[time-nuts] GPS 18 behavior

[David McGaw]

The GPS-18 does NOT output 1PPS until it acquires lock.  Then the 1PPS 
stays on even if lock is lost, running from the internal crystal.  I 
have not checked, but once it reacquires lock I presume it jumps to the 
correct second.  All outputs including 1PPS are CMOS levels, so it 
drives both low and high.

David


On 7/21/13 9:02 AM, Jim Lux wrote:
> I don't have a GPS-18 in front of me, and I'm modifying some software 
> remotely, and I ran across an issue that someone on this list probably 
> knows off the top of their head.
>
> Does the GPS-18 put out 1pps pulses even if it hasn't got a fix yet? 
> That is, when you apply power, does it just start putting out 1pps 
> pulses (at some random phase), and then when it gets a fix, it snaps 
> to the correct timing.
>
> The manual says that it puts out 1pps pulses until power is removed, 
> so I assume that when the GPS signal goes away, it keeps on as best it 
> can.  I would assume that the clock isn't great, but it's reasonably 
> close to 1pps.
>
> What's the behavior when it reacquires?  Say the free-running 1pps is 
> something like 1.1 Hz, so in 10 seconds, we've got an extra 1pps. Does 
> it then skip pulses, or what?
>
> And a sort of background hardware question...
>
> The manual is also not very forthcoming on the electrical 
> characteristics.. is it a pull up with a switch pulling it down to 
> ground? or does it source significant current? That is, if you wanted 
> to "wire or" the output from the GPS-18 with another 1pps source, 
> would it be "diodes" or "resistors" you'd use.
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