On Monday, January 27, 2014 10:40:26 AM UTC-5, Phil Hobbs wrote:   
   > On 01/24/2014 09:21 AM, ggherold@gmail.com wrote:   
   >   
      
   >   
   > A lock-in is exactly equivalent to a symmetrical bandpass filter   
   > followed by phase-sensitive downconversion to DC.  Most of the time you   
   > want it to track the signal phase, but once in awhile it's useful in   
   > measuring the noise floor, or asynchronous signals.  (You usually want a   
   > 2-channel lock-in for that, so you can get the total amplitude, as in   
   > the aforementioned AM radio. ;) )   
   >   
   Hi Phil,  I did this experiement years ago.  I set up a system with the signal   
   was bigger than the noise.  I then looked at the output of   
   Lockin -> low pass.. with signal on and off   
      
   And then   
   Bandpass filter->amplitude det.-> low pass.   
   With things set up so that both had the same Q.  (This was with a gain   
   switched lockin and not a multiplying lockin so I've always worried a bit that   
   I didn't get all the factors correct.  (Q lockin = LP time constant/switching   
   period.)   
   anyway the signal to noise looked pretty much the same for both setups.  The   
   output with no signal was different!)   
      
   George H.   
      
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