[time-nuts] Delay through GPS antenna splitter/amplifier -- an answer, and a question
John Ackermann N8UR
jra at febo.com
Mon Mar 12 12:04:06 EDT 2007
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?
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
>> 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
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