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

Normand Martel martelno at yahoo.com
Tue Mar 13 01:51:57 UTC 2007


One is much better to take the filter's delay in
account rather than NOT USING a filter!!! i can hardly
imagine a GPS receiver/antenna without any form of
preselection, and, unfortunately, they're many of
those filterless units on the market!

73 de Normand VE2UM
Montreal, Qc. Canada
--- John Ackermann N8UR <jra at febo.com> wrote:

> 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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