[time-nuts] GPS SDR

[Attila Kinali]

On Thu, 02 Feb 2012 07:49:53 -0800
Jim Lux <jimlux at earthlink.net> wrote:

[Limiting / Downmixing converter]

> Yes, if you need lots o'bits, but a single bit sampler with wide 
> bandwidth is easy (which is why they do it).  It's basically a D-latch 
> at the end of the amplifier/limiter chain.

Yes, but you lose IIRC about 3dB of performance compared to a 2bit ADC.

> 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.

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.

> 40MHz gives you a sample bandwidth of 20 MHz, so you could probably 
> sample slower, but I think having more samples/chip makes the tracking 
> easier (if nothing else, oversampling is like having more bits in your ADC)

Yes.

[Antennas]
> > Crossed dipole are narrow band and not easy to build as dual band designs
> > at least at those frequencies. Quad helix needs quite a precision to get
> > the right frequency and dual band designs (stacked helixes) get even more
> > difficult.
> 
> I suspect that you're right.. the actual antenna may be simple, the 
> design is hard.  The antennas we use for multiband look like a crossed 
> dipole on the surface of a hemisphere, but the actual elements are a 
> very odd shape: generally a wide strip, but there are some lumps and 
> bumps in the outline.

I thought about combining an antenna simulator with a genetic algo
to see whether it produces any usable shapes. But i havent had time
for this yet (and it's actually way down in my priority list).

> I'm going to guess that they were designed with some FEM code, and then 
> iterated by hand.  If you knew the shapes, it would be pretty easy to 
> build, though: copper foil tape on an appropriate substrate.  As you 
> note, precision is important.

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'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.


		Attila Kinali
-- 
Why does it take years to find the answers to
the questions one should have asked long ago?