[time-nuts] GPS simulator,

[Bob Camp]

Hi

My guess is the 1 bit DAC is nothing more than the FPGA output pin. About all you really need is enough of a buffer to stay ahead of the USB bus. That may not be much with a modern USB chip set and a hand made driver on the PC.

Bob

Sent from my iPhone

On Nov 26, 2010, at 10:34 AM, jimlux <jimlux at earthlink.net> wrote:

> Luis Cupido wrote:
>> Hi,
>> The fact they refer that atmospheric s/n degradation and dropouts etc being replayed precisely lead me to think this is just a spectrum rec/play machine (no mod/demod of any kind)
>> like the ham-oriented "time machine" but for GPS.
>> http://www.expandedspectrumsystems.com/prod2.html
>> Luis Cupido.
>> ct1dmk.
> 
> 
> But they mention software to generate sequences.  It's actually a fairly clever idea.  It's easy to stream 16.3xx Mbps from a modern computer, so you could generate an arbitarily complex signal off line..
> 
> 
> For a lot of applications this would be great.  A nice test source that you could replay on command.
> 
> The only question I'd have is whether the 1 bit DAC can adequately represent the real-life complex signal.  I suppose, intuitively, since your receiver is digitizing it with a 1 bit ADC...
> 
> And, I have a question whether the 1 bit DAC can produce a high enough fidelity waveform for a higher performance receiver that uses a multibit digitizer.
> 
> Since the clock you are recording and playing back with isn't necessarily as good as the one on the satellite, you probably couldn't use this for testing your precision receiver.  It would be like introducing a random propagation variation or coax length variation in the system.
> 
> It is intriguing though.. a homebrew version could be quite simple.. you can get the USB interface and a small FPGA pretty easily.  One of those 10.23 MHz crystals from Rick might be handy...
> 
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