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

From: Bruhtesfa Ebrahim
Subject: [Discuss-gnuradio] Re: Frequency dependent problem after syncronizing , Please help!
Date: Thu, 12 Feb 2009 22:20:08 +0100

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

> 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.
> Johnathan

Thank you Johnathan!

What makes it strange is the dependence on gain.when the linear gain in 
the spectrum panel(usrp_fft.py)is set to 0,the signal at the center(the 
DC)is 5dB higher than the noise level.When I increase the gain, the 
difference between the DC and noise level keeps decreasing and finally 
the DC level is equal to the noise level.
Specifically, for a linear gain >=20, the signal around DC is not 
visible(it is equal to the noise level).

is this gain dependence normal?

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