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Entry  Fri Mar 25 01:27:29 2011, mevans, Frogs, Green Locking, digital frequency counting 
    Reply  Fri Mar 25 11:16:19 2011, josephb, Frogs, Green Locking, digital frequency counting C1GFD.png
Message ID: 4442     Entry time: Fri Mar 25 01:27:29 2011     Reply to this: 4444
Author: mevans 
Type: Frogs 
Category: Green Locking 
Subject: digital frequency counting 

Today we tried the Schmitt trigger DFD, and while it works it does not improve the noise performance.  At least part of our problem is coming from the discrete nature of our DFD algorithm, so I would propose that an industrious day job person codes up a new DFD which avoids switching.  We can probably do this by mixing the input signal (after high-passing) with a time-delayed copy of itself... as we do now, but without the comparator.  This has the disadvantage of giving an amplitude dependent output, but since we are working in the digital land we can DIVIDE.  If we mix the signal with itself (without delay) to get a rectified version, and low-pass it a little, we can use this for normalization.  The net result should be something like:

output = LP2[ s(t) * s(t - dt) / LP1[ s(t) * s(t) ]],

where s(t) is the high-passed input and LP is a low-pass filter.  Remember not to divide by zero.


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