[time-nuts] GPS active antenna delay ?

Magnus Danielson magnus at rubidium.dyndns.org
Sun Feb 8 12:12:28 UTC 2015


Attila,

On 02/08/2015 11:11 AM, Attila Kinali wrote:
> 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.

I've seen a few different approaches.

>> 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?

Indeed.

LC filters should avoid too high Q in pass-band, but should have a bunch 
of zeros a bit further out to punch out the stop-bands properly.
SAW-filters should similarly avoid high Q notches, but there it is easy 
to achieve higher degree systems such that you achieve the filtering 
without going to high-Q systems.

>> 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).

Works great unless you have a radio amateur doing L-band (23 cm) 
transmissions. Here in Sweden you can transmit 1 kW in that band, just a 
handfull of MHz from L2.

Cheers,
Magnus



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