# USRP Receive problem. (20MHz sample rate)

**URL:** <https://www.ruby-forum.com/t/usrp-receive-problem-20mhz-sample-rate/237715>\
**Category:** GNU Radio\
**Created:** [November 26, 2013, 3:48pm UTC](https://www.ruby-forum.com/t/usrp-receive-problem-20mhz-sample-rate/237715 "2013-11-26T15:48:57Z")\
**Posts on this page:** 1\
**Page:** 1

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**Author:** ![musicdenotation](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/ea666f/32.png) [@musicdenotation](https://www.ruby-forum.com/u/musicdenotation)\
**Post date:** [November 26, 2013, 3:48pm UTC](https://www.ruby-forum.com/t/usrp-receive-problem-20mhz-sample-rate/237715/1 "2013-11-26T15:48:57Z")

</div>

I want to use USRP to receive real 802.11a/g wifi signal, so I set  
sample rate 20MHz and appropriate centre frequency. But frequency  
spectrum of the receiving OFDM signal is strange. (see picture:  
fft\_router2422\_20m.jpg)

Besides that, I did another experiment. using a USRP generate a 300kHz  
bandwith OFDM signal, anther receiving end also using 300kHz sample  
rate, and we can see a perfect OFDM frequency spectrum wave.(see  
picture: fft300.jpg)

Does anyone know why sample rate increase, OFDM frequency in receiving  
end turn into this shape?

USRP: N210  
Daughter board: xcvr2450