In several lock-in amp tutorials and experiments (including at the end of “Physics 111: Low Light Signal Measurements” on Youtube), the lock-in amp X or Y output is applied directly to a spectrum analyzer. I have two very basic questions about this:

  • After the lock-in amp mixer section the signal is rectified – how can the spectrum analyzer display be accurate with such a rectified signal?
  • The rectified signal is low-pass filtered before the lock-in amp output – so the spectrum analyzer is only seeing whatever high frequencies that are able to pass through the filter, correct?
  1. The demodulator can be very accurate. It's not just a diode- it may be a precision analog multiplier or a digital DSP function. I've gotten ppm accuracy using digital methods.

  2. Yes, the spectrum analyzer will only see frequencies from approaching DC to the cut-off frequency of the low-pass filter. Of course the filter will not be infinitely sharp so there will be some attenuation of frequencies approaching the cutoff frequency and some higher frequencies will get through.


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