[time-nuts] GPS SDR

[Jim Lux]

On 2/2/12 9:39 AM, Attila Kinali wrote:
> On Thu, 02 Feb 2012 07:49:53 -0800
> Jim Lux<jimlux at earthlink.net>  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.