Re: [Discuss-gnuradio] Three different USRP2 nodes are transmitting with almost exactly 1 MHz frequency offset

[Jason Abele]

On Wed, Feb 08, 2012 at 06:39:24PM -0500, Nazmul Islam wrote:
> Hello,
> 
> I am running the benchmark_tx.py codes and looking at the spectrum of the
> signals using uhd_fft.py. I am using the latest image of GNU radio
> (GNUradio 3.5) and I have XCVR2450 daughterboards. I ran the
> benchmark_tx.py code in three transmitter nodes and surprisingly, all of
> them are transmitting with 1 MHz frequency offset! I have attached two
> screenshots with the email (I hope that they go through). I give the
> following input parameters to run the benchmark_tx code.
> 
> ./benchmark_tx.py -f 2.4G -r 1M -m gmsk -M 10 -a "addr = 192.168.10.2"
> 
> Both uhd_fft.py and the spectrum analyzer of my laboratory show that the
> received signal is centered at 2.401 GHz. I varied the frequency to 2.45
> GHz, 2.41 GHz, but this 1 MHz frequency shift persists.
> 
> When I run the benchmark_rx.py code at the receiver nodes, they don't
> receive/detect any packets (due to the frequency offset, I guess). I even
> tried to run the transmitter at 2.4 GHz and the receiver at 2.401 GHz.
> However, that did not help either!
> 
> I will try to modify the control loop gain parameters using Tom's blogs
> suggestions and see if that helps. However, I am really surprised to see
> how all three different transmitter nodes can transmit with almost exactly
> 1 MHz frequency offset. Any suggestion will be appreciated.
> 

Can you tell us which version of UHD you are using? 
(uhd_usrp_probe --version)

We have heard reports of such an issue and my best guess is that it was
related to an accidental swap of I and Q in the XCVR2450 transmitter
code.  This went in after the 3.3.2 release and is fixed on latest UHD
master since 837437c65ce36d418cceb3df5b093f9497b3af5f

Jason