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Re: [Discuss-gnuradio] Frequency dependent problem after syncronizing ,

From: Johnathan Corgan
Subject: Re: [Discuss-gnuradio] Frequency dependent problem after syncronizing , Please help!
Date: Thu, 12 Feb 2009 10:33:16 -0800

On Thu, Feb 12, 2009 at 5:43 AM, Bruhtesfa Ebrahim <address@hidden> wrote:

> I syncronized my USRPs(one is v4.3 and the other v4.5) by using the USRP
> clocking notes on http://www.gnuradio.org/trac/wiki/USRPClockingNotes,
> to make them frequency and phase syncronized.


> ...Then,I run usrp_fft.py on the master USRP.I
> see that the master USRP receives the 5GHz transmission from signal
> generator without problem, but when i change the center frequency to
> 4.96GHz to receive from the slave, I got a weird signal.The signal is
> not stable, the noise(whole spectrum) level decreases and increases very
> fast and it has only a very small peak at the center frequency.Moreover,
> the spectrum changes very much with gain.)

It sounds like, since you have perfect synchronization between
transmitter and receiver, that what you have after downconversion in
the daughterboard is DC + noise.  That is, the transmitted carrier
(plus noise and multipath from the channel) is being mixed with a
synchronized local oscillator, resulting in only the noise components
being left in the complex baseband signal GNU Radio is receiving.
There will also be a DC component based on the path delay and
wavelength causing a phase shift between the transmitted carrier phase
and and the receiver LO phase.


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