On 2/8/15 2:11 AM, Attila Kinali wrote:
On Sat, 7 Feb 2015 10:07:44 -0800
Tom McDermott <tom.n...@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.
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