[time-nuts] GPS SDR (was: FE-.5680A trimming resolution)

Attila Kinali attila at kinali.ch
Wed Feb 1 18:12:26 UTC 2012


On Wed, 1 Feb 2012 09:27:30 -0800
Chris Albertson <albertson.chris at gmail.com> wrote:

> I thought it might be interresting but then found out you need to buy
> $2,000+ worth of hardware for even start experimenting.    Open Source
> SDR needs to run on a common affordable platform or it will never gain
> the critical mass of users that it take to make the project live
> longer then a few months.

That's because the URSP is a general purpose system. It is designed
to do many things. That makes it expensive. And being expensive,
it has a low production volume, which makes it even more expensive.

I think, a specialized GPS SDR can be build for less than 500 USD
in low (a dozen at max) volumes.

I guestimate, that the RF/ADC part would cost somewhere between
100 to 200 USD in parts. The big uncertainty here is the FPGA.
I have no clue how much logic space for a GPS SDR would be needed
at minimum and how much would be desirable. Hence i have no guess
what the FPGA would cost (could be anything from a cheap 20USD
FPGA to a 300 USD one).

> I think the way to go is to find a commercial GPS chip that has a low
> level interface and then build the uP controller using a common
> development system.   Both the chip and the uP board need to be,
> common, well documented and cheap.

There are no common, well documented and cheap GPS frontend chips
out there. All chips that are still in production are for high volume
stuff. Without knowing someone inside those companies, you will not
be able to get them at single pieces. I searched quite a while some
time ago, and couldn't find anything that is not EOL. Finally i came
to the conclusion that it is easier to build a custom frontend from
scratch, from the available HF parts.

>   Then with this you build an open
> source thunderbolt type device.     An SDR that samples the microwave
> RF is going to be un-affordable, even mixing and down converting
> microwaves is not so simple as doing the same on HF ham bands.  But
> there might be low level GPS chip available for cheap.

It might not be as simple as doing in the HF ham bands (which anyone
who does GHz electronics considers as DC anyways ;-), but it's possible.
Today we have so much electronic that works in the 2.4GHz band that
we have many devices at our disposal. Yes, working with them requires
more than just a bread board and a few wires. You have to design a PCB
(correctly!) and have to have the equipment to solder and test it.

And this is where the real difficulty lies:
The components on the RF side will be all SMD, often in nasty cases
like QFN. Considering that most people do not dare to solder a SOIC
with it's wide 1.27mm pitch, much less TSOP (0.63mm) or QFP (0.5mm),
how would you design a device that can be build by a normal hobbist?

If you say that home soldering is not an option, you have to start
producing them in batches of >100. Anything else will be just too
expensive (think NRE).

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



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