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

[Magnus Danielson]

From: John Ackermann N8UR <jra at febo.com>
Subject: [time-nuts] Delay through GPS antenna splitter/amplifier -- an answer, and a question
Date: Mon, 12 Mar 2007 11:03:12 -0400
Message-ID: <45F56BB0.90009 at febo.com>

John,

> 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.

Hmm... I would have zoomed in closer to only have a +/- 20 MHz span or so.

I have an E5071A at work.

> 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.

As expected.

> 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?

The electrical length of the splitter needs to go into the cable delay
compensation.

The C/A signal has it's main peak at 1,57542 GHz (with doppler +/- 6 kHz) and
the first nulls is at +/- 1,023 MHz of that peak. The secondary peaks is at
+/- 2.046 MHz but much reduced in amplitude and additional peaks roll of fairly
quickly. There are some fancy plots showing this in the literature, I don't
have the numbers from the top of my head.

Most commercial receivers have a +/- 1,023 MHz window so the delay differances
outside of that is not really relevant. The flatness of their groupdelay within
that window is however an issue.

I'd expect the effect to be somewhat similar to multipath-effects.

> 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.

Indeed. You should also ask yourself this question about the input filter and
MF1, MF2 and maybe even MF3 filters of the GPS receiver. What is the net
effect of these skews?

Cheers,
Magnus