[time-nuts] Question regarding 1PPS timing accuracy and GPSDO

Chris Albertson albertson.chris at gmail.com
Tue Jan 11 12:06:51 EST 2011


On Tue, Jan 11, 2011 at 8:14 AM, Tijd Dingen <tijddingen at yahoo.com> wrote:

> When a GPS devide XYZ datasheet quotes a "timing accuracy" on the 1PPS
> output of lets say 1 us RMS, would it be useful? I.e would /you/ consider
> using it? What if it was 100 ns RMS on the 1pps? Or 10 ns RMS? Or ... At
> roughly what number would you consider the 1pps as a candidate for your
> GPSDO?

Buying a better GPS unit is the most cost effective why to improve the
total system performance.  Everything else you can do has a llower
"bang per buck" ratio.  For example going from a  1us to a 10ns GPS
cost nearly nothing but s 100X improvement

A lot of this decision has to do with cost.  If a 10ns GPS cost $10K
I'd go for a lower spec'd GPS but with current prices on eBay you
don't need to think about it.  For Example you can buy a Moterola MT12
with a <10ns spec for $50 shipping included.  Or you can buy a
Thunderbolt for $150 or less.

I'm currently setting up a system and I'm finding that the GPS unit
itself is the lowest price component.  Little things like putting up a
mast and running coax cable cost 4X more than the GPS.

I'm currently experimenting, I'm moving the antenna to better
locations and using different cable.  I've found that location really
does matter and you really do want  a view clear down to the horizon
except to the north where it does not matter. and then I had to run 50
feet of coax cable and found the attenuation killed the signal, so I
either put the antenna back in the poor location where much of the sky
is blocked or buy much more expensive cable that has lower loss or
find a way to locate the GPS near the antenna, possibly in a
weatherproof box.  (It turns out I can do a combination of all three.
)  Ok, the point is that other things matter more and GPSes are dirt
cheap, even good ones.  So don't try to save $25



-- 
=====
Chris Albertson
Redondo Beach, California




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