Hi everyone,
Since a few days, I started using the USRP B200 for transmitting and receiving IQ-samples to and from a spectrum analyzer/signal generator. For this purpose, I am using UHD's example functions "rx_samples_to_file" and "tx_samples_from_file". To test the RX and TX characteristics of the B200, I am generating sine-waves with specific frequencies and write/read the IQ-samples of the wave to/from binary data files.
I have noticed a difference in the effect of the sampling rate that I can tune in the UHD-functions mentioned above:
If I send the IQ-samples to the signal generator with a specific sampling rate (e.g. 20MHz) and use the B200 for RX, I need to tune the sampling rate of the USRP to twice the sampling rate of the signal generator (e.g. 40MS/s) for correct downconversion of the signal. So I am assuming the sample rate for the USRP is defined as 1 sample for I and 1 sample for Q, meaning 2 samples for 1IQ-value. Is this correct? With this setting I see the peak of the FFT-plot of the received signal at the frequency of the sine-wave that I generated (as expected).
However, for the counterpart of sending the IQ-data with the USRP (TX) to a spectum analyzer, I need to tune the sampling rate of the USRP to exactly the same sampling rate as for my analyzer (e.g. 20MHz at analyzer, 20MS/s at USRP) to observe the correct FFT-plot. If I tune the USRP sampling rate to 40MS/s (like above for RX) the peaks of my FFT-plot are at twice the frequency of the sine-wave (I receive the wave with twice the frequency).
So my question: Is the sampling rate of the UHD RX and TX functions defined differently? I tried using only the signal generator and spectrum analyzer for TX/RX and it works using the same sampling rate. So there has to be a difference in the meaning of the rate option in the UHD functions for TX and RX. Or am I missing something fundamental?
Best regards,
Felix
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