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[Discuss-gnuradio] sending sine wave, but created another unwanted frequ

From: Yang Liu
Subject: [Discuss-gnuradio] sending sine wave, but created another unwanted frequency component
Date: Sun, 21 Jan 2018 12:56:13 -0500

Dear all,

In this application, I am trying to send a sine wave at a specific frequency to usrp x310:
  sine wave generator ---> usrpx310

For the sine wave generator, I use blocks.sig_source_c from gnuradio. The parameters at the transmitter are in the following:
center frequency: 1e9 (usrp tuning frequency)
sampling_rate: 5e6
waveform frequency:  -2.4193125e6 (at the boundary of this frequency band).

At the receiver side, I tuned the usrp to 1e9, and used 10e6 to sample the received data. According to the spectrum I observed, there are two frequency components, one is at -2.4193125e6, another one is around at -2.4193125e6 + 5e6 (not very sure if they are exactly symmetric). Actually, this happens when the sine wave is very close to the boundary (near -2.5e6 or 2.5e6). As I moved the waveform frequency to the center (1e9), the second frequency disappeared.  Firstly, I thought that it is the power issue, however, after I decreased power level, the second component is still there. 

According to the function (blocks.sig_source_c), what it generates is a exp(j*2*pi*f_waveform/f_s). Therefore, there should not exist any second frequency component. I feel very confused about why this can happen. 

Any thoughts about this will be greatly appreciated!


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