Decoding FM radio Signals using Software Defined Radio

I am an RF enthusiaist, I was a signals and systems engineer working with RF testing. I have worked in this field for a while and I recently purchased a Software Defined Radio to play with. This is one of my first pet projects that I am taking on. My setup is Ettus usrp B210 radio with 2 receive ports and 2 trancseive ports. I will be using a Vert900 antenna to capture the radio waves. The software tools used is GNU radio. This tool has has signal processing blocks that can be used to encode or decode RF signals. The processing blocks can be customised using python.

Setup

GNU radio DSP block

First of all we need to include the source of our signal. In this case, the signal is generated by the USRP so we need to include the USRP Source block. The radio signal is modulated with a high-frequency signal, with the carrier signal, and each radio station uses a different carrier, so we need to tune the USRP in the corresponding carrier frequency. To do that we need to configure the Center Frequency, in my case I want to hear the radio station that uses the carrier 92.99 MHz. Keep this in mind because now we have to configure the sampling frequency, and it is only 5 MHz.Then the output signal sampled at 5 MHz will be passed through a low pass filter. Audio signals have a spectrum of 20 Hz to 20 kHz, so we can attenuate all the frequencies above 20 kHz. In this case, I have configured a cut frequency of 100 kHz, with a sampling rate of 5 MHz. Also, I have used this filter to decimate the signal, that is, divide the number of samples by n. In this case, the decimation factor is 20, so the output rate will be 5 MHz divided by 20, which is 250 kHz. Next, we need to include the Wave Band FM demodulator, with a quadrature rate that is the same as the sampling frequency, 250 kHz. Finally, the sound card of most computers can manage signals with a rate of 48 kHz, so we need to resample the signal to pass from 250 kHz to 48 kHz. This block performs the operation input_samples * Interpolation / Decimation, which has as a result 96 kHz. The output of this block will be connected to the Audio sink block, and the FM receiver is complete.

Results

The next figure shows the frequency spectrum of the output signal. That signal contains the sound that the speakers will reproduce.