On Tuesday, January 28, 2014 8:26:39 PM UTC-5, gghe...@gmail.com wrote:   
   > On Tuesday, January 28, 2014 12:28:26 PM UTC-5, haitic...@gmail.com wrote:   
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   > > > A lock-in is exactly equivalent to a symmetrical bandpass filter    
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   > > > followed by phase-sensitive downconversion to DC.  Most of the time you    
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   > > > want it to track the signal phase, but once in awhile it's useful in    
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   > > > measuring the noise floor, or asynchronous signals.  (You usually want a    
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   > > > 2-channel lock-in for that, so you can get the total amplitude, as in    
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   > > > the aforementioned AM radio. ;) )   
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   > > snip snip   
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   > > Phil, Here is the way I see lock-in amplifiers. They are ways to 'mark' a   
   signal so that the noise cannot sneak into the part you want to measure.   
   Consider a square wave which has a complex 16 bit pattern on it, there are   
   various codes which are low    
   probability of appearing in noise. These codes are used in CDMA, spread   
   spectrum, pseudo noise, Huffmann codes, Barker codes, etc. In this case, a 16   
   bit binary code will out-perform a simple square wave because its a rare   
   pattern and the information    
   gained, the SNR boost, is directly related to the Szilard formula for   
   information and entropy. So if you demodulate a rare long bit pattern, it is   
   like a demodulator for spread spectrum sequences, and it will reject noise   
   well.    
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   > > Lock-in amps have been built with "Barker" codes of 13 bits, but according   
   to the tenets of information theory, a longer sequence of bits might even do   
   better - A practical advantage of this approach is it can be done by   
   microprocessor with minimal    
   part count - See Cappels.org.   
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   > Hi jb,  this is wierd.  In one post you say there is no 'return' from   
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   > information theory.  And above is perfect use.     
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   > I don't know anything about these longer bit sequences. (but I can imagine.)   
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   > There's certainly a bandwidth/ averaging time trade-off.  (it takes longer   
   to average a long bit sequence.)   
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   > George H.   
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   > > jb   
      
   No No, I didn't mean there is no return, just that it didn't turn out the way   
   originators thought...as a major scientific breakthrough that would change or   
   add to the fundamental structure of physics. Of course, there are scientists   
   around working on a    
   reformulation of relativity, quantum theory based on IT, but haven't seen   
   anything yet.    
   And again, I will stick my neck out and say that information theory doesn't   
   really say what information IS, it just describes some circumscribed behaviors   
   of defined systems.    
   And - to be a further troublemaker, information theory is not a theory, and it   
   is not science. Why? Because there is no specific, formulated hypothesis that   
   can be disproven. (and if I am wrong about this, tell me what it is.)   
   Of course, this is to say that many "theories" are not valid as science. But   
   this does not mean they are wrong, just that they are not science. Many   
   theories are extremely valuable, like the theory of feedback systems used in   
   cybernetics, biology, and    
   electronics. But I have never heard any scientific "laws" come from this area   
   that can be disproven in a global sense.    
   And this could be my ignorance regarding cybernetics.    
   respectfully,   
   John B   
      
   Evidence-Based Medicine Review of the parachute: "It is an interesting   
   technology, but unproven. Given that no double-blind clinical trials have been   
   done, we cannot recommend it for safety and efficacy."   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)