[time-nuts] GPS active antenna delay ?

[Attila Kinali]

On Sat, 7 Feb 2015 10:07:44 -0800
Tom McDermott <tom.n5eg at gmail.com> wrote:

> While compensating for cable delay is relatively straight forward by
> measuring the length and compensating for
> the velocity factor, a question is: how much amplifier / filter group delay
> is to be expected within the antenna itself?

The usual way is to calibrate the whole setup, including antenna, LNA,
cable and receiver. Ie. you drive to the national lab, set up your whole
system, then measure the timing difference of your GPS receiver to the
one of the lab, drive back home, and apply the correction.
 
> Looking through GPS SAW filter datasheets seems to show none with group
> delay specifications.

Not surprising. Group delay is not considered of any importance in most
RF designs.
 
> googling leads to some research papers with delays of about:
> 
> L1 - 20 MHz wide SAW filter has about 15 nsec of group delay
> L1 - 2 MHz wide SAW filter has about 65 nsec of group delay
> L1 - LC filter - can't find anything, but suspect it's probably just a few
> nanoseconds.

I would be very much interested in those papers. Could you list their titles
and authors at least?


> I'm not sure a consumer grade antenna even has a SAW filter, it may simply
> be an LC filter.

Unlikely. LC filters are not sharp enough and difficult to build reliably
at those frequencies. I would rather assume that there are no filters
at all (beside the antenna characteristics).


			Attila Kinali

-- 
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