[time-nuts] Choke Ring Pictures

Bruce Griffiths bruce.griffiths at xtra.co.nz
Tue Mar 16 02:46:15 UTC 2010


WarrenS wrote:
>
> Bruce said:
>> "Unless you have a CSO or a hydrogen maser ... can't do that."
>
> So I would generally answer if one can not measure any ills effects, 
> Then it does not mater to THAT person AND it is good enough.
>
Being unable to measure something doesn't mean that it doesn't matter.
> But, Sounds like you could use a little more Practical experience.
> Cause it is so simple to measure, most anyone can do it, even if it 
> does not mater to them.
> I do it all the time and have nothing better than a HP10811
You first need to establish the stability of the 10811A, they aren't all 
equal.

>
> The First indirect way is to look at the position dispersion when in 
> position tracking mode.
> OR
> For a more direct way, set the GPSDO tracking TC very fast say at 10 
> sec or so which will then allow plotting of  the GPS ADEV noise.
> The goal is to see how the GPS noise changes with different Antennas 
> and the ultimate goal is to see where it drops below the internal Osc 
> noise.
> You can See Tom's site  for more on that.
High pass filtering the relative phase noise of the OCXO and the GPS 
receiver PPS output like that doesnt allow ADEV to be accurately 
measured without correction for the effect of the high pass filter.

>
> And if you want to do it without ANY external equipment, It can be 
> done with some of LH plots and graphs good enough,
> by placing the Osc in disable and ploting the Phase noise which for 
> short time periods will be the GPS noise.
Not particularly useful unless you select which SVs are used, so the 
antenna performance for low elevation SVs (for which multipath 
interference is more likely) can be explored.

> More info on request.
> ws
>
> **********************
>
> Bruce Griffiths wrotre
>
> Unless you have a CSO or a hydrogen maser absolute measures of ADEV and
> phase noise arent feasible for the range of Tau of interest.
> Even an indirect method such as measuring the location of the apparent
> minimum in ADEV between the GPS SV constellation observables and the
> OCXO when it is undisciplined depend heavily on the ADEV characteristics
> of the OCXO being used.
>
> Bruce
>
> ********************
> WarrenS wrote:
>>
>> yeah,
>> So many variables, ALL the more reasion to just see what the overall 
>> effect is on the more common type of GPSDO receviers at a few sites.
>> So did you have a better plan?
>>
>> ws
>>
>> ****************
>> Bruce Griffiths Added:
>>
>>> WarrenS wrote:
>>>
>>> Bruce wrote:
>>>> "Which antenna performance metric do you have in mind?"
>>>    Could do GPSDO hold over performance, but that would not be much of
>>> a test of the antenna.
>>> How about the antenna's effect on the ADEV Osc noise and Phase noise.
>>> What else does the Time Nut care about?
>>>
>>>> Since the better timing receivers use carrier phase ...
>>>    I don't remember you ever finding ANY Time Nut that is now using 
>>> one.
>>> So may be simpler for now to just stick to the more common type of
>>> GPSDO in use.
>>>
>> The Motorola M12+T and iLotus M12M use carrier phase smoothing of the
>> code phase observables.
>>
>> Its highly likely that a number of the better performance GPS timing
>> receivers also use carrier phase smoothing.
>>
>> Thus whether one is aware of it or not the antenna carrier phase
>> properties are likely to be of some importance.
>>
>> In the absence of complete information on how your particular GPS timing
>> receiver uses carrier phase and code phase observables, the best you can
>> do is compare the performance of a range of antennas using a given
>> timing receiver.
>>
>> Such results will only apply to a particular site and receiver.
>> Specifying the pertinent characteristics (eg isolated on a flat plain,
>> surrounded by a set of hills, mountains which obscure the sky below 10
>> degrees, surrounded by trees  that obscure everything below 40 degeees
>> elevation, etc) of your antenna location and the particular GPS receiver
>> used will be helpful to others in selecting an antenna that suits their
>> budget, receiver, antenna location constraints, etc.
>>
>> Bruce
>>
>>> ws
>>>
>>> *************
>>> Bruce said:
>>>
>>> Which antenna performance metric do you have in mind?
>>> There are several, some of which are considered in the paper:
>>> http://www.novatel.com/Documents/Papers/effectofantenna.pdf
>>>
>>> Since the topography surrounding the antenna, its height and 
>>> location on
>>> the Earth all affect measured performance any comparative measurements
>>> should use the same receiver and antenna location.
>>>
>>> Some estimates for the effect of multipath on code phase receivers can
>>> be found:
>>>
>>> http://bbcr.uwaterloo.ca/~wzhuang/papers/iee95_gps.pdf
>>>
>>> http://bbcr.uwaterloo.ca/%7Ewzhuang/papers/iee95_gps.pdf
>>>
>>> Since the better timing receivers use carrier phase smoothing of the
>>> code phase timing, both the carrier phase and code phase performance of
>>> the antenna are important.
>>>
>>> A phased array antenna like the one in the following papers may provide
>>> better performance than alternative antennae:
>>>
>>> http://www.navsys.com/Papers/0001002.pdf
>>> http://www.congrex.nl/07c12/papers/day1_s1_paper05_Konovaltsev.pdf
>>>
>>> Some measurements with geodetic antennae:
>>> http://www.fig.net/pub/fig2008/papers/ts05g/ts05g_03_eventzur_shaked_2816.pdf 
>>>
>>>
>>>
>>> Comparison of code phase and carrier phase time transfer:
>>> http://tycho.usno.navy.mil/ptti/ptti2004/paper41.pdf
>>>
>>> Bruce
>>>
>>> ******************
>>> WarrenS wrote:
>>>> Brian wrote:
>>>>
>>>>> "There were also comments about surveying and timing antennas."
>>>> Those may of been from me, unsuccessfully trying to make a point of
>>>> the difference between what is 'Best' and what is 'GOOD enough'.
>>>>
>>>>> "about every national timing laboratory uses choke ring antennas.
>>>>> ... for timing stability reasons."
>>>> Then again they also have multiple CS and Just their Antenna budget is
>>>> likely more than the annual income of most time nuts.
>>>> Can you do a test to show IF there is ANY improvement for the AVERAGE
>>>> time nut when compared to a well setup (Tbolt) GPSDO using a TacoSalad
>>>> antenna?
>>>>
>>>> Would be interesting to see a plot of cost vs. performance for the
>>>> various antenna types,
>>>> Scaled to show the performance improvement that the average Time nut
>>>> would see.
>>>> The TacoSalad antenna, originally cost me a total of $7.95, And took
>>>> under 30 seconds to build.
>>>> That cost should be discounted because those parts had been considered
>>>> just throw away junk up until now.
>>>>
>>>> ws
>>>>
>>>> **********************
>>>>
>>>> ----- Original Message ----- From: "Brian Kirby"
>>>> <kilodelta4foxmike at gmail.com>
>>>> To: "Discussion of precise time and frequency measurement"
>>>> <time-nuts at febo.com>
>>>> Sent: Monday, March 15, 2010 2:09 PM
>>>> Subject: Re: [time-nuts] Choke Ring Pictures
>>>>
>>>>
>>>>> Dr. Clark passed on a tip that I used.  Put the funnel in a microwave
>>>>> oven and run it and see if the funnel warms up.  If it warms up, you
>>>>> do not use it.  I do not know what type of plastic the funnel was
>>>>> made out of; it was white, semi-transparent.
>>>>>
>>>>> There were also comments about surveying and timing antennas.  If you
>>>>> investigate about every national timing laboratory uses choke ring
>>>>> antennas.  Some enclose the antenna unit and they temperature control
>>>>> it. They do this for timing stability reasons.
>>>>>
>>>>> The commercial timing antenna is bullet shaped and is operated
>>>>> without a ground plane.  They are patch antennas.  When there is not
>>>>> ground plane, the antenna picks up best from the overhead and less
>>>>> towards the horizon. These antennas usually have a lot more gain
>>>>> (30-50 db vs most normal antennas in the 15-25 db range).
>>>>> Also in surveying, we cut off the horizon at 15 degrees in software.
>>>>> A free Army Corp of Engineering manual on GPS Surveying is at
>>>>> http://140.194.76.129/publications/eng-manuals/em1110-1-1003/toc.htm
>>>>> The main difference in surveying and timing is in surveying they use
>>>>> the carrier phase method, were in timing most use a solution derived
>>>>> from the processing of the coarse acquisition code, in were the
>>>>> receiver is in a fixed over-determined position .  Some timing labs
>>>>> are using carrier phase method, when they need more resolution.
>>>>>
>>>>> Brian - KD4FM
>>>>>
>>>> ****************
>>>>>> warrens wrote:
>>>> ...
>>>>>> Preliminary results for the Taco Dish GPS antenna as an indoor
>>>>>> antenna are  looking good.
>>>>>> Certainly worth considering if your GPS antenna is stuck indoors,
>>>>>> 'Out of the rain in the living room'.
>>>>>> I find it best to rise it up near the ceiling such as on an upper
>>>>>> shelf with nothing above it.
>>>>>> It would be hard to tell the difference between the GPSDO
>>>>>> performance obtained from this or the Best outdoor antenna if using
>>>>>> a Tbolt set to the  standard default settings.
>>>>>> Picture attached
>>>>>>
>>>>>> ws
>>>>>>
>>>>> **************
>>>>
>>>
>>
>
>
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