[USRP-users] uhd_cal_tx_iq_balance skips wide frequency range

Urban Hakansson uhakansson at tecore.com
Tue Oct 21 09:58:06 EDT 2014


Michael, 

Thanks for your reply. 

We changed the threshold to 15 and now we get calibration data written to file for each freq step. 

Q: What is the reason the threshold is set to 30? Is there any reason I should not set it to 15 or any other value? Is there is a reason this parameter value is hard coded and is not something you can pass in as an argument? 

Regardless, the original problem I wanted to solve by calibrating in finer frequency steps still remains, namely, the residual IQ imbalance is too large in certain bands. 

I calibrated the entire N210+SBX frequency range from 400 to 4400 MHz in 200kHz steps, thinking that should do it. 

To check for any residual IQ imbalance I then generated a complex exponential with amplitude 0.1 and frequency f = 1.25 MHz, sampled at Fs = 6.25 MHz, and set tx gain to 0dB (using GnuRadio flowdiagram). 

I varied the center frequency fc (LO) in 100MHz steps from 700 MHz to 3000 MHz, measured the power-ratio between fc+f and the undesired replica at fc-f which is due to residual IQ imbalance. 

fc (MHz) P_(fc-f)/P_(fc+f) (dB) 
700 -40 
800 -23 
900 -17 
1000 -22 
1100 -27 
1200 -40 
1300 -12 
1400 -15 
1500 -40 
1600 -40 
1700 -21 
1800 -13 
1900 -40 
2000 to small to measure (-infinity) 
2100 -12 
2200 -14 
2300 to small to measure (-infinity) 
... ... 
3000 to small to measure (-infinity) 

I thought that by calibrating the residual IQ imbalance would result in a power ratio of at least < -40dB between the undesired replica at fc-f and the original signal at fc+f. 

Maybe my expectations are too high, or perhaps I have a bad SBX daughterboard? 

Q: What results should I realistically be able to expect using the N210 + SBX combination? 

Regards, 

Urban 

----- Original Message -----

From: "Michael West" <michael.west at ettus.com> 
To: "Urban Hakansson" <uhakansson at tecore.com> 
Cc: "USRP-users at lists.ettus.com" <usrp-users at lists.ettus.com> 
Sent: Thursday, October 9, 2014 3:03:08 PM 
Subject: Re: [USRP-users] uhd_cal_tx_iq_balance skips wide frequency range 




Hi Urban, 

If I am understanding you correctly, you are saying that there is no calibration data for those frequencies in the resulting file. That does not mean the frequencies are being skipped by the utility. That only means that a suppression of over 30 dB could not be achieved for those frequencies, so no correction data was saved. You can set a lower threshold for the minimum suppression by changing the value in the following line in each IQ calibration utility: 

if (best_suppression > 30){ //most likely valid, keep result 

Best regards, 
Michael 



On Thu, Oct 9, 2014 at 8:57 AM, Urban Hakansson via USRP-users < usrp-users at lists.ettus.com > wrote: 


Hi everybody, 

I am trying to calibrate my N210 SBX daughterboard using the three calibration scripts uhd_cal_rx_iq_balance, uhd_cal_tx_iq_balance,and uhd_cal_tx_dc_offset using different values for freq_start, freq_stop, and freq_step. 

uhd_cal_tx_iq_balance utility that is not behaving as I expected. 
(uhd_cal_rx_iq_balance and uhd_cal_tx_dc_offset give me no problems). 

If I run uhd_cal_tx_iq_balance with any other values besides the default values it will skip approximately 300MHz of frequencies from 790 MHz to ~ 1100MHz. 

Here are a few of my attempts. 

uhd_cal_tx_iq_balance --freq_start 500000000 
uhd_cal_tx_iq_balance --freq_start 600000000 
uhd_cal_tx_iq_balance --freq_start 700000000 
uhd_cal_tx_iq_balance --freq_start 785000000 
uhd_cal_tx_iq_balance --verbose --freq_step 100000 
uhd_cal_tx_iq_balance --verbose --freq_step 110000 
uhd_cal_tx_iq_balance --verbose --freq_step 225000 
uhd_cal_tx_iq_balance --verbose --freq_step 900000 
uhd_cal_tx_iq_balance --verbose --freq_step 1000000 
uhd_cal_tx_iq_balance --verbose --freq_step 1100000 
uhd_cal_tx_iq_balance --verbose --freq_step 2000000 
... 
uhd_cal_tx_iq_balance --verbose --freq_step 7300000 

uhd_cal_tx_iq_balance always stops just before 790MHz and just sits there, but does not abort, and after a really long time continues at ~ 1100MHz and continues all the way to 4370MHz without any further gaps. 

I attach the calibration scripts I am using and the three csv files for one example, uhd_cal_tx_iq_balance --verbose --freq_step 225000. 

General background information about my environment: 
Fedora 17 Linux; GNU C++ version 4.7.0 20120507 (Red Hat 4.7.0-5); Boost_104800; UHD_003.007.001-0-unknown 

N210 informationfrom uhd_usrp_probe: 
| Device: USRP2 / N-Series Device 
| _____________________________________________________ 
| / 
| | Mboard: N210r4 
| | hardware: 2577 
| | mac-addr: 00:80:2f:0a:e6:15 
| | ip-addr: 192.168.2.199 
| | subnet: 255.255.255.255 
| | gateway: 255.255.255.255 
| | gpsdo: none 
| | serial: F4A09C 
| | FW Version: 12.4 
| | FPGA Version: 10.1 

What could the cause of this behaviour be? Are there only certain combination of values that work? What do I need to do to make uhd_tx_iq_balance not hang right before 790MHz? 

Thanks for you consideration. 

Regards, 

Urban Hakansson 
Senior Software Engineer 
Tecore Networks 
Phone: +1 410 872 6315 
Fax: +1 410 872 6010 
Email: uhakansson at tecore.com 


Urban Hakansson Senior Software Engineer Tecore Networks Phone: +1 410 872 6315 Fax: +1 410 872 6010 Email: uhakansson at tecore.com 

Urban Hakansson 
Senior Software Engineer 
Tecore Networks 
Phone: +1 410 872 6315 
Fax: +1 410 872 6010 
Email: uhakansson at tecore.com 

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