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2

In addition to the clock leakage mentioned in the other answer, a DC offset spike can also show up in the downmixed IQ if the mixer and/or the ADC aren’t perfectly balanced (exactly equal gains for both positive and negative voltage signal swings).

4

Internal interference in the radio. There's a clock ticking at that frequency in order to downmix the signal, and spurious emissions from that clock is being picked up by other circuitry. The clock is at the center frequency because it's being converted from your center frequency to a lower one. You get a spectrum around 100MHz from your SDR by this clock ...

0

In addition, the $S_{11}$ parameters of the both TX/RX antennas should be typically less than -15 dB for a better measurement in the frequency interval that you sweep. Then, you will have a better observation about how much the received signal power decreases as the operating frequency increases. Otherwise, your measurements will be affected by the ...

0

The edges of your spectrum is where the performance of the receiver is hitting the limit. there is a trade-off between flatness and total bandwidth. they could have made the bandwidth smaller to keep it nice and flat but figured, a little extra bandwidth might be nice to have. It's not like this is a spectrum analyzer.

0

Answer to part 2: The USRP SDR uses an downconverting IQ modulator before the ADCs. An error in quadrature phase or quadrature levels of the two IQ modulator clock signals or gain and delay balance of the IQ modulator(s) can result in DC artifacts in the spectrum of the IQ ADC data. The ADCs themselves can have a small DC offset in their sampling. Both of ...

-2

Answer to your first question: As far as I know, the USRP device that you use has a maximum gain of 30 dB. So, I recommend you decreasing the gain a little. Second one: As you may know, the USRP devices firstly amplify and downconvert the received signal, then sample it. The two first RF stages have a part called "DC choke (maybe RF coupling) " ...

2

Not at all. The file sink, as any sample-processing block in GNU Radio, is not aware of RF properties – all it sees are the baseband samples. So, the representation of things being centered around 0 Hz is correct; that's all the information there is in the signal. You will need some other mechanism to save such information. SigMF is one option, filenames ...

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