[time-nuts] To use or not to use transmission line splitters for GPS receivers

Bob Camp lists at rtty.us
Tue Oct 9 12:48:23 EDT 2012


Hi

If you look at the way NIST sets up one of their "time modem" installations,
they do indeed worry a lot about this sort of stuff. There's a major choke /
isolator between the antenna and the feed line. The claim is that they see
in building grunge causing trouble without it. I'm sure that will be a
variable depending on your building.

The next claim is that without the isolation and a choke antenna, there is a
possibility of multi-path issues. Since most of us do not have a choke ring
antenna the isolator may be overkill. The NIST site in Bolder is definitely
multipath challenged. They probably have some pretty good data on that.

If you are after sub ns level timing, things are a bit different than if you
are happy with tens of ns error. Few of us have an adequate survey of our
location to *really* worry about sub ns numbers. If you are one of those
lucky few that can worry about sub-ns, yes mismatch and voltage and a whole
long list of things matter. The temperature coefficient of your antenna also
gets onto that list at some point. 

Receivers are often designed with an "I'm by my self" approach. Some designs
do indeed feed crud back up towards the antenna. Digitization clocks and all
sorts of other things can be a source of these signals. Given the high gain
of the antenna preamp, they can get away with a certain degree of sloppy
design. I suspect that there are cases of GPS A not liking the spurs from
GPS B. 

For most of us, none of this matters. If we're 10 ns off, we'll never know
it. If our whole setup varies 2 or 3 ns over a day, we simply don't have the
gear to spot the problem. 

It also does not matter to most of the people who use GPSDO's in systems. I
have yet to see a surplus GPSDO arrive with a non-zero cable delay in it's
eeprom. I doubt that people have zero delay cables. Without measuring the
cable delay and compensating for it, you can easily be off 100's of ns.

Yes, you can get out your TDR and come up with a cable number to 100ps or
less. You can get a survey that's good to centimeters. You can get a good
antenna (cheap if you are lucky or patient). Do all that and more, you can
get into the sub ns range. Calculated cables based on length, estimated
location based on self survey, easy to get antennas, not going to cut it. 

As always, the answer is "it depends on what you are doing".

Bob

-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of Edgardo Molina
Sent: Monday, October 08, 2012 10:31 PM
To: Discussion of precise time and frequency measurement
Subject: [time-nuts] To use or not to use transmission line splitters forGPS
receivers

Dear Group,

Good evening. I just arrived home after the first day of conferences at the
Electrical Metrology Forum 2012 at Mexico's metrology center CENAM. I
attended several presentations of time and frequency, very interesting
indeed. At last I understood some concepts hard to land in theory. I will be
attending the whole week long and half of next week. I will have the
pleasure and honour to meet Dr. Judah Levine from NIST, who has shown a very
nice attitude towards me and accepted to talk a little bit around my thesis
in network synchronization. 

Now to the point if you kindly allow. I got involved in a round table
discussion around the use of GPS antennas for time and frequency GPS
receivers. I tried to make some points from my personal perspective. I got
resistance from the audience and the topic went hot very quickly and didn't
set at the end. Honestly I would like to share my doubts and opinions with
you in order to enhance my experience about the topic. In the end it could
also be beneficial to close this debate tomorrow while attending to CENAM's
time and frequency forum.

Facts and thoughts:

1. The time and frequency attendees at CENAM`s time and frequency forum  is
reluctant to use GPS antenna splitters for a number of reasons I couldn't
conceive. 
	1.1 They argued that cross talk could happen among ports. I doubt it
with the newer models. I have experience with HP and Symmetricom units and
they state their products cancel cross talk.
	1.2 They argued that there could be problems from the power feeding
of the antenna and mismatches at the receiver antenna port voltages. Again I
doubt it if one uses receivers in the same voltage range

2. I have been experimenting with GPS constellation coverage with different
brands of antennas. I have found different gains, different radiation
patterns and as a result different satellites in view for identical GPS
receivers.
	2.1 I have found that using a single antenna and a two port HP
splitter I get the same radiation pattern, gain and identical satellites in
view for a set of identical receivers. For comparison purposes I feel this
is an adequate scenario. 
	It is easier for me to take care of the transmission line length and
errors caused by phase differences, attenuation and delays. 
	2.2 Two identical GPS receivers each one with it's own antenna,
could eventually cause spatial diversity reception for a system of two
receivers conceptually set as one for comparison purposes. Different
satellites being tracked on
	each receiver if not connected to a common antenna. Even if antennas
and transmission lines are identical.

Question is: Am I wrong doing the above mentioned assumptions while
considering the use of GPS transmission line splitters? I which scenarios
are the splitters recommended? In which cases they are rather not to be
considered?

Better ask as to start buying more antennas or feel comfortable with my
original RF distribution design using splitters. 

Your kind comments and expert advise is always welcome. Thank you!


Best 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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