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

Timeok timeok at timeok.it
Tue Oct 9 03:58:54 EDT 2012


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.



> 	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.

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.


> 	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

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.
>
> 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.

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

> 	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?

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




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