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Re: [Discuss-gnuradio] gr-sounder tx and rx results


From: Martin Dvh
Subject: Re: [Discuss-gnuradio] gr-sounder tx and rx results
Date: Mon, 28 Jan 2008 23:21:59 +0100
User-agent: Icedove 1.5.0.14pre (X11/20071018)

Johnathan Corgan wrote:
> On 1/24/08, Dev Ramudit <address@hidden> wrote:
> 
>>         I've been trying out the gr-sounder code to see if I can use it for
>> some research. I modified it slightly and graphed the output in
>> realtime.
> 
> Let me work with you offline on this.  The code does work over the
> air, but it is probably not giving you what you expect.
> Unfortunately, I'm tied up for a day or two.
> 
I have been working with the sounder-code too.
WHat I found is that since the clocks of the USRP boards are not synchronised 
you have two problems.
The LO's of the RFX boards are not phase-coherent so you have an RF frequency 
offset between the boards of a few kHz.
This results in not getting a spike but a kind of smeared out LF-modulated wave.

This problem should be much less with basicRX/TX or LFRX/TX boards

The other problem is that the 64 MHz clocks are not synchronised.
This produces a little smearing of your results but is much less a problem.

Both problems get bigger as you lengthen the peudo-random codelength.

You could solve this by using very precise (GPS-tamed) external 64 MHz clocks 
on both sides.
You can also solve it by giving one USRP a clock-out and the other a clock-in 
and connecting them together (so they run of the same 64 MHz clock)
This requires a clock-cable between the two USRPs so this is only feasable for 
short distances.


An other solution I see is to extend the code with some PLL which recovers the 
symbol clock and uses that to compensate for both problems.

I don't know what solutions Jonathan has, but I would like to know them too.

Greetings,
Martin




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