Attila,
On 02/08/2015 11:11 AM, Attila Kinali wrote:
On Sat, 7 Feb 2015 10:07:44 -0800
Tom McDermott <[email protected]> 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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