0
\$\begingroup\$

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?
\$\endgroup\$
1
\$\begingroup\$
  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.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.