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Digital Lock-In Amplifier

I have a few questions regarding lock-in amplifiers.  In my application, I do not have a reference signal because the sensor excitation is hetrodyned by an external source.  The signal of interest is the difference (or sum) of the excitation and the external source.  Both of these frequencies are however very well known.  Historically we have used the FFT of the incoming signal to extract the single tone and plot this amplitude versus time. 

My first question, would a lock-in amplifier be beneficial in this application?  It seems that the bandwidth could be reduced to much less than 1 Hz (which it currently is with the FFT) which would reduce the noise that is integrated into the signal.


Second question, is it possible to use a software PLL to generate the correct reference signal for a lock-in amplifier since I already know the frequency?


Any examples or advice would be greatly appreciated.





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This definitely sounds like an application for a lock-in amplifier, which should also allow to get a bandwidth of 1 mHz or lower.  I am not quite clear on your setup but could you not just feed in your external signal as the reference signal to get the difference between your excitation and the external source?  If that is not the case, it should definitely be possible to generate a reference signal if you know the frequency.

Doug Farrell
Solutions Marketing - Automotive
National Instruments

National Instruments Automotive Solutions
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I have a very similar problem.  We want to lock-in to a side band our signal after it is interfered with another frequency wave.  It is basically psuedoheterodyne (we are not mixing different frequencies of light, just different modulations of the light). 


In short, our signal is at 260 KHz (that's the modulation of it) and we have a reference leg (as part of an interferometer) where the reference beam is modulated at 260 Hz.  The interference of these two beams creates a sideband.  We want to lock-in at the sideband, but how do you mix these signal in labview to tell it that the sideband frequency is actually the reference?


Did you ever figure this out.  I guess it's been a while.  Thanks!



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