# Uhd\_fft.py and USRP N210

**URL:** <https://www.ruby-forum.com/t/uhd-fft-py-and-usrp-n210/207656>\
**Category:** GNU Radio\
**Created:** [May 31, 2011, 2:37am UTC](https://www.ruby-forum.com/t/uhd-fft-py-and-usrp-n210/207656 "2011-05-31T02:37:34Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![luislavena](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/l/0ea827/32.png) [@luislavena](https://www.ruby-forum.com/u/luislavena)\
**Post date:** [May 31, 2011, 2:37am UTC](https://www.ruby-forum.com/t/uhd-fft-py-and-usrp-n210/207656/1 "2011-05-31T02:37:34Z")

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Hi,

I was running uhd\_fft.py on USRP N210 with TVRX2 daughter-board. I get  
mysterious peak at every 100 MHz center frequency i.e. 100MHz, 200 MHz,  
even though no antenna is connected. Please help me what is wrong.

Thanks you,  
Rahul

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**Author:** ![rahul\_sol](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/r/9dc877/32.png) [@rahul\_sol](https://www.ruby-forum.com/u/rahul_sol)\
**Post date:** [May 31, 2011, 2:44am UTC](https://www.ruby-forum.com/t/uhd-fft-py-and-usrp-n210/207656/2 "2011-05-31T02:44:21Z")

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> Hi,
> 
> I was running uhd\_fft.py on USRP N210 with TVRX2 daughter-board. I get  
> mysterious peak at every 100 MHz center frequency i.e. 100MHz, 200  
> MHz, even though no antenna is connected. Please help me what is wrong.
> 
> Thanks you,  
> Rahul

Two things:

There is almost always an anomaly near DC, so it’s often best to tune  
your target frequency so  
that it’s away from DC in the baseband. So, let’s say your signal of  
interest is at exactly  
500MHz (just to use the example from problem.png), tune 50KHz away,  
500.050MHz, so that  
DC appears outside your passband.

The clock oscilator on the USRP2/N2XXX is 100MHz, so there will be small  
spurs of this clock  
appearing, but which are usually easily drowned out once you put an  
antenna on the input.

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**Author:** ![rahul\_sol](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/r/9dc877/32.png) [@rahul\_sol](https://www.ruby-forum.com/u/rahul_sol)\
**Post date:** [June 3, 2011, 8:15am UTC](https://www.ruby-forum.com/t/uhd-fft-py-and-usrp-n210/207656/3 "2011-06-03T08:15:35Z")

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On 06/02/2011 03:06 AM, rahul sol wrote:

> got peak at other frequencies, which didn’t happen with XCVR.
> 
> Moreover, SBX is also giving weird results. Although the network  
> analyzer is transmitting at 952 MHz, I got peaks at 948 & 952 MHz.  
> It’s like mirror image around the center frequency. I tried in all  
> possible band, in both TVRX2 & SBX but the results are same.
> 
> Thank you,  
> Rahul

Rahul:

Your TVRX2 plot looks very much like you’re driving it into the clipping  
region, which will produce  
massive amounts of harmonic content. Try turning down the gain, and  
reducing the output of your  
signal generator.

For the SBX, I’m not sure what might be going on there, but again, you  
may be running into a clipping  
issue.