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Re: [Discuss-gnuradio] USRP DDC usage

From: Achilleas Anastasopoulos
Subject: Re: [Discuss-gnuradio] USRP DDC usage
Date: Thu, 26 May 2005 16:10:46 -0400
User-agent: Mozilla Thunderbird 1.0.2 (Windows/20050317)

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

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...

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

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