On 2/2/12 9:39 AM, Attila Kinali wrote:
On Thu, 02 Feb 2012 07:49:53 -0800
Jim Lux<[email protected]> wrote:
[
There is a sampling rate around 38-39 MHz that works out nicely for all
three bands (actually, any rate in that range probably works..I haven't
looked).. It helps that the 3 GPS frequencies are related to a common
base.
Only if you sample them seperately. Which requires seperate, sharp
filters for all of them. Also something that isn't that easy to do.
The filters don't have to be all that sharp. What you typically do is a
chain of amp/filter/amp/filter/amp/filter, etc, for about 6 stages.
I'll ask around about the filters, but I suspect they're a pretty
standard ceramic thing (it's a bit high frequency to be a SAW), and
since GPS frequencies are "standard" it's likely to be a "catalog part".
Also do not forget that Galileo E1 signals have about a 20MHz Bandwidth.
The combined E5 frequencies have about 50MHz. I think i've read somewhere
that you can get away with 8MHz for the E1 signal. Don't know how
the E5 behaves if you limit its bandwith.
yes, that might be tricky
[Antennas]
That's why i said that probably a patch antenna build out of PCBs
is the best solution. You can get the copper sheet at 0.1mm precision
which would define frequency and polarity properties quite well.
The only thing that would have to be done by hand would be the distance
from the ground plate. I guestimate that this value is not as critical
and that 0.5mm variation should be ok.
I've seen dual band patches that were pretty simple. One was air
dielectric, so the interplate spacing was set mostly by the spacers.
I'd go hunting through patents assigned to Dorne& Margolin. (part of
EDO, these days, I think). Or even maybe looking at their datasheets.
There's also what they call the "helibowl" antenna which is some form of
helix in a bowl shaped reflector/ground plane. googling that might turn
up something.
From my understanding of antenna theory (which is very little),
these are mostly variations on the directivity characteristic
(ie to get a more favorable distribution), but do not change
much the frequency characteristics. Ie if you don't have the
frequency characteristics right with a straight design, there
wont be much chance to get them right with a "shaped" design.
True in some designs.. however, in general "fat" elements have wider
bandwidth. Adding oddball protrusions and notches can flatten out a
response quite nicely.
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