# Questions about calibration examples in UHD utility

**URL:** <https://www.ruby-forum.com/t/questions-about-calibration-examples-in-uhd-utility/225758>\
**Category:** GNU Radio\
**Created:** [January 28, 2013, 9:23pm UTC](https://www.ruby-forum.com/t/questions-about-calibration-examples-in-uhd-utility/225758 "2013-01-28T21:23:14Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![SSkonishi\_takurou\_S](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/bb73d2/32.png) [@SSkonishi\_takurou\_S](https://www.ruby-forum.com/u/SSkonishi_takurou_S)\
**Post date:** [January 28, 2013, 9:23pm UTC](https://www.ruby-forum.com/t/questions-about-calibration-examples-in-uhd-utility/225758/1 "2013-01-28T21:23:14Z")

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Hi, guys,

I am reading the codes for uhd\_cal\_rx\_iq\_balance.cpp.  
When doing calibration by setting the RX and TX antenna as “CAL”, is  
the signal from TX directly switched to RX?  
And I noticed that in the function set\_optimum\_defaults(), the tx\_gain  
is 0 but the rx\_gain is 25! Is that too high? I heard that when  
directly connecting TX and RX, we should use attenuators. So why does  
it set 25 rx\_gain? Thanks.

Best,  
Gang

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**Author:** ![figo2476](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/f/71e660/32.png) [@figo2476](https://www.ruby-forum.com/u/figo2476)\
**Post date:** [January 28, 2013, 10:16pm UTC](https://www.ruby-forum.com/t/questions-about-calibration-examples-in-uhd-utility/225758/2 "2013-01-28T22:16:24Z")

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Hi,

Calibration uses full duplex and should only be done without anything  
plugged into the ‘tx/rx’ or ‘rx2’ ports. There is no direct connection,  
just measurement of the residual rf escaping.

Mike  
M0MIK

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**Author:** ![figo2476](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/f/71e660/32.png) [@figo2476](https://www.ruby-forum.com/u/figo2476)\
**Post date:** [January 31, 2013, 4:48pm UTC](https://www.ruby-forum.com/t/questions-about-calibration-examples-in-uhd-utility/225758/3 "2013-01-31T16:48:58Z")

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Hi, Mike,  
Thanks very much for your reply. I also searched online about the  
usrp calibration. I found a post and some information:

“During cal, the TX is switched away from the TX antenna into the  
“closed” switch port, where it leaks into the RX which is listening to  
the TX/RX antenna. So termination on RX2 should have no effect as that  
port is already unused in calibration.”

Here is the link:  
[http://lists.ettus.com/pipermail/usrp-users\_lists.ettus.com/2012-April/004028.html](http://lists.ettus.com/pipermail/usrp-users_lists.ettus.com/2012-April/004028.html).

I am a little confused. Since calibration uses full duplex, why is  
there no direct connection between RX2 and TX/RX? I thought they are  
connected inside the usrp during calibration.

Best,  
Gang