Forum: GNU Radio Offset Problem with UHD and USRP LFTX

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,

Tom.

> 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?
>

Hi Tom,

There was a recent change to disable automatic DC offset correction on
the LFRX board since it seemed to give a lot of folks a hard time. In
any case, I think merits a GRC options for the offset correction.

In the meantime try adding my_usrp_souce.set_auto_dc_offset(True) to the
generated python code. Does that work for you?
http://gnuradio.org/cgit/gnuradio.git/tree/gr-uhd/...

I suppose that you could do the equivalent operation (of what the FPGA
is doing) in software with a subtract block and an average block with a
nice long averaging time. :-)

-josh