SOURCEFORGE » RALF » 2015 Quantitative analysis of RF signals

3. Moving from simulation to acquisition of real signals¶

To replace the generated signal by a real signal received by an antenna, we added the USRP. We first had a lot of problems to know how to set it well so we were not able to observe a good constellation.
In fact, when we receive signals with the USRP they are first down-converted to baseband, then filtered by a low-pass filter of 20 MHz-bandwidth and finally sampled.
The block we are using to compute the power spectrum calculates it between 0 and Fs/2. But because of the filtering, it is useless to compute the power spectrum over a range higher than 20
MHz. Thus, the maximum sampling rate will be set to 40 MHz. In order to display the power spectrum of the signal at the input of the USRP, a frequency shift is added to the power spectrum just computed to compensate the down-conversion. The most complicated is to find a signal with a low symbol rate because of this maximum sampling rate.

For the analog part, we need to measure the power of the signal and the power of the noise with two different carrier frequencies of the USRP. The problem is that we can’t do these two computations at the same time. So we first need to launch the program with a carrier frequency corresponding to the noise (being in the tab “Power Spectrum Noise”) and after we launch it again with the carrier frequency of the signal (being in the tab “Power Spectrum Signal”).