[time-nuts] Three-cornered hat on timelab?

Tom Van Baak tvb at LeapSecond.com
Wed May 3 00:33:42 EDT 2017

Hi BobS,

> I've also made the timelab file (compressed by 7z) available here:
> http://evoria.net/AE6RV/Timelab/GFSvsCS.

Thanks for sharing that. To follow-up on recent emails...

When you start to push the limits of your own test equipment it's good to employ tricks such as 3-corner hat in order to get a few dB better measurement. But there's a limit to that. And you probably don't want me to suggest that you run 3 GPSDO, 3 counters and 3 cesiums all simultaneously.

I've attached two plots showing 4+ days of your GPSDO data and 4+ days of a TBolt (factory defaults, *untuned*)

bob-gpsdo-3.gif -- Notice how much your GPSDO (blue trace) wanders all over the place.

bob-gpsdo-5.gif -- ADEV comparison. See below for comments.

When I look at the ADEV for your data set it's pretty clear that the limiting factor on the left (short tau) is the resolution of your 5370A counter. That's why the TBolt looks better. Don't worry about that.

In the middle of the ADEV plot it would appear that both your 5370A counter and your PRS-45A cesium reference are sufficiently good to properly measure your GPSDO. That's good. The TBolt looks bad in the middle but that's simply because it was using a way-too-short default time constant. With proper tuning the red trace would be mostly flat out to 1000 seconds.

>From the look of the phase plot and the ADEV plot it almost seems to me like your PRS-45A is the limiting factor on the right (long tau). If so, it would explain the lack of diurnal and sidereal effects in your data. If you look at:
http://leapsecond.com/pages/tbolt-8d/sigma1.gif from http://www.leapsecond.com/pages/tbolt-8d/
you'll note the sidereal "blip" is below 4e-14. Your ADEV doesn't get near that level.

I say this because a rule-of-thumb is that a good M12+T timing receiver should get you down to 3 ns per day RMS. And a good ublox-6T can get you to 3 or 2 ns per day RMS. Your data shows something more like 5 ns -- which for a GPSDO is worse than no GPSDO at all. So something's wrong. What set of experiments can you perform to locate the problem? Or, what experiments can you perform that don't require buying expensive gear?

My first suggestion is for you run a standalone GPS timing receiver like a M12+T or ublox-6. Put all your GPSDO away. Just use that OEM board, and your TIC, and your PRS-45A. Collect sawtooth data too. You should get down to 2 or 3 ns RMS.


----- Original Message ----- 
From: "Bob Stewart" <bob at evoria.net>
To: "Discussion of Precise Time and Frequency Measurement" <time-nuts at febo.com>; "Magnus Danielson" <magnus at rubidium.se>
Sent: Thursday, April 27, 2017 9:48 AM
Subject: Re: [time-nuts] Three-cornered hat on timelab?

Hi Magnus,
Try as I might, the weather and the local power company had other ideas about my long term capture. I'm running everything but the 5370 from a UPS. I guess I'm going to have to get batteries for my other UPS and run the 5370 from that. A one second power loss was all it took to stop the test.

Anyway, I did manage to get 376,238 points of data. The data is captured on a 5370A. The external clock input and the STOP channel are fed by the 10MHz from my PRS-45A. The START channel is fed by the 10MHz from one of my GPSDOs. The EXT channel is fed by the 1PPS from another of my GPSDO units. "EXT ARM" is enabled. So, essentially, at every 1PPS pulse, the phase difference between the two 10MHz feeds is captured.

I've attached a screenshot of the phase plot which can also be found here:http://evoria.net/AE6RV/Timelab/Screenshot.png
I've also made the timelab file (compressed by 7z) available here:

So, back to my question: Where are the large ionospheric phase moves? This question has been causing me doubt since I started on this project. Or don't I still have enough data collected for this to happen?




      From: Magnus Danielson <magnus at rubidium.dyndns.org>
 To: Bob Stewart <bob at evoria.net>; Discussion of precise time and frequency measurement <time-nuts at febo.com> 
Cc: magnus at rubidium.se
 Sent: Tuesday, April 18, 2017 1:09 AM
 Subject: Re: [time-nuts] Three-cornered hat on timelab?
Hi Bob,

That is a good solution indeed. Good luck with that measurement run!

One of the fun stuff with Timelab is that you can walk by and check the 
developments. I've found that very useful for long measurements (as in 
hours and days).

I prepared a cesium for one vendor, and initially they did not care so 
much, but then they saw more deviations between the receivers, so they 
wanted to sort it out, but discovered that they could not cancel out the 
common mode of GPS signals (and its shifts), so then firing up that 
cesium was the right thing. I remember writing support emails while 
waiting for the airplane in Madrid airport, happy that they was doing a 
first run for the right measurement reason. :)


On 04/18/2017 04:25 AM, Bob Stewart wrote:
> Hi Magnus,
> Today I started a long run against my PRS-45A. Maybe this time I won't have a power outage. I'll see what it tells me in a few days.
> Bob
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