Crosstalk? With the same signal?

On Tue, Oct 9, 2012 at 4:31 AM, Edgardo Molina xe1xus@amsat.org wrote:

I was wondering about that myself, but my guess is the crosstalk would
be from whatever grunge was coming from the other GPS. Every amplifier
has reverse parameters, so a small amount of the crud (circuitry noise)
from one GPS will reach the other GPS. Not much, but some people are
nuts about time.

This is a bigger problem with radios, where the locals from one radio
can reach the input to the other radio.

I was also confused on the notion of a transmission line splitter. Is
this a Wilkinson or something else?

On 10/9/2012 12:40 AM, Azelio Boriani wrote:

Hi all,

In my shack I have a single antenna with two power splitter in series
because I need several ports for the four GPSDO and spare port for
occasional testing.

any cross talk depends on the circuit and layout of the splitter. Some
crosstalk can happen with two close antenna cable.
I suppose the first question have to be : What is my time stability
target?
Important is a stable electrical and mechanical configuration and port
connection.

I have the same dubt and for this reason I have powered my Agilent
splitter not from the GPS but from separate port is present (ac and
backup du power).This also for the smart splitter.

I agree, the new receivers have comparable performance.Some more
difference are in PLL OCXO and firmware.

I think a standard calibration lab have to setting up with two antenna
system and two receivers as recovery plan in case of fault. One system
can be single antenna plus GPSDO (recovery reference). The other can use
a splitter connected to second receiver and several more ports for
testing. All my ports are with a blocking capacitors in series to avoid
any antenna or active splitter interaction.

Luciano
timeok

This article discusses timing errors due to mismatch and multiple
reflections in transmissin lines:
http://www.dtic.mil/dtic/tr/fulltext/u2/a508044.pdf

Adrian

Edgardo Molina schrieb:

Is there any difference between what a GPS receiver can receve via
crosstalk or receive directly from the antenna? In my opinion crosstalk is
absolutely less than the last argument about GPS antenna splitters.

On Tue, Oct 9, 2012 at 9:58 AM, Timeok timeok@timeok.it wrote:

I agree,
Luciano
timeok

Il 2012-10-09 10:41 Azelio Boriani ha scritto:

--
timeok

When GPS first started to be fitted to light aircraft it was found that LO leakage from some VHF navigation recivers blocked the GPS when the NAV was on certain channels. You can buy a BNC "T" adaptor where the leg of the T is a 1.5GHz coax stub notch filter. They go on the NAV RX antenna connector.
 
Robert G8RPI.

From: gary lists@lazygranch.com
To: time-nuts@febo.com
Sent: Tuesday, 9 October 2012, 8:51
Subject: Re: [time-nuts] To use or not to use transmission line splitters for GPS receivers

I was wondering about that myself, but my guess is the crosstalk would be from whatever grunge was coming from the other GPS. Every amplifier has reverse parameters, so a small amount of the crud (circuitry noise) from one GPS will reach the other GPS. Not much, but some people are nuts about time.

This is a bigger problem with radios, where the locals from one radio can reach the input to the other radio.

I was also confused on the notion of a transmission line splitter. Is this a Wilkinson or something else?

On 10/9/2012 12:40 AM, Azelio Boriani wrote:

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Lots of comments. Indeed it sounds like a great discussion for pizza and
beer. The more beer the more lively. Did they bring beer?
Fact
I have used a 8 way splitter Sat/TV for 5 years now. Port to port loss is
something like 16 db or 26 db as I recall. It has dc blocking on all but 1
port built in. The loss was as advertised. The cost was pretty high at $7.
To make up for the loss I used a amplifier. A Mar circuit and only enough
gain to cover the splitter loss since the single antenna has 30db of gain
and feed 1/2" hardline. So if all of things discussed are happening its not
at all apparent from the 6 rcvrs on the system. Some old like odetics
austrons some newer like 3801s and Tbolt...Plus I never have to hunt for a
port for experimenting.
There is one catch and this can apply to all splitters some rcvrs need a dc
load so that they think they have an antenna. I think about 430 ohms. As I
say its been 5 years and it just works.
Total investment $10??
Though it doesn't say HP on it.
Regards
Paul
WB8TSL

On Tue, Oct 9, 2012 at 7:14 AM, Robert Atkinson robert8rpi@yahoo.co.ukwrote:

Paul,

I am convinced your realization work very well and it is a lower cost
in the market.

But depend what kind of user have to use the device.
For a standard laboratory or a company I am sure is not sufficent your
realization, for an hobbist yes, can be.

Business or research company  want to have a datsheet with temperature
range, technical characteristic, a repair service. In one word a
professional package.

Me for example, am an hobbist and I am started from the tv sat splitter
but now,  I have bought on ebay a low cost professional splitter.It is
better than mine at least as mechanical realization ad impedance
matching.

That's all.

All the people in time-nuts community want to improve day by day the hw
and sw they have at home.

yes, yesterday night I have bring a bier. Cirio!

Luciano
timeok

Il 2012-10-09 15:15 paul swed ha scritto:

--
timeok

On 9 Oct, 2012, at 12:48 , "Bob Camp" lists@rtty.us wrote:

I think you can get sub-nanosecond time (if you can arrange for a proper
equipment calibration) and sub-centimeter positioning on your own using
the IGS products and GPS Precise Point Positioning techniques.  The gotchas
are that you need to have a high-priced dual-frequency, carrier phase
tracking receiver and the software you need seems to only be available to
the very rich (though there are free online services which will process
your data to determine the location for you).

The antenna temperature thing is kind of indicative of just how much lore
and black art seems to be involved in arranging equipment for fine timing,
however.  I have the ITU 2010 Handbook for "Satellite Time and Frequency
Transfer and Dissemination".  In Chapter 12, when discussing GPS Common
View techniques, the document says this about antenna temperature

12.5.2 Temperature stabilized antennas

It is now well documented, and generally admitted, that GPS time-receiving
equipment, and more specifically its antenna, is sensitive to environmental
conditions [Lewandowski and Tourde, 1990]. For conventional GPS time-receiving
system this sensitivity could be expressed by a coefficient of about
0,2 ns/°C and can approach 2 ns/°C. This was a major precluding obstacle,
as it did, the goal of 1 ns accuracy announced earlier for GPS time transfer.

and goes on to recommend using an antenna with an oven keeping the temperature
of the electronics constant.  In Chapter 13, on the other hand, when discussing
GPS PPP, it says this:

There have been some poorly supported claims of strong variations of
geodetic clock estimates with temperature changes in some GPS antennas,
together with recommendations to use temperature-stabilized units. While
this might apply to certain low-end, single-frequency units, direct tests
of a standard AOA Dorne Margolin choke ring antenna have failed to detect
any sensitivity of the clock estimates to antenna temperature variations.
Ray and Senior [2001] placed an upper limit of 2 ps/°C on the short-term
(diurnal) temperature sensitivity and later extended this to <10.1 ps/°C
for any possible long-term component [Ray and Senior, 2003]. Even smaller
sensitivities, 0.17 ps/°C or less, were determined by [Rieck et al., 2003]
for an Ashtech choke ring model.

So Chapter 13 says that what Chapter 12 said is bogus.  It appears that Chapter 12
may have written been written by a European while Chapter 13 is an American
effort, so this may be some sort of cultural thing.  Chapter 13 does later go
on to point out how crappy the Canadian IGS stations are in the winter and
blames this on snow and ice in the near field below the antenna, so even Chapter
13 does find a use for heating at the antenna.  Both chapters do agree that keeping
the temperature of the receiver constant is good.

I think the antenna splitter thing is probably the same kind of issue.  Someone,
somewhere, may have had a problem with an antenna splitter and published a paper
on that, and this in turn reinforces the conservative assumption that you should
leave anything out that doesn't absolutely need to be there, so it has become
common wisdom that you should avoid splitters.  Or something.

Dennis Ferguson

Boy all I can say is I measured the $7 satellite splitter and it matched
the specs for fwd and rtn loss. Port to port loss using an HP network
analyzer. So what can I say it worked and well. Actually surprisingly so.
Regards
Paul.

On Tue, Oct 9, 2012 at 2:38 PM, Dennis Ferguson <dennis.c.ferguson@gmail.com

Hi

In the context of the original post, probably the right question is: do you
have a hydrogen maser and an ensemble of cesiums to compare it to? That's
the environment that gets front and center at these conferences.

Bob

-----Original Message-----
From: time-nuts-bounces@febo.com [mailto:time-nuts-bounces@febo.com] On
Behalf Of paul swed
Sent: Tuesday, October 09, 2012 2:43 PM
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] To use or not to use transmission line splitters
for GPS receivers

Boy all I can say is I measured the $7 satellite splitter and it matched
the specs for fwd and rtn loss. Port to port loss using an HP network
analyzer. So what can I say it worked and well. Actually surprisingly so.
Regards
Paul.

On Tue, Oct 9, 2012 at 2:38 PM, Dennis Ferguson <dennis.c.ferguson@gmail.com

our

gotchas

antennas,

American

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Here's a link to a USNO paper that measured the tempco of three GPS
amplifiers: http://www.dtic.mil/cgi-bin/GetTRDoc?AD=ADA490830

They found that amplifier filtering was the prime cause of tempco, and
the narrowest bandpass amplifier they looked at had a group delay range
of 4 nanoseconds over the range of -15 to +45 degrees C.

And for what it's worth, I used a VNA to measure the electrical length
of an ~80 foot piece of LMR-400 laying on the dark roof of my Georgia
house, and saw no measurable difference between cool pre-sunrise and hot
mid-afternoon on a sunny summer day.

There's also a brief paper about coax tempco from Haystack:
http://www.haystack.mit.edu/tech/vlbi/mark5/mark5_memos/069.pdf

John

On 10/9/2012 2:38 PM, Dennis Ferguson wrote:

Dear Edgardo,

On 10/09/2012 04:31 AM, Edgardo Molina wrote:

Cross-talk can occur, indeed. The main issue should be the
connected/unconnected reflections, only very secondary would be leakage
of signals from one GPS receiver to another. Both is being handled by
measuring and require sufficient port isolation.

There can be. Therefore you should select to use antenna splitters that can

1. Select which port to be feeded from.
2. Back-signal the loss of antenna to the GPS receiver.

There are double frequency GPS splitters able to do that. I will soon
get some for my private lab. I will measure one I borrowed tomorrow.

A third issue would be that passive splitters will give you loss of
signal level. Therefore is active splitters recommended.

Just using a transmission line style Wilkinson power splitter alone is
not recommended. The isolation effect between ports is terrible. Using
three-port couplers allows for the ports reflection to be loaded into a
separate resistor. I think I recall seeing that on the splitters that I
have openend up. Thus, it is fairly trivial to isolate the splitter.

The Agilent L1 splitters uses resistive power-dividers, which has better
isolation at the cost of higher loss, but then they include an amplifier
to overcome that, but the amplification is at the input side.

If you require isolation between ports, it can be designed in and if you
get the devices I have seen, it should be fair isolation.

Similarly, the power require similar isolational effects, and again
commercial devices addresses these issues.

When in doubt, measure the effect. Network analyzers for 1.2-1.7 GHz
isn't hard to come by these days. I'll toy around a little with the one
at work.

The joy of using multiple antennas is one thing, but if you have a good
antenna up, then good cabling in combination with good splitters should
work well.

My 2 öre contribution to the discussion.

Cheers,
Magnus

On 10/09/2012 09:27 PM, John Ackermann N8UR wrote:

This is a good paper. I've read it before. It presents three strategies
for GPS amplifiers:

1. Wide-band amplifier, represented by the AOA Wideband amplifier
2. Narrow-band amplifier with peaks, represented by the AOA narrow band
   amplifier
3. Narrow-band amplifier with no peaks, represented by the KW microwave
   phase-stable narrow band amplifier.

The wide-band amplifier has around 4 ns group delay, and it is fairly
flat and stable. Since there isn't much delay to start with, it doesn't
change a whole lot either. Since the amplifier isn't very flat, it also
has some variations in group delay. It's fairly natural. The downside is
that it has no suppression of interference, so we should do some damping.

The second case tries to achieve just that, but in order to create steep
slopes around the pass-band, they have used two resonances, one on each
side of the pass-band. You see the peaking effect on the gain curve of
figure 1, but oh... they show up clearly in the group delay measurement
of figure 2 too. This is expected from the theory, as these two
pole-pairs has fairly high Q, their group delay will show this property
in the direct vicinity of their respective resonances, just as their
contribution to gain will do. So, nice steep slopes and good
suppression, but lots of group delay, and by that higher sensitivity to
environmental effects, i.e. temperature.

The third example shows wider but much flatter amplitude response, and
essentially flat group delay. This is what you expect from maximum flat
group delay filters such as Bessel/Thompson. No wonders those are
specified as measuring filters for digital transmission. Lesser delay,
and lesser sensitivity. The downside is that the cost of steep slopes
comes from a higher number of needed poles/zeros.

Just as I expect from traditional signal theory.
Again, you get what you pay for.

Now you know why I want a network analyzer reaching this area at home.

Cheers,
Magnus

Dear Friends,

Thank you all for all your replies and points of view regarding this thread. I was unable to reply to you individually as I got too many answers in a very short time. StilI had the chance during the day to summarize all your information and presented what is more than representative of the fine work of this enthusiastic and knowledgeable discussion group. Time Nuts is now well known at CENAM (National Metrology Center). All of today's round table participants were pleasantly surprised by all the replies to this topic. In the end the round table discussion was way better than yesterday and for your information it was set democratically as follows:

a. The use of GPS splitters along the transmission lines is a practice avoided in high end metrology labs. They rather keep things simple and predictive. Including elements in the GPS signal chain is undesirable as they represent complex variables and as the traditional definition of it, they are outside the control of the experimenter.

b. If the characteristics of the splitters are known in deep, there is no reason why they should be avoided in general. The use of splitters is generally a matter of convenience, and as such they should be taken in account for their impact when doing precision work.

c. The decision of using splitters should be considered in a case by case basis. It is as important as deciding the quality and type of transmission line and the associated RF connectors.

d. As a side benefit, it gives food for thought for the amateur time and frequency community and their experimentation (this part was included by me as a side comment).

I toured the time and frequency labs at CENAM today. If it is appropriate,  I could upload the pictures I took to an image hosting server, showing a little bit of Mexico's time scale UTC (CNM) and the current work with Cesium fountain and frequency comb clocks. My apologies beforehand if my offer could be a little of topic and probably way off the general interest of the group. Please kindly advise.

I wish you all a nice evening and a pleasant Wednesday. Thank you!

Regards,

Edgardo Molina
Dirección IPTEL

www.iptel.net.mx

T : 55 55 55202444
M : 04455 20501854

Piensa en Bits SA de CV

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On Oct 9, 2012, at 2:39 PM, Magnus Danielson magnus@rubidium.dyndns.org wrote: