On 11/17/17 11:05 AM, Jerry Hancock wrote:

I don't think so..
GPSDOs are really more of a syntonization (same frequency) rather than a
synchronization (same phase) box.

You might have two boxes of the same kind that have some well defined
and stable phase relationship between, say, the 10 MHz sine and the 1pps
(if the 1pps is derived by dividing the 10 MHz, for instance)

Hi

Most GPSDO’s use a divider to get straight from the 10 MHz to the PPS. There are some that
derive both the PPS and the 10 MHz off a higher frequency, they are not common in the eBay
world (yet). The divider is going to have a fixed delay based on it’s design. That might be 35 ns,
it might be 3.5 ns. It all depends on how they did this or that. There easily could be an inversion
that moves everything 50 ns.

If the GPSDO wanders around over a 30 ns range, that’s a pretty big phase shift at 10 MHz. If
they both are doing this you are up to (maybe) half a cycle or more.

Now toss in devices that “skip” a 10 MHz cycle to keep the PPS aligned. This is pretty uncommon
in normal operation. Uncommon does not mean that it never happens.

So, if you have two of the same model / same firmware GPSDO’s, the 10 MHz phase is probably
“in the ballpark”. It’s certainly not close in terms of < 1 degree or something like that. If you have
GPSDO’s from multiple outfits …. who knows what you will see.

Bob

Hi,

First of all, the receiver varies around some average phase.
The average of two receivers may not be the same, even if sharing antenna.
Their average can be offset from "absolute phase" because of offsets in
the receiver, antenna cables and antenna.

If you are not to careful, a GPSDO will do nicely for many applications.
The more you care, the more hassle.

Oh, and two GPSDOs next to each other can have significant common mode
disturbance, so you can't really evaluate a GPSDO by measuring up to
another GPSDO.

Cheers,
Magnus

On 11/17/2017 08:05 PM, Jerry Hancock wrote:

Thanks, understand completely.

You mention “sharing an antenna”.  Is there a problem with connecting the same antenna to two units?  I am referring to my REF0/REF1 pair.

Thanks

Hi,

The reason I mention sharing antenna is that it takes out the difference
that is natural from having different antennas and the difference in
multipath.

Sharing antenna has its own set of problems, so a suitable antenna
splitter was assumed. Effectively you need to bypass DC to power the
antenna, you need to amplify the signal to compensate for the loss due
to the splitter, and it needs to look like an antenna to the GPS
receiver, so the antenna alarm does not goes off.

Cheers,
Magnus

On 11/17/2017 09:01 PM, Jerry Hancock wrote:

I figured, what the heck, and just used a BNC T and it’s working.

Can work, can go wrong.

One should be careful and think it though.

Cheers,
Magnus

On 11/17/2017 09:25 PM, Jerry Hancock wrote:

I agree.  I disconnected it after wondering about the active antenna bias.