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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
ad9361-phy

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
in_voltage_rf_bandwidth

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 AD-FMCOMMS3-EBZ 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.

Playback

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_play tool will playback the desired channel on the speaker of the HDMI monitor:

iio_fm_radio_play 95.5

To directly get access to the audio waveform data the iio_fm_radio tool can be used. This tool outputs the audio data (in 16-bit mono at 48kHz) on standard output from where it can be taken and further processed. E.g. the following command plays back the stream on the headphone jack of the ZED board.

iio_fm_radio 95.5 | aplay -D default:CARD=ADAU1761 -r 48000 -f S16
resources/tools-software/fm-radio.txt · Last modified: 14 Mar 2016 22:29 by rgetz