[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

> 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,

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