[ntp:questions] garmin 18x and linux

unruh unruh at wormhole.physics.ubc.ca
Sun Sep 11 03:56:07 UTC 2011

On 2011-09-11, Chris Albertson <albertson.chris at gmail.com> wrote:
>>> 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.

Except that measuring the time it takes to travel down the cable is not
what was wanted or discussed. 

> 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.

The interrupt processing time is NOT easy to measure, because it is
highly variable. It depeds on what the computer is doing at the time.
It can fluctuate by usec. 

> 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

The clock in this case is the internal system clock in the PC, which is
a complex combination of timer interrupts and cpu couters, or hpet, or
whatever you computer uses. And none of those signals is delivered out
onto some measuring point. 

> 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.

But that still does not get the time inside your computer which is where
it is wanted. 

> Chris Albertson
> Redondo Beach, California

More information about the questions mailing list