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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