[time-nuts] Why using HP5370 ext-ref is (maybe) a bad idea

[John Seamons]

On Mar 2, 2014, at 7:05 AM, Pete Lancashire <pete at petelancashire.com> wrote:

> Idea. On the next go around for the board put the copper down and holes for
> a couple small daughter cards and any support logic needed to interface
> with the BBB.
> The the only additional cost would be limited to the daughter board I/O
> since my guess it would be SMT hence a bit hard to leave it unpopulated.

Good idea. Also the Beagle spec allows for multiple, stacked interface boards ('capes' they call them). So for a backwards compatible solution an experimental GPSDO + backup power cape could be interposed between the Beagle and 5370 board. I say experimental because I have no idea if any of the SDGPS projects out there would be ultimately suitable for a DO.

This brings up a question I have about how the PPS edge is actually derived by a GPS receiver. Does it originally come from the NAV data stream and then get corrected by the (fixed-mode) positioning solution to account for transmission/system delays? I know about the issue of alignment with the VCTCXO clock, but I'm talking about upstream of that.