the PSD just mixes the signal and references through multiplying the reference and signal, why can't this just be done in software instead of requiring a specific piece of very expensive equipment?

finally, what I got from a bit of reading was that the critical parameters are the carrier and signal frequency and the phase between the signal and carrier; the frequency can be largely ignored since we assume it's the same for both, and the phase dependency can be ruled out if we perform two separate detections (one in phase, one with a 90 degree shift); is this about right?

again, if so, why is this not done in software (especially with two phase lock in amplifiers? why not just take one detector input and perform two sets of calculations?)
/r/AskElectronics Thread