On Thursday, January 23, 2014 12:33:56 PM UTC-5, haitic...@gmail.com wrote:   
   > On Tuesday, January 21, 2014 12:29:51 PM UTC-5, Phil Hobbs wrote:   
   >    
   > > On 01/21/2014 12:00 PM, haiticare2011@gmail.com wrote:   
   >    
      
   > > Highly recommended if you're new to lock-in type measurements,>    
   > > especially the "TRY IT" sections, which are meant to build intuition and    
   > > experience very quickly.   
   > >     
   > > Cheers   
   > > Phil Hobbs   
   >    
   > Yes, Mark Johnson has a chapter on this. Good overview, but few circuit   
   details. Since we are on this subject, I'll add a few others.   
   >    
   > "Simple Lock-i Amplifier using a four-quadrant analog multiplier and Phase   
   lock loop.' Ramani, Rev Sci 52(9) 1981   
   >    
   > They claim to measure proton resonance signals. Shows circuit.   
   >    
   > A really simple one, using one op-amp, is:   
   >    
   > "The Lock-in amplifier: A student experiment." Wolfson, Am J Phys 59(6) June   
   1991     
   >    
   > This shows a circuit using an op amp pus silicon switch for synchronous.   
   >    
   > The "Art of Experimental Physics" Appendix A has a section on lock-ins.   
   >    
   > Cappel.org has interesting circuits done with low cost chips.   
   >    
   > "Low Frequency Digital Lock-In amplifier"  Probst Rev Sci Instr 56(3)   
   descibes an all-digital version.    
   >    
   > EGG&G has a mongraph describing 20 successful lock-in projects   
   >    
   > Jim Williams books on analog design has some circuits which use a bandpass   
   filter to do lock-in. He gets 20 dB increase, but a regular lock-in can do   
   better.   
   >    
   > There are several books on the subject -    
   >    
   >    
   >    
   > =============================   
   >    
   >    
   >    
   > I have read the above, but as I was searching around on Google, I saw a   
   Wikipedia article on Lockin amplifiers. This has some good references, but   
   some are private journals, like Review of Scientific Instruments.    
   >    
   > The Wikipdia article is a good place to start. I was particularly interested   
   in a reference about using an AM radio as a LI amplifier. A moment's thought   
   will convinve that a lock-in and a radio do the same thing: detect a signal   
   modulated on a    
   particular frequency.   
   >    
   >    
   >    
   > Note also that lock-in amplification benefits the SNR by narrowing the band   
   width and sometimes achieving phase lock with a signal.    
   >    
   Hmm A lock-in is always phase locked.  (Well if it looses lock then it's not a   
   lock-in.)   
      
   This is not meant to refer to you, But in grad school and as a post doc. I   
   often found that students would take their signal and immediately plug it inot   
   the lockin.  Where if they had spent some time trying to clean up the original   
   signal.. optimizing    
   the signal strength and reducing the noise they would have benefited greatly.     
   (Just sorta thinking out loud..)    
   As I see it the lockin (LI) has two advantages when compared to say a narrow   
   bandpass filter and amplitude detector.     
   1.) you can get much narrower bandwidths with a LI.     
   2.) When the signal is below the noise level then the LI rocks!  The output of   
   the LI in the absence of signal is zero, whereas the output of a BP +   
   amplitude detector is the noise level...    
      
   George H.   
      
   >    
   > It should be distinguished from other SNR-boosters like chopping, and the   
   "signal averaging" that I posted i this thread originally. Signal averaging,   
   which does not deal with a signal and is not averaging, is yet another   
   strategy to increase SNR. The    
   two may be combined, as seen in Horowitz AOE.   
      
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