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Re: [Discuss-gnuradio] USRP DDC usage

From: Krzysztof Kamieniecki
Subject: Re: [Discuss-gnuradio] USRP DDC usage
Date: Thu, 26 May 2005 16:33:15 -0400
User-agent: Internet Messaging Program (IMP) 3.2.2

Quoting Achilleas Anastasopoulos <address@hidden>:
> Quoting Achilleas Anastasopoulos <address@hidden>:
>  >> I have a question/observation on this issue:
>  >>
>  >> Since the DDC multiplication is complex and since the internal DDC
>  >> oscilator is noncoherent, I do not see a difference between the MUX
>  >> settings: 0x33221100 and 0xf3f2f1f0.
>  >>
>  >Is the difference between them a phase shift of 45 degrees and a gain 
>  >ratio of
>  >sqrt(2)?
> Yes about the sqrt(2), but I guess the 45 degrees are irelevant here 
> since the operation is noncoherent so we cannt resolve the phase.
> (see also comment below)
>  >> Similarly, with the setting 0x00003210 you will never be able to
>  >> separate the two channels coming from ADC0/ADC1 and ADC2/ADC3.
>  >>
>  >You can seperate ADC0/ADC1 and ADC2/ADC3 if you set the DDC frequency 
>  >to zero (I think you also have to never have set it to a non-zero 
> value >after powerup, since you can't reset the DDC phase)
> Even if you set the frequency to 0, there is a "random" initial phase 
> that is unknown, and thus you cannot separate the two channels.
> If x0(t) and x1(t) are the two inputs from ADC0 and ADC1 respect, then 
> the output is
> [x0(t)+j x1(t)] exp(j w_cordic t + phi),
> where phi is unknown...
I can't find the message, but I though that Matt Ettus stated that "phi" is zero
at powerup, but it cannot be accessed thru software.


>  >> Is this right?
>  >>
>  >> Thanks
>  >> Achilleas

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