Running rpitx on the Raspberry Pi 4

Tuesday November 16 2021
radio c++ sbc

The program rpitx is a piece of software that uses some of the hardware already in the Raspberry Pi CPU to generate radio frequency signals on the GPIO4 pin of the Raspberry Pi. I've used this before on earlier Raspberry Pi products. In late 2020 support was added for the Raspberry Pi 4.

Changes to make rpitx work on the Raspberry Pi 4

The Raspberry Pi 4 uses the Broadcom 2711 chip, which is different than prior Broadcom chips used. So the librpitx project has been updated to take this into account. The peripheral base address of the BCM2708 is retrieved using a library function now called bcm_host_get_peripheral_address.

    //Channel DMA is now hardcoded according to Raspi Model (DMA 7 for Pi4, DMA 14 for others)
    uint32_t BCM2708_PERI_BASE =bcm_host_get_peripheral_address();

Github link

This winds up getting checked against a value 0xFE000000 to determine if the device is a Raspberry Pi 4. Interestingly if the value returned is zero the code now also assumes that it is running on Raspberry Pi 4. So presumably prior versions of the board have some other peripheral base address. There is a member variable called pi_is_2711 that is set to true on the Raspberry Pi 4.

    if(BCM2708_PERI_BASE==0xFE000000) // Fixme , could be inspect without this hardcoded value
    {
               dbg_printf(0,"RPi4 GPIO detected\n");
        pi_is_2711=true;  //Rpi4
        XOSC_FREQUENCY=54000000;
    }
    if(BCM2708_PERI_BASE==0)
    {
        dbg_printf(0,"Unknown peripheral base, switch to PI4 \n");
        BCM2708_PERI_BASE=0xfe000000;
        XOSC_FREQUENCY=54000000;
        pi_is_2711=true;  
    }

The value pi_is_2711 is checked when doing frequency calculations in functions like clkgpio::SetFrequency and clkgpio::GetMasterFrac

Setup and installing rpitx

To run rpitx, I put Raspberry Pi OS Lite on a microSD card. Then I enabled ssh by creating the file /boot/ssh in the configuration.

You can download the latest version of rpitx from GitHub. Once you have it downloaded you can run the install.sh script to build and copy the files to the appropriate locations. It also automatically modifies /boot/config.txt to have the following lines

gpu_freq=250
force_turbo=1

These change system behavior at startup time, so you'll need to reboot your Raspberry Pi after running install.sh. You can verify these changes took effect by using vcgencmd get_config to make sure they are set.

raspberrypi:~ $ sudo vcgencmd get_config gpu_freq
gpu_freq=250
raspberrypi:~ $ sudo vcgencmd get_config force_turbo
force_turbo=1

Power supply

I first tried running rpitx on the Raspberry Pi 4 with this power supply, which I thought would be sufficient.

It crashed most of the times I tried something, so I switched to using a CanaKit 3.5A USB-C power supply.

Antenna

Although you can technically run rpitx without any additional hardware it isn't very useful

The pin right beside GPIO4 is a ground, so the combination of the two pins acts like a very simple dipole antenna.

On the frequencies below the 2 meter band I had to add some additional wire to make the antenna more efficient

If you know the frequency you're going to operate rpitx on you can use a dipole antenna calculator to figure out the ideal lengths of the wires.

To verify the pi was actually transmitting I used my handheld Yaesu FT-60R with the original antenna. For the frequencies below 2 meter I used my Yaesu FT-450D, the antenna was just about 6 ft of wire strung across my desk.

Test results

Disclaimer: Unlicensed RF emissions are generally illegal in most of the world. In my case I have a license from the FCC or used an ISM band to run most of my tests. You must comply with all laws in your jurisdiction.

Frequency Modulation

These tests were performed using the testnfm.sh script. This transmits narrow band FM modulation that is equivalent to what amateur radio handie talkies use.

Parameter	Operating Frequency	Observed Center Frequency	Notes
434.0e3	434.0 MHz	434.01 MHz	OK
146.400e3	146.4 MHz	146.4 MHz	OK
108e3	108.0 MHz	N/A	pi locked up
222.5e3	222.5 MHz	222.5 MHz	OK
927.1e3	927.1 MHz	927.1 MHz	OK, weak signal
915e3	915.0 MHz	915.0 MHz	OK, weak signal
52.53e3	52.54 MHz	52.540 MHz	OK with but strange popping in audio
29.600e3	29.6 MHz	29.6 MHz	Great quality audio and strong signal

Single Side Band

These tests were performed using the testssb.sh script. This transmits upper side band.

Parameter	Operating Frequency	Observed Frequency	Notes
52.54e6	52.54 MHz	52.54035 MHz	audible, but with ghostly noises in the background
40.68e6	40.68 MHz	40.67830 MHz	audible, but with ghostly noises in the background
28.333e6	28.333 MHz	28.33290 MHz	OK
24.925e6	24.925 MHz	24.92510 MHz	OK
1.95e6	1.95 MHz	1.950 MHz	good quality
473e3	475 kHz	N/A	pi locked up

Reception range

Since I had everything setup, I was curious to see what kind of transmission distance I could get out of this simple setup. I set rpitx to transmit on 448.2 MHz FM. I used by my Baofeng UV-3R handheld as the receiver. The range was around 308 meters. That is long enough to be a useful transmitter.