[time-nuts] References for GPSDO measurement (was Choke Ring Pictures)

John Miles jmiles at pop.net
Tue Mar 16 01:57:42 UTC 2010


Actually an HP 5065A in good condition is adequate to characterize the best
GPSDOs beyond t=10s or so (see green trace).  5065As aren't that much
noisier than passive masers until you get past t=1000 seconds!

On the other hand, a telecom-grade rubidium is nowhere near good enough for
GPSDO tweaking (blue trace).

-- john, KE5FX

> -----Original Message-----
> From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com]On
> Behalf Of Bob Camp
> Sent: Monday, March 15, 2010 6:22 PM
> To: Discussion of precise time and frequency measurement
> Subject: Re: [time-nuts] Choke Ring Pictures
>
>
> Hi
>
> One could always use the ever popular super cooled sapphire
> resonator oscillator and an equally handy optical ion standard.
>
> --------------------
>
> Anything that involves comparing same to same is vulnerable to
> all sorts of common mode effects. Ensembles of cesiums all made
> on the same line can have issues. Even having something like
> (yikes!) WWVB to throw in the mix will help rule out part of the
> errors that creep in.
>
> One thing we haven't talked about (but could) are time transfer
> setups via common view GPS. Back when we had a NIST box it did
> them under robotic control. Judging from the data plots it did a
> very good job.
>
> There really wasn't a lot in the silly box. Their system did
> indeed use a (at that time) fancy antenna and a pretty good
> receiver. It's not clear to me that the 1980's technology in the
> box could not be duplicated today for a lot less than they put
> into it. Anything you can toss into the mix is a good thing. With
> good enough time transfer you could use the H-Maser in somebody
> else's basement .....
>
> Bob
>
>
>
> On Mar 15, 2010, at 8:55 PM, Bruce Griffiths wrote:
>
> > 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_shak
> ed_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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>
>
>
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