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Re: [Discuss-gnuradio] USRP with Basic_RX

From: Josh Blum
Subject: Re: [Discuss-gnuradio] USRP with Basic_RX
Date: Tue, 01 Jun 2010 08:38:14 -0700
User-agent: Mozilla/5.0 (X11; U; Linux i686; en-US; rv: Gecko/20100423 Thunderbird/3.0.4

You are seeing the lower bound clipping in the nlog10 block. Try looking at it with the scope sink, you should see some bits twiddle in the wind. -Josh

On 05/31/2010 03:03 PM, Marcus D. Leech wrote:
I'm trying to "carve off" about 250Khz of bandwidth around 38.5MHz,
(actually any frequency
   between about 25MHz and 40MHz) using a Basic_RX and a USRP2.

In GRC, I created a simple flow-graph with a USRP2 source, and an FFT sink.

I set the tuning parameter of the source to 38.5MHz, and what I get on
the FFT is a flat-line at
   -410dB!  Now, granted, I don't have the inputs connected to anything
yet, but I would expect there
   to be *some* noise, and -410dB seems, well, like bad physics :-)

Cribbing from hfx2.py in gnuradio/*examples*/python/apps  it seems that
this should "just work".

Am I missing something?  I thought that the FPGA would "do the right
thing" in the case of the Basic_RX,
   and use the DDC to pull the "target" frequency down around DC, etc, etc.

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