[USRP-users] IQ balance calibration problem

Josh Blum josh at ettus.com
Mon Feb 20 20:16:58 EST 2012



On 02/20/2012 08:30 AM, Christophe ALEXANDRE wrote:
> Hi list,
> 
> i'm trying to use the calibration utilities with
> 
> N210, WBX, UHD 3.4.0, Ubuntu 11.04
> 
> i have troubles using :
> 
>     uhd_cal_rx_iq_balance
>     uhd_cal_tx_iq_balance
>     uhd_cal_tx_dc_offset
> 
> 1) if i use it with default options, i often get some errors
>     while running : 
> 
>     Error : RunTimeError : no control response
> 

This is a dropped control packet. I cannot say the cause. However, I
have pushed a changeset to the code with retry-on-failure for control
packets. Perhaps this can help.

>     or
> 
>     OError : Unexpected error code 0x8

This is overflow. I can think of 2 causes:

1) Do you see a buffer resize warning. Packets may be dropped if the
kernel buffer is too small.

2) Or, this is the same dropped packet issue as for control packets.
Consider trying a different Ethernet card or PC, if that may fix the
situation.


> 2) my second problem is the efficiency of this calibration.
>     i've created a QPSK test signal in order to visualize the correction
>     with a scatterplot. The result of my tests is that the costellation is from far better
>     without calibration than with it.
> 

Am am curious, which of the IQ calibrations are causing the distortion?
Can you isolate the problem to just the TX or just the RX?

How is the constellation distorted (ex: does it become trapezoidal)? Can
you attach screen captures?

Thanks for the feedback,
-Josh




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