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" <[EMAIL PROTECTED]> > To: "Discussion of precise time and frequency measurement" > <[email protected]> > 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 >> >> _______________________________________________ >> time-nuts mailing list >> [email protected] >> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >> > > > > _______________________________________________ > time-nuts mailing list > [email protected] > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > > _______________________________________________ time-nuts mailing list [email protected] https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
