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Re: [Discuss-gnuradio] questions about USRP2 sink block and upconversion

From: Marcus D. Leech
Subject: Re: [Discuss-gnuradio] questions about USRP2 sink block and upconversion
Date: Thu, 04 Nov 2010 01:11:39 -0400
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On 11/04/2010 01:02 AM, Steve Mcmahon wrote:
> Marcus:
> Thank you for your reply. Let me ask you another question to make sure I'm 
> understanding this correctly. Remember, I have a USRP2 board with a WBX 
> daughterboard. If I construct a flow graph where a signal source block 
> generates a 7 khz cosine at a sample rate of 200 khz (100e6/500), and if this 
> cosine is fed into a USRP2 sink block with its "frequency" parameter set to 
> "900M", then will I see a spike in the WBX output spectrum at 907 MHz, or at 
> 893 MHz, or both? (assuming perfect precision of the internal reference clock 
> on the USRP2) What if it were a sine instead of a cosine? Thanks again for 
> your help. I really appreciate your time.
> Steve McMahon
You'll see a spectral spike at 900.007Mhz.  Regardless of the phase. 
You keep making the
  "907MHz" mistake in your posts.  Remember that upconversion doesn't
"expand" the frequency
  components.  So a spectral component at 7Khz will still be at 7Khz
relative to the upconverted
  "DC" frequency.

Your complex baseband signal has spectral components that stretch from
-bandwidth/2 to
  +bandwidth/2.   If you have a component at +7KHz, it'll still be at
  when it's finished being upconverted.

Interpolation also doesn't change the relative spacing of spectral
components, only the time-domain
  resolution of the samples of those spectral components.  So your +7Khz
signal sampled at
  200KHz will still be at +7Khz when sampled at 100MHz.


Principal Investigator
Shirleys Bay Radio Astronomy Consortium

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