October 04, 2015

Simple FM Receiver with GNU Radio and RTL-SDR

Introduction


I recently started playing around with software defined radio using a USB TV tuner dongle utilising the popular RTL2832U chipset. After playing around with software like CubicSDR and gqrx I was somewhat frustrated at the opaqueness of what is going on under the hood. As such I resolved to learn GNU Radio so I can do the signal processing myself. Starting with a basic FM receiver seems like a good idea, since one of my goals is to receive NOAA APT transmissions, which are FM modulated at 137 MHz.

The Radio


The image below is the FM radio (source code) I built in GNU Radio Companion. GNU Radio Companion is part of GNU Radio that makes it pretty easy to graphically put together a custom signal processing chain and "make" a radio in software. I am not usually a big fan of graphical programming, but in this particular instance I have to admit the going was a lot easier than if I had to do this textually. There is built in support for documenting each block, which is nice, though I wish I had more control over the font used and the size of the text boxes.

FM radio created in GNU Radio Companion.

It looks more complicated than it is. Basically, the output from the receiver, which is centred around the central frequency, is down-sampled from 2.4 MHz to 500 KHz, then low-pass filtered and then passed to a FM demodulator module. The output of the FM demodulator is then resampled to 48 KHz and piped to an audio sink, i.e. a sound card.

All the extra stuff is are GUI controls and visualisations to aid in understanding and debugging.

This is what it looks like running, tuned a local station at 99.1 MHz, which is BBC Radio 1.

Radio in operation. Top-left: FFT of the signal from the receiver; Top-right: resampled signal; Bottom: Low-pass filtered signal.
There is some rudimentary control over GUI position and size, but it is otherwise brutally utilitarian. However the resulting python file can be modified to fine tune appearances and such I believe. Nonetheless, it is very useful for figuring out where you went wrong.

Next Step

The current goal is to make a NOAA APT specific radio and keep trying for a clean image.

Cheers,
Steve