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[Discuss-gnuradio] unexpected aliasing tx output

From: Anisha Gorur
Subject: [Discuss-gnuradio] unexpected aliasing tx output
Date: Tue, 16 Oct 2012 15:04:01 -0400

Hello All,
I am trying to transmit a random sequence from one USRP N210 using the basic TX daughterboard at 40Mhz IF to an external rf frontend. When I look at the output of the basic TX on a spectrum analyzer (before any external filtering, etc), I see a very weird copy at 20MHz. I would expect to see one at 60MHz, which you do, however the copy at 20MHz is very strange. This doesnt happen when I use tx_waveforms.cpp, where I just see a spike at whatever frequency I'm transmitting at. To transmit random numbers, I'm am essentially using exactly the same tx_streamer setup as tx_waveforms, the only major difference is here:
       for (size_t n = 0; n < buff_tx.size(); ++n){             

buff_tx[n] = std::complex<float>((float)rand()/(float)RAND_MAX, (float)rand()/(float)RAND_MAX);//pilot_source(n);
//fill buffer with random complex floats
I just use a random generator to fill the tx buffer. Everything else is the same as tx_waveforms. I use set_tx_rate(2e6), though I get similar results with 4e6, 5e6 and 6.25e6. 
Is there any reason that this copy is appearing? 
Thank You,

Anisha Gorur
Class of 2012
Electrical Engineering

Attachment: randomoutput_40mhz.jpg
Description: JPEG image

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