Controlling the transceiver and transferring data

PlutoSDR runs Linux. The device drivers allowing you to control the transceiver and capture samples are therefore part of to the Linux Industrial IO (IIO) framework. IIO is a kernel subsystem for analog to digital or digital to analog converters and related hardware. IIO communicates with user space via sysfs and a character devices. From a plain user interaction point of view this is quite intuitive, since everything is just a file. However when controlling the device from software, this can be quite painful, since you simple want to call a function or method, instead of doing string manipulation and file IO. LibIIO fills this gap, provides all sorts of device abstraction and handles all kind of IIO internals. LibIIO is cross platform and also provides different language bindings, so that you can control IIO devices from C, C++, C# or Python.

If you’re not familiar with IIO, please start reading here:

• Libiio

• About libiio

• IIO Linux Kernel Documentation sysfs-bus-iio-*

• IIO Documentation

• IIO High Speed

• Video from FOSDEM of how IIO is used in SDR applications

LibIIO - A Library for Interfacing with Linux IIO Devices - Dan Nechita, Analog Devices Inc

Video

LibIIO - A Library for Interfacing with Linux IIO Devices - Dan Nechita, Analog Devices Inc

https://www.youtube.com/watch?v=p_VntEwUe24

Controlling the transceiver

The code snippet below is a minimalistic example without error checking. It shows how to control the AD936x transceiver via a remote connection.

1. Create IIO IP Network context. Instead of ip:xxx.xxx.xxx.xxx it’ll also accept usb:XX.XX.X

2. Get the AD936x PHY device structure

3. Set the TX LO frequency (see AD9361 device driver documentation)

4. Set RX baseband rate

#include <iio.h>

int main (int argc, char **argv)
{
    struct iio_context *ctx;
    struct iio_device *phy;

    ctx = iio_create_context_from_uri("ip:192.168.2.1");

    phy = iio_context_find_device(ctx, "ad9361-phy");

    iio_channel_attr_write_longlong(
        iio_device_find_channel(phy, "altvoltage0", true),
        "frequency",
        2400000000); /* RX LO frequency 2.4GHz */

    iio_channel_attr_write_longlong(
        iio_device_find_channel(phy, "voltage0", false),
        "sampling_frequency",
        5000000); /* RX baseband rate 5 MSPS */

    receive(ctx);

    iio_context_destroy(ctx);

    return 0;
}

Receiving data

1. Get the RX capture device structure

2. Get the IQ input channels

3. Enable I and Q channel

4. Create the RX buffer

5. Fill the buffer

6. Process samples

int receive(struct iio_context *ctx)
{
    struct iio_device *dev;
    struct iio_channel *rx0_i, *rx0_q;
    struct iio_buffer *rxbuf;

    dev = iio_context_find_device(ctx, "cf-ad9361-lpc");

    rx0_i = iio_device_find_channel(dev, "voltage0", 0);
    rx0_q = iio_device_find_channel(dev, "voltage1", 0);

    iio_channel_enable(rx0_i);
    iio_channel_enable(rx0_q);

    rxbuf = iio_device_create_buffer(dev, 4096, false);
    if (!rxbuf) {
        perror("Could not create RX buffer");
        shutdown();
    }

    while (true) {
        void *p_dat, *p_end, *t_dat;
        ptrdiff_t p_inc;

        iio_buffer_refill(rxbuf);

        p_inc = iio_buffer_step(rxbuf);
        p_end = iio_buffer_end(rxbuf);

        for (p_dat = iio_buffer_first(rxbuf, rx0_i); p_dat < p_end; p_dat += p_inc, t_dat += p_inc) {
            const int16_t i = ((int16_t*)p_dat)[0]; // Real (I)
            const int16_t q = ((int16_t*)p_dat)[1]; // Imag (Q)

            /* Process here */

        }
    }

    iio_buffer_destroy(rxbuf);

}