[time-nuts] Long period variation of GPS PPS timing?

Hal Murray hmurray at megapathdsl.net
Wed Jul 14 02:24:07 EDT 2010


> It ought to be a bit better than this, but the 10uS PPS width is a bit too
> short for a steady timing of centroid through a soundcard at 192k samples/
> sec, I need to stretch the pulse a factor of 10 or so.

The PPS on my TBolt was also too short for a PC running Linux/ntpd to catch 
it.  (It works on another system with different hardware.)

Here is how I kludged it...

>From the TBolt PPS pin:
  resistor (current limiter)
  diode
  cap to ground
  resistor across cap (decay)

I think I used 50 ohms for the current limiting resistor and 0.1 uF for the 
cap.  I forget the value of the other resistor.

That's far from symmetric so finding the "centroid" may get interesting 
and/or may not work at all.

I was connecting it to pin 1 (DCD) on the serial port.  That's where ntpd 
expects PPS and it's unused on the TBolt.  It all fits on the bottom side of 
the board inside the TBolt box.


Or you could try the FatPPS from TAPR:
  http://www.tapr.org/kits_fatpps.html
Or look at the manual in for ideas.  It uses a 74HC123.




-- 
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