[time-nuts] crunching numbers from XOR phase detector

John Beale beale at bealecorner.com
Wed Jan 4 00:56:26 UTC 2012


Previously I have been comparing 10 MHz frequencies using TvB's picPET 
device plus a picDIV divider to get a 1 PPS signal, but I wanted more 
resolution for comparing relative drift of two Rb references. I got square 
wave outputs from my references (see my previous posts) and I made a simple 
XOR phase detector from a single XOR gate (74LVC1G86) :

https://picasaweb.google.com/lh/photo/ofFwP8Eo1qFAzNObq69iCtMTjNZETYmyPJy0liipFm0

I have read about how nonlinear the XOR PD becomes at the endpoints (0 and 
180 phase shift) although this one seems to work pretty well, and the 
output looks reasonably triangular:

https://picasaweb.google.com/lh/photo/swKVhhP7NerRvMKdnW8rjtMTjNZETYmyPJy0liipFm0

There is some nonlinearity but it seems consistent from cycle to cycle. I 
might be able to reduce the bumps with better circuit layout, shorter 
wires, terminated lines etc. But just for playing around with my initial 
data, I think I can model the shape of the response and get a more accurate 
reading of instantaneous phase angle vs time. I could write some code for 
this, but I suspect this wheel has been invented before... is there any 
reference I should consult?  I think something similar is done inside the 
PIC-TIC to calibrate its response?



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