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Re: [Discuss-gnuradio] LO of the USRP2 motherboard

From: Marcus D. Leech
Subject: Re: [Discuss-gnuradio] LO of the USRP2 motherboard
Date: Wed, 10 Nov 2010 02:39:39 -0500
User-agent: Mozilla/5.0 (X11; U; Linux i686; en-US; rv: Gecko/20101027 Fedora/3.0.10-1.fc12 Thunderbird/3.0.10

On 11/10/2010 02:20 AM, Jorge Miguel wrote:
> Hi,
> I have an easy question.
> When I use my basic T/Rx daughterboards I can tune the receiver to an
> specific frequency. Since the basic T/Rx doesn't have any LO, I would
> like to know where is physically that LO in the USRP2 motherboard. I
> cannot find anything in the schematics. Where is it? Is that LO the
> one which translates from IF (coming from the daughterboards) to real
> base band?
> Thanks in advance,
> Jorge
There is no "LO", per-se on the USRP2 motherboard, only the various
digital clocks used by
 parts on the motherboard, including the ADCs/DACs, and the FPGA.

When you're using a BASIC_RX/LF_RX and you "tune", the FPGA uses a DDC
(Digital Downconverter)
  to digitally downconvert your desired frequency range to baseband.  On
the USRP2, the
  BASIC_RX or LF_RX is sampled at 100MHz, so the FPGA "sees" from
roughly DC to roughly
  50MHz.  Within that range, the FPGA DDC converts whatever you ask it
to into a complex
  baseband, which is what the host computer sees.

Principal Investigator
Shirleys Bay Radio Astronomy Consortium

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