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Re: [Discuss-gnuradio] Fwd: Problem in uhd_fft

From: Marcus D. Leech
Subject: Re: [Discuss-gnuradio] Fwd: Problem in uhd_fft
Date: Fri, 05 Jul 2013 08:42:17 -0400
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On 07/04/2013 11:51 PM, Karan Talasila wrote:
Hi Josh,
              can you explain how and why the frequency translation occurs? secondly when the same basic tx chip is used on a  usrp N210, the translation occurs at 50Mhz and after. So for USRP N210, the entire range from 0-250 Mhz is represented by alternating between -50Mhz to +50Mhz. Why is it that way?

The BASIC_RX and LF_RX have no downconversion hardware in them at all.  They are basically just "buffers" for the ADCs.  The BASIC_RX has an
  analog reponse that starts to fall off at about 250Mhz, which is why it's rated to 250Mhz.

In the USRP1/B100/E1XX systems, the sampling clock is at 64MHz by default.  That means the first aliases start to show up at the 1st Nyquist
  frequency of 32Mhz (half of sample rate).   Similarly in the N2XX, which a 100MHz sampling clock, those aliases start to show up at 50Mhz.

My suggestion would be to look into so-called bandpass sampling, and also, importantly, Nyquist Sampling Theorem.

Marcus Leech
Principal Investigator
Shirleys Bay Radio Astronomy Consortium

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