On 2/2/12 1:28 AM, Attila Kinali wrote:
On Wed, 01 Feb 2012 21:53:16 -0800
Jim Lux<[email protected]> wrote:
You don't need the ADC: you just need a limiter/comparator.
Yes, but this degrades sensitivity quite a lot.
You don't need insane sampling rates. Think in terms of subharmonic
sampling.
This requires that you have an ADC that has the anlog bandwidth of
the signal. And ADCs with a analog BW in the GHz range are damn expensive
and hard to get.
Also a problem is to get the sampling frequency right if you want to
sample more than one band. Downmixing solves both of these problems
at the cost of higher complexity and a bit more noise.
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.
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. A few minutes work with an Excel spreadsheet trying frequencies
will probably find something that works: You want the carrier to alias
to about a quarter of the sample rate (so the entire signal is in the
sample bandwidth without aliasing), but not exactly in the center
(because having some known frequency offset means your Doppler tracking
doesn't have to go through zero)
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)
Is there a publically-available antenna design that's easy to
fabricate, has a stable phase center, covers 1100--1600 MHz, and has a
good pattern over this band with low cross-polarization? Even a large
choke-ring design would be okay if it's fully specified.
I think there are some crossed dipole designs around. What about quad
helix?
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'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.
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.
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