[time-nuts] Looking for off-the-shelf device to timestamp multiple PPS inputs

[Kevin Rosenberg]

On Sep 28, 2011, at 12:19 PM, Hal Murray wrote:
> What sort of accuracy and/or resolution do you want/need?

Whoops, left out that critical piece of information!
It's for my son's project, he'll be looking at deviations probably
no shorter than one 1 sec, so milliseconds will be plenty for him.

>> 7) Use a box like in #5, and capture the PPS from the PRS-10 as well and
>> using that to scale the other PPS inputs that arrive around the same time. 
> 
> If you have a Linux box handy, that one is easy to try.

I agree. That idea was listed last because I came up with it while describing
the other options that I considered. I do have a linux box with a 4-port PCI
serial port that seems to be supported by the kernel's PPSAPI.

> After the 60Hz grab-every-cycle experiment, I started collecting every-cycle 
> data for PPS signals.  I have a box  with 3 PPS signals: one from a Z3801A 
> and 2 from low cost GPS units.
> 
> I can send you some files if you want to investigate.  (5-6 megabytes per day)

Thanks, I would be interested. One of the PPS signals will be 60 Hz mains, but 
probably will use two ICs to divide it by 60 to simplify the analysis for him
and keep all his signals nominally 1 Hz.

> I'll have to make some graphs.  The ballpark is several microseconds of 
> noise, but that's on top of your system clock offset.  I think it would be 
> easy to see 10 microseconds of offset.  You would have to work harder to see 
> smaller numbers.

That'd be more than enough resolution for him. The main thing I was hoping for 
was something simple to use, without much construction, so that he can do the 
project himself.

> Reminder: Your system may have troubles catching 10 microsecond PPS signals.  
> TAPR says "no longer available" for the FatPPS.  I poked John.  He was going 
> to see if they could build another batch.

Your quite right about the PRS-10 PPS pulse, 10 us was obviously too fast for the
Soekris with a kernel timing frequency of 1000 Hz. I wasn't aware that FatPPS
had gone "no longer available". Makes me glad that I got mine last year along
with a TADD-1 which was also discontinued. That was considerate of you to mention
it to John to assist others who have such brief PPS pulses.

Thanks the great thoughts, Hal!

Kevin