I am new at gnuradio companion and USRP and I am a little bit confused with UHD:USRP Sink block. I am trying to build an FMCW radar in USRP N210 with CBX40 daughterboard. I want to generate a sweep from 2GHz to 3GHz. First, I’ve generated the sweep in baseband which I want to upconvert to my desired frequency range. My grc diagram is the following:

enter image description here

At the output of the Hilbert block I can see the baseband frequency sweep. However, I connect my USRP to an Spectrum Analyzer and I only see a tone at 2.2GHz (the Center Frequency I set in the UHD:USRP Sink block).

I expected the frequency at the output of my usrp to be the sum of the baseband frequency and the center frequency.

Is there anything I am misunderstanding? Should I modify any of the settings?

Thank you in advance,


1 Answer 1


You've got a few very concerning conceptual and technical problems there, many of which will be adressed by the console output, which you should probably read!

  • You can only sweep within your nyquist bandwidth. That is at most 25 MS/s (at 16 bit sample depth) or 50 MS/s (at 8 bit sample depth) for the N210
  • You want a single-tone sweep from negative baseband to positive baseband frequencies. So, generate a complex oscillation from the start; don't use a real-valued oscillator and a Hilbert transformer.
  • 200 kHz is far too low a sampling rate. Try 2 or 20 MS/s. Again, this defines the bandwidth you can sweep in. (read the console)
  • The 30.72 MHz master clock rate is impossible for an N210 – leave that field blank, you'll get 100 MHz (the only possible rate). (read the console)
  • 0 gain: Practically zero output
  • The bandwidth of the CBX is fixed at 40 MHz, your bandwidth setting will be ignored(read the console)

I'll allow myself the assessment that you're not really aware of what complex baseband is, and haven't really realized how bandwidth of a system relates to sample rate. I'm afraid that's something you should really read up on if you're doing SDR.

  • $\begingroup$ Thank you for your response Marcus. $\endgroup$
    – Fati
    Sep 24, 2020 at 14:41
  • $\begingroup$ I modified the question above, so that you can see the new diagram. $\endgroup$
    – Fati
    Sep 24, 2020 at 15:03
  • 1
    $\begingroup$ That's really a new question and should be asked in a new post! $\endgroup$ Sep 24, 2020 at 15:12
  • $\begingroup$ Hi Marcus, it sounds like your answer is based on the previous image? I see that @Fati did change some things. $\endgroup$ Sep 24, 2020 at 19:26
  • 1
    $\begingroup$ @MikeWaters The answer is consistent with the current state of the question. $\endgroup$ Sep 24, 2020 at 19:57

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