[time-nuts] Delay through GPS antenna splitter/amplifier -- an answer, and a question

Rob Kimberley rk at timing-consultants.com
Mon Mar 12 13:29:38 EDT 2007


I used to be bothered when I was working on this stuff full time, especially
when we were testing equipment in labs for customers on a "mission". Now, as
long as all my clocks are on the same second.....

:-)

Funny story however.. Was at NPL on Thursday for our bi-annual Time &
Frequency Club meeting. There was a small exhibition running and one of the
companies - Time & Frequency Solutions had a natty new T&F analyser - nice
box - touch screen etc. The sales guy wanted to demo it and show me how it
checked the delay on a sample piece of cable (with BNCs and a BNC T on the
end). He asked me to guess the delay. I looked at it, did a mental
calculation based on the assumed length and the connectors and told him 6nS.
He put it on the machine, and guess what....6nS. He said I'd been watching
and knew the result! Had to explain that I'd been around the T&F "block" a
few times and that sometimes experience (and an inspired guess!) count more
than hardware!!

Cheers

Rob

-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of John Ackermann N8UR
Sent: 12 March 2007 16:04
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] Delay through GPS antenna splitter/amplifier -- an
answer, and a question

But we're time-nuts... we DO worry about those things. :-)

While we were at it with the network analyzer, we did FDR (frequency domain
reflectometry) to measure the cable delay to the antenna, and I spent
yesterday making up six matched cables to go from the splitter to the
receivers -- they all test within about 1 nanosecond of each other.

Obsessive compulsive?  Me?

John
----

Rob Kimberley wrote:
> From my experience, your position and hence derived time is based on 
> the antenna centre. Cable, splitter, connector, and antenna filter 
> delays all need to be taken into account when looking at  very accurate
"nanosecond"
> timing applications.
> 
> For most applications in the microsecond or tens of microsecond region 
> it isn't worth worrying about.
> 
> Rob K
> 
> 
> ----- Original Message -----
> From: "John Ackermann N8UR" <jra at febo.com>
> To: "Discussion of precise time and frequency measurement" 
> <time-nuts at febo.com>
> Sent: Monday, March 12, 2007 3:03 PM
> Subject: [time-nuts] Delay through GPS antenna splitter/amplifier -- 
> an answer, and a question
> 
> 
>> I had a chance recently to look at the performance of the two-port 
>> and eight-port HP GPS antenna splitters on a super-duper network
analyzer.
>> Screenshots of the results are at
>> http://www.febo.com/time-freq/pages/gps-splitter.
>>
>> In short, the minimum delay (at the center of the passband) from 
>> antenna port to output port is around 15 nanoseconds for the eight 
>> way unit, and about 22 nanoseconds for the two way one.  The delay 
>> seems consistent on all the ports, with less than 1 nanosecond variation.
>>
>> However, there is also a hump in the delay near the edges of the 
>> passband, about 12 MHz above and below the center.  The delay at the 
>> edges increases by perhaps 5 nanoseconds, though depending on the 
>> port, it's not always symmetrical.
>>
>> So, an interesting question for any of you *real* GPS experts is what 
>> effect a variation in group delay of the RF input has on the timing 
>> solution?  Is the true "length" of the amp/splitter some average of 
>> the delay across the passband, or, given the spread spectrum nature 
>> of the signal, does it not really matter?  In fact, is the "length" 
>> of the splitter even related to the measured group delay?
>>
>> This also raises the issue that any GPS antenna that has RF filtering 
>> is likely to have similar delays; I've never seen that sort of data 
>> published.
>>
>> John
>>
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> 
> 
> 
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