# Mirrored spectrum around tuning freq

**URL:** <https://www.ruby-forum.com/t/mirrored-spectrum-around-tuning-freq/242953>\
**Category:** GNU Radio\
**Created:** [August 10, 2015, 6:25pm UTC](https://www.ruby-forum.com/t/mirrored-spectrum-around-tuning-freq/242953 "2015-08-10T18:25:36Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![Rafa\_F](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/r/958977/32.png) [@Rafa\_F](https://www.ruby-forum.com/u/Rafa_F)\
**Post date:** [August 10, 2015, 6:25pm UTC](https://www.ruby-forum.com/t/mirrored-spectrum-around-tuning-freq/242953/1 "2015-08-10T18:25:36Z")

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Hi all,

why when I use:  
uhd::tune\_request\_t tr(tuningFreq);  
USRP\_DEVICE-\>set\_rx\_freq(tr);

I get a spectrum that is “mirrored” around the tuninFreq ?  
See: [http://picpaste.com/sp-zJquS0qK.png](http://picpaste.com/sp-zJquS0qK.png)  
with tuningFreq = 149.8 MHz, input signal at F = 150 MHz (from signal  
generator)

If I use a LO offset different from zero the mirror effect disappear.

Why ?

Thanks,  
Luca

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**Author:** ![lucapette](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/l/58f4c7/32.png) [@lucapette](https://www.ruby-forum.com/u/lucapette)\
**Post date:** [August 10, 2015, 6:32pm UTC](https://www.ruby-forum.com/t/mirrored-spectrum-around-tuning-freq/242953/2 "2015-08-10T18:32:37Z")

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My first thought was “IQ imbalance”, but it’s pretty strong. Which USRP  
and daughterboard do you have?

M

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<div class="post-metadata">

**Author:** ![lucapette](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/l/58f4c7/32.png) [@lucapette](https://www.ruby-forum.com/u/lucapette)\
**Post date:** [August 10, 2015, 6:40pm UTC](https://www.ruby-forum.com/t/mirrored-spectrum-around-tuning-freq/242953/3 "2015-08-10T18:40:13Z")

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Could be intermodulation due to driving the RX amplifier into  
nonlinearity;  
Luca, what gain are you using? Can you further reduce it? Can you reduce  
the amplitude of your test signal?

Best regards,  
Marcus