[time-nuts] A first MDEV plot...

Bert, VE2ZAZ ve2zaz at yahoo.ca
Tue Mar 16 13:37:03 UTC 2010


OK, thanks Bruce.

The sample size is actually 28800 seconds (8 hours). I presume the KE5FX tool plots only the portion of curve that has sufficient confidence level. In any case, I will launch a more lengthy test and see what I get down at the bottom.

Bert.


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Message: 1
Date: Wed, 17 Mar 2010 01:32:49 +1300
From: Bruce Griffiths <bruce.griffiths at xtra.co.nz>
Subject: Re: [time-nuts] A first MDEV plot...
To: Discussion of precise time and frequency measurement
    <time-nuts at febo.com>
Message-ID: <4B9F7A71.3090108 at xtra.co.nz>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed

Bert, VE2ZAZ wrote:
> Hi Everyone,
>
>
> This is my first Allan Deviation plot. It consists of the PPS output of an Oncore GT+ (Navigation-grade) GPS vs. an Ovenaire 10MHz OCXO divided down to 1Hz via an HP 5328A counter. The HP 5370A is free running on its 10811-60101 OCXO. The control/plotting is done by KE5FX's T.I. utility software.
>
> The curve clearly shows the limitations of the GPS at Tau=1s... But what else can be said of the GPS, or of the test setup when looking at the plots?
>
> One thing I see is that the bottom valley is much more pronounced when plotting MDEV, whereas with ADEV the curve barely breaks. How much more revealing is the MDEV curve in this case, and what effect is shown here? What dominant factor causes the valley. Is it hitting a noise floor, a true GPS performance deterioration or is is simply a calculation artefact?
>
> I use the HP 5328A as a divider by 10 million. This is a very useful feature if you own one of these counters. I presume that a straight line plot gives a warm feeling about the divider performance, but it would be great if someone on this list who owns a HP 5328A could characterise the Timebase Output when dividing by 10 million. I will try some measurements using the 5370A and one of my OCXOs, but I am not sure I own what is necessary to make valuable measurements.
>
> So much to learn! So much to try!
>
> Thanks,
>
> Bert.
>
>    
Bert

If you only accumulated data for a bit over 10,000 seconds you cant 
place much reliance in the plot much beyond about Tau=3000 secs as the 
error bars become a litle large.
Accumulating data for longer (eg 1 day or more) will improve the 
confidence in the plto at around 10,000 seconds.

Eventually for sufficiently large tau the noise contribution from the 
gps receiver will be equal to that of the OCXO.
For larger tau the noise from the OCXO will dominate and the curve will 
turn up as the OCXO instability eventually increases for increasing Tau.

The plots probably indicate that the divider noise isn't worse than the 
GT specification with SA on.
For your particular antenna and SA off the noise should be a little lower.

To get a better idea of the divider noise would require using a time 
interval counter like your 5370A.

I'd connect the PPS from the divider to the START input of the 5370A and 
the divider input signal to the STOP input.
This minimises the effect of the 5370A timebase error.

If possible save the measurements so you can calculate ADEV for the setup.

If the divider has low jitter you'll end up measuring the ADEV of the 
5370A itself.

Bruce




------------------------------

Message: 2
Date: Tue, 16 Mar 2010 05:59:32 -0700
From: jimlux <jimlux at earthlink.net>
Subject: Re: [time-nuts] frying pan antenna
To: Discussion of precise time and frequency measurement
    <time-nuts at febo.com>
Message-ID: <4B9F80B4.2040107 at earthlink.net>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed

Bob Camp wrote:
> Hi
> 
> I certainly won't disagree with the data. I have noticed over the years that past a few MHz antennas out in the fresh air always seem to work better than indoor antennas. It could easily be that there is a *lot* of water in your typical roof. Certainly the water is far from pure, so it's going to be pretty low conductivity. 
> 
> Bob
> 

there's also an amazing amount of metal and other junk in the usual 
composition roof construction.  Nails to hold the shingles on, for instance.

But I think the major absorption will be the moisture in the wood sheathing.

By this logic, a thatch roof in a dry climate is the best thing going.




------------------------------

Message: 3
Date: Tue, 16 Mar 2010 13:54:54 +0100
From: mike cook <michael.cook at wanadoo.fr>
Subject: Re: [time-nuts] frying pan antenna
To: time-nuts at febo.com
Message-ID: <4B9F7F9E.60400 at wanadoo.fr>
Content-Type: text/plain; charset="iso-8859-1"; Format="flowed"

Yes Bruce, I also think that azimuth dependent masking would be useful.

I have been suffering from strong reflections from facing buildings at 
50m to 100m, which cause sharp departures from the average PPS TI 
(HPZ3801A/ Z38XX app.) values. I can remove the problem by carefully 
watching the satellite azimuth/elevation display  and  sending the 
receiver a command to ignore the relevent SV when it passes  to the 
south (antenna on north facing window ledge). I think that it would be 
feasible in somethig like Z83XX and LH  to set up an azimuth dependent 
mask and monitor the SVs predicted positions to selectively 
ignore/include them when they pass out of and into line of site.

I did post in december on this problem but add here a TI stats view to 
show the magnitude of the errors seen.

Mike

Le 16/03/2010 03:30, Bruce Griffiths a ?crit :
>
> A pity that the GPS receiver elevation mask can't generally be made to 
> be azimuth dependent.
> You could then have a higher elevation limit in the direction of those 
> trees.
>
> However, it is possible in principle with an M12+T, M12M T or similar 
> timing receiver which provides the relative timing contribution of 
> each SV to the solution, to back out the contribution of a given SV 
> and compute the corresponding timing correction for software 
> correction of the PPS pulse timing.
>
> Bruce
>
> Mike Naruta AA8K wrote:
>>
>> My aluminum frying pan ground plane does a nice job.
>>
>> The nut welded on for the handle is a convenient
>> place to attach the lightning ground wire.
>>
>> Snow accumulation hasn't been a problem yet.
>>
>> The non-stick underside seems to discourage insects.
>>
>> You should have seen the look on my wife's face when
>> I told her that I would like to bolt a frying pan
>> on the peak of the roof.
>>
>>
>> Mike - AA8K
>>
>>
>> WarrenS wrote:
>>>> Next on my list of things to try is a smaller open taco maker deep 
>>>> dish metal pan.
>>>
>>
>>
>>
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>
>
>
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