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[Discuss-gnuradio] Offset Problem with UHD and USRP LFTX

From: Tom Hendrick
Subject: [Discuss-gnuradio] Offset Problem with UHD and USRP LFTX
Date: Fri, 26 Oct 2012 11:21:29 -0700 (PDT)

Hello all,

I have an older version of gnuradio (pre 3.6.2) on one of my laptops.  When I run a simple script I made in GRC which has a USRP source block and a WAV file sink block it seems to work fine.  With nothing connected on the LFRX RF connectors, I get a signal with basically 0 amplitude with 0dB and 10dB gain.  This version uses the USRP source block.

On my other laptop I have the newest version of GnuRadio.  I made the same exact script using the UHD source block in GRC.  When I run this script with the same exact hardware and nothing connected on the RF connector, I get a DC offset in the recorded signal.  For 0dB gain, I get basically 0 amplitude, but for 10dB gain I get about -0.05, and for 15dB gain I get about -0.15.  This is on a +1/-1 scale.

The settings on the USRP source block and the UHD source block in the newer GRC version are the same.  The difference is there are a couple of extra parameters in the new UHD block that I just leave as default (for instance the bandwidth, etc).  Is there a reason I am seeing this offset with the newer version which uses the UHD block?  I'm wondering if I need to specify additional parameters in order to make it act like the USRP source block from the older version?

Thank you,

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