[ntp:questions] garmin 18x and linux

Chris Albertson albertson.chris at gmail.com
Sun Sep 11 02:42:48 UTC 2011

>> 50 nanoseconds.  Can you even begin to time the steps between the
>> arrival of the PPS in your GPS receiver and the updating of the
>> computer's clock?

Yes.  You can find any number of older HP Universal Counters in eBay.
Some are cheap.  I paid $35 for one recently.  Shipping cost as much.
 Better ones are about $250.    With one of these I can measure the
time it takes for a pulse to travel down a few meters of coax cable.
I connect the "start" and "stop" triggers with a length of rg58 coax
cable and then place a signal generator on the "start" using a "tee".
The pulse starts the timer same pulse delayed by the cable stops it.
I can measure the length of the cable this way with accuracy better
than a foot.   The better (affordable) counters work at the 100 pico
second level.  This is 80's vintage test equipment that goes for cheap
on eBay.

The PPS interrupt processing time is dead easy to measure assuming you
have a counter.  So is the delay cause by the GPS antenna feed line
and the RS232 cable that goes from the GPS to the computer.  All these
cable delays are "huge" if you are working at the nanosecond level.

If you have one of these counters the next thing to buy is a very
stable frequency reference.   It is not hard to set up a oscillator
with Allan deviation of about 10e-12.  (12 zeros is a lot)  then using
the counter you measure the phase of the GPS' PPS signal relative to
your clock.   Believe me many people have done this.  My equipment
here at home can measure to 250 pS but I don't have a good enough
local clock to take advantage of this.    Next on my shopping list is
a good a couple clocks first ovenized quartz crystal oscillator.
Second on the list is a Rubidium oscillator.  Rubidium is the "entry
level" atomic clock and they've come down in price.  Serviceable units
go for $150.

Chris Albertson
Redondo Beach, California

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