[time-nuts] New NTBW50AA

[Bob Camp]

Hi


On Sep 9, 2013, at 10:31 PM, quartz55 <quartz55 at hughes.net> wrote:

> OK, did a bit more reading.  I already understand the difference between accuracy and stability however.
> 
> I thought ADEV was some sort of measurement of accuracy, but I understand now it is a measure of stability over time.  I'm supposing now that I can assume that the best frequency accuracy I can imagine is what is specked in the book for the unit, <.8x10^-10.  

ADEV as calculated is a fine thing. The next issue is that you are not doing the measurement that goes into the calculation the right way in a GPSDO like this. Even a stability measure needs a reference. In this case you don't have one. There is only one signal into the box (GPS). You need a second signal into it to do a comparison. There is no second signal present. LH is not doing anything wrong, it just does not have the data to do it completely right. 

> That should be good enough for me.  Although most seem to say the GPSDO units are good for .1Hz at 10GHz which I think would be 10^-13 no?

If you are talking about a multi day average - sure. If you are talking about "what frequency am I at right now" not so much. The OCXO drifts / ages / wanders in much less than a day (or hour). You are doing pretty well to keep frequency under 1.0 x 10^-11. Some GPDSO designs are running fine when producing < 1.0x10^-10. In each case they likely would report very good numbers while looking only at themselves. 

These boxes were designed as time references rather than frequency references. The frequency requirements on a base station are much looser than the time requirements. The net result is that frequency can move around a bit as they push the pps to stay where it belongs. 

> 
> Yeah, I've read through the h...cpp and a lot of it is greek to me, I'm no programmer, but I can pull a bunch of stuff out of it.  But it doesn't explain the acronyms or the meanings of them.

There is no formal manual for LH and no magic help function. The archives of this list are probably the best place to find information on what this and that means. 

> 
> I've lowered the el mask to 20 and I get plenty of sats now.  When it was at 43, lots of times it was down to 2, now it's generally up to 6.

You should be able to see 8 to 10 sats most of the time with a good location. I'd try to find someplace where you don't drop below 5. If you are running through trees, multi path may become an issue. Since multi path means a longer path delay it throw the time (and then frequency) of your GPSDO off.

>  I'll see how it does, especially if it rains, and yes the trees really cover the antenna.  I am getting 30-40 or more dBc however which is what I had when it was more in the clear.  I can move it to the west about 30' on the chimney where my UHF/VHF beams are and it's a lot more open straight up and especially to the south.  The position where it is now is just real convenient and it's only maybe 25' from the unit.  Plus I didn't have to get up on that part of the roof that's 7 in 12.

The antenna has an amp in it. It should have around 36 db of gain. If you keep the cable loss below 20 db, distance should not matter. For most quad shield RG-6 that will let you run 200 feet. 

> 
> Yes, I notice the gis for our county seems to have a slightly different co-ordinate system, they don't line up with google or the GPS which seem to agree as far as I can zoom in on our location.  I'd say the GPS and google are within a foot or 2.

There are a number of different systems. Google picked the same one as GPS. The way they do their maps, you can be off by 30 feet or … depending on where you are.
> 
> I guess I can just turn off the temp chart if it's not going to report right and stop looking/worrying about it.  As long as the green lock light is on.  I wonder if I could trick the Nortel unit into thinking it's seeing the CM though, so the top green light would come on instead of the yellow one.  But that doesn't matter.

If you feed it the "right stuff" over the RS-485 links and cross tie the right lines to another uint, you can get the light to go out. 

Bob

> 
> Thanks for all your help, I'll hang around for a while.
> 
> Dave
> N3DT
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