[time-nuts] Delay through GPS antenna splitter/amplifier -- an answer, and a question
John Ackermann N8UR
jra at febo.com
Mon Mar 12 11:03:12 EDT 2007
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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