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JANVIER, Thibault, 12/15/2015 01:46 AM
2.2. Measurements regarding the useful signal and approximation of the signal-to-noise ratio
To carry out measurements on the desired carrier, the USRP has to be reconfigured. To do so, the program needs to stop running. Then, the selection of the tabs “USRP Configuration” and “Power Spectrum Signal” in the “Analog” thumb index will allow configuring the USRP and displaying the results.
The USRP down-converts the incoming signal according to the local oscillator frequency that is indicated as an input by the control command “carrier frequency”. Then it samples the down-converted signal at a sample rate given by the control command “IQ sampling rate”. To display the power spectrum of the entire channel, a down-conversion with a local oscillator frequency of 1,6 GHz has been chosen. Then the sampling rate has been set to 40 MHz to have a span of 20 MHz on the displayed power spectrum (from 1,6 GHz to 1,62 GHz). While the program is running, it is possible to configure in real time the settings of the power spectrum of the signal. The value “-1” that is set for the resolution bandwidth is a default value that chooses automatically the best resolution bandwidth possible taking into account the other parameters. The results are shown below.
Figure 11: Power Spectrum Density displayed on the user interface of the SDR signal analyser
Figure 12: Interface to change the settings of the power spectrum that is displayed
Figure 13: Power Spectrum Density displayed by the Rohde&Schwarz Signal Analyser
From Figure 11, we can notice that the actual resolution bandwidth of the SDR signal analyser has been set to 100 kHz. At equal resolution bandwidth, we can see that the power spectrum of the SDR signal analyser matches the one of the Rohde and Schwarz signal analyser. Thus, the total bandwidth of the channel measured from the SDR signal analyser matches the theoretical one and the one measured from the Rohde&Schwarz signal analyser.