[time-nuts] GPS active antenna delay ?

[John Ackermann N8UR]

I think that the surplus HP/Agilent GPS splitters may have an SAW 
filter.  If so, measuring the delay of one of those could yield at least 
an approximation.

I may have that data laying around; I'll do some digging.

John
----

On 02/08/2015 05:11 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.
>
>> 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
>