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

[Rob Kimberley]

>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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