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HowTo: observe the FM band with an AD9361-equipped board

About FM radio

FM band:

  • In Europe and Africa: 87.5 MHz to 107.9 MHz
  • In America: 87.9 MHz to 107.9 MHz
  • In Japan: 76.0 MHz to 90 MHz

FM channels:

  • either 200 kHz wide, with a frequency deviation limited to 150 kHz total,
  • or 100 kHz wide, with a frequency deviation limited to 75 KHz total.

The sysfs interface

It is possible to configure the AD9361 chip directly from sysfs. The interesting files are located in /sys/bus/iio/devices/iio:deviceX where X is the number of the device whose name is ad9361-phy:

root@linaro-ubuntu-desktop:~# cat /sys/bus/iio/devices/iio:device1/name

This directory contains several files, some of which can be used to read raw values from the hardware, while others are used to configure the ad9361 chip.

root@linaro-ubuntu-desktop:~# ls /sys/bus/iio/devices/iio:device1
calib_mode                              in_voltage_rf_dc_offset_tracking_en
calib_mode_available                    in_voltage_rf_port_select_available
dcxo_tune_coarse                        in_voltage_sampling_frequency
dcxo_tune_fine                          name
dev                                     out_altvoltage0_RX_LO_frequency
ensm_mode                               out_altvoltage1_TX_LO_frequency
ensm_mode_available                     out_voltage0_hardwaregain
filter_fir_config                       out_voltage0_rf_port_select
in_out_voltage_filter_fir_en            out_voltage1_hardwaregain
in_temp0_input                          out_voltage1_rf_port_select
in_voltage0_gain_control_mode           out_voltage2_raw
in_voltage0_hardwaregain                out_voltage2_scale
in_voltage0_rf_port_select              out_voltage3_raw
in_voltage0_rssi                        out_voltage3_scale
in_voltage1_gain_control_mode           out_voltage_filter_fir_en
in_voltage1_hardwaregain                out_voltage_rf_bandwidth
in_voltage1_rf_port_select              out_voltage_rf_port_select_available
in_voltage1_rssi                        out_voltage_sampling_frequency
in_voltage2_offset                      power
in_voltage2_raw                         rx_path_rates
in_voltage2_scale                       subsystem
in_voltage_bb_dc_offset_tracking_en     trx_rate_governor
in_voltage_filter_fir_en                trx_rate_governor_available
in_voltage_gain_control_mode_available  tx_path_rates
in_voltage_quadrature_tracking_en       uevent

The program we will use, IIO Oscilloscope, use the sysfs interface to configure the ad9361, and use the special character device /dev/iio:device1 to read the raw values.

Equipment used

This test has been realized with a Zed board, coupled with a FMCOMMS2 module.

Monitoring the FM band

We will monitor the most used FM band, from 87.5 MHz to 107.9 MHz. The IIO Oscilloscope can be configured like this:

  • RX LO Frequency: the base frequency at which the hardware will be tuned. Set to 87.4 MHz (and not 87.5, since 87.5 is the middle of a 200 KHz channel);
  • RF Baudwidth: the size of the band you want to capture. Here, we want at least a band of 107.9 - 87.5 = 20.4 MHz
  • Sampling rate: the number of samples you want to capture (in millions per second). Here we set this to 22, so that we get a resolution of approximatively one hertz.

Lanch a capture, and you will be able to see peaks in the frequency domain:

In this screenshot, it is clearly possible to identify several channels: at 95.5 MHz, 96.3 MHz, 97.5 MHz, 98.4 MHz among other frequencies.


To verify if those frequencies actually correspond to something, we can use the iio_fm_radio tool. But first, it is necessary to change the gain control:

echo fast_attack > /sys/bus/iio/devices/iio:device1/in_voltage1_gain_control_mode

Then, the iio_fm_radio tool will playback the desired channel:

iio_fm_radio 95.5
resources/tools-software/fm-radio.1413603901.txt.gz · Last modified: 18 Oct 2014 05:45 by Jie Yan