Hello Radio: How to Listen to FM Radio Using LimeSDR

LimeSDR is an open source software defined radio which can support a continuous frequency range from 100KHz to 3.8GHz. Many wireless systems such as 2/3/4G, Wifi and FM radio are covered in this frequency range, which makes LimeSDR a fairly useful tool.

To test a software defined radio, usually the first experiment to run is to listen to FM radio. FM radio is the equivalent of Hello World in the universe of the software defined radio. Before starting to describe how to run FM radio test, we have a list of HW/SW needed:

1. LimeSDR board with antenna connected

2. FM transmitter which pairs with a smart phone

3. Lime Suite GUI (version 20.10.0)

4. GNU Radio (version 3.7.11)

The test is done using Ubuntu 18.04.4 LTS.

Step 1: LimeSDR configuration file generation

In the test, we will listen to a FM radio channel set at 89.0MHz. Therefore, LimeSDR needs to generate a tone at 89.0MHz, multiply this tone with the input radio signal, and down-convert the ratio signal to the baseband for further processing. To generate this configuration, in Lime Suite GUI -> SXR, Freq needs to be set as 89.0MHz, then calculate and tune. By pressing the "Save" button at the top left of the GUI, the configuration file will be save as fm_receiver_89Mhz.ini for future usage.

Step 2: GNU Radio set up

GNU Radio is a popular tool for software define radio. To make GNU Radio work together with LimeSDR, one needs to install both GNU Radio and gr-limesdr, a LimeSDR library for GNU Radio. After that, FM_receiver.grc file from /gr-limesdr/examples folder can be loaded into GNU Radio. A few changes need to be made before starting to run it. First, device serial #, .ini file and RF Frequency need to be modified in the LimeSuite Source (RX) module. Device serial # is the serial number for the LimeSDR device and it can be read from Lime Suite GUI; The .ini LimeSDR configuration file has been generated in step 1; RF freq is to be set as 89.0MHz. Baseband Freq in the WX GUI FFT Sink module also needs to be updated to 89.0MHz. Then by pressing the green triangle "Execute the flow graph" button, GNU Radio starts to run. 

Step 3: Run Test

However, you often only hear noise even if the frequency tuned to is a valid local radio channel. The issue is likely due to antenna setting. A rule of thumb is that the antenna size should be in similar dimension as the wavelength of the wireless signal received. For FM radio at 89.0MHz frequency, the wavelength is 300,000,000/89,000,000 = 3.37 meter. But the antenna bought together with LimeSDR is only 4.5 cm long. The antenna size is too small for FM radio. Based on the antenna length, it is designed for multi-GHz freq signal instead of for FM radio which ranges from 88MHz to 108MHz. There are at least two ways to fix it. The first way is to use a different antenna which is much longer. The second way is to move LimeSDR closer, much closer, to the radio transmitter or the other way around. We use the second way. We bought a FM transmitter which can be plugged into the smart phone and tune it into 89.MHz. Then by putting FW transmitter almost right next to the receiver antenna (shown in the second photo below, the first photo shows the my LimeSDR/antenna and their Lego housing), we can now hear the music. 

Task accomplished!