[time-nuts] GPS active antenna delay ?

[Jim Lux]

On 2/8/15 2:11 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.

One has to be "very" careful about reading group delay specs on wideband 
devices. Sometimes, the group delay (or its flatness/deviations from a 
straight line) is measured ONLY over the frequency band of interest, 
which might not be the filter passband.

You could have wild fluctuations of phase vs frequency somewhere, but as 
long as dphase/dfreq is constant in the desired area, the 
filter/amplifier meets spec.





>> 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).
>

There might be a wideband (500MHz) filter in front of the LNA, and then 
separate narrow band filters for each of the three frequencies.  The 
wideband filter could be LC or coupled microstripline equivalents.