[time-nuts] Thunderbolt PPS Modification

Arthur Dent golgarfrincham at yahoo.com
Sun Sep 26 12:07:50 UTC 2010

Here is a link to a
photo of the 5V 10uS wide pulse. It is there but catching it on some scopes can
be a problem because of its narrow width and low repetition rate.




Hal Murray: “The
problem for me was that the PPS pulse wasn't wide enough for one of my PCs to
catch.  (Another one does catch it.)

I changed the wire into a diode, resistor, and cap.  It's a kludge, but it

I can see that there
might be a problem relying on an RC time constant if you are interested in consistent
pulses with minimum delay. What I had done on my Z3801 about 10 years ago was
use an inverter to trigger a 555 pulse stretcher. I made the pulse about a
50-50 duty cycle because I was also driving an LED for indication the pulse was
there. If you lower the values of the 1 megohm resistor and the .47uF capacitor
you can narrow the pulse width you get at pin 3 of the 555 timer.  A 10Kohm resistor and a .001uF capacitor will
give about a 100uS wide pulse. You can check an online reference of the 555 datasheet
if you want a specific pulse width. The consistent propagation delay thru the
circuit is probably in the vicinity of 250nS.





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