Raspberry Pi bareback LF/MF/HF/VHF FT8 transmitter

Makes a very simple FT8 tx from your RasberryPi by connecting GPIO
port to Antenna (and LPF), operates on LF, MF, HF and VHF bands from
0 to 250 MHz.

Compatible with the original Raspberry Pi, the Raspberry Pi 2/3, and
the Pi Zero.

******
Installation / update:
******
  Download and compile code:
    sudo apt-get install git
    git clone https://github.com/sehugg/Ft8Pi
    cd Ft8Pi
    make

  Install to /usr/local/bin:
    sudo make install

  Uninstall:
    sudo make uninstall

******
Usage: (ft8 --help output):
******

  If running from the console, recent versions of Jessie cause WsprryPi to
  crash when the console screen blanks. The symptom is that WsppryPi works
  for several transmissions and then crashes. The fix is to add "consoleblank=0"
  to /boot/cmdline.txt.
  JamesP6000/WsprryPi#10

  Usage:
    ft8 [options] freq message
    ft8 [options] freq -- message
    ft8 [options] freq "message"
      OR
    ft8 [options] --test-tone freq

  Options:
    -h --help
      Print out this help screen.
    -p --ppm ppm
      Known PPM correction to 19.2MHz RPi nominal crystal frequency.
    -s --self-calibration
      Check NTP before every transmission to obtain the PPM error of the
      crystal (default setting!).
    -f --free-running
      Do not use NTP to correct frequency error of RPi crystal.
    -r --repeat
      Repeatedly, and in order, transmit on all the specified command line
      freqs.
    -x --terminate <n>
      Terminate after n transmissions have been completed.
    -o --offset
      Add a random frequency offset to each transmission:
        +/- 1000 Hz
    -t --test-tone freq
      Simply output a test tone at the specified frequency. Only used
      for debugging and to verify calibration.
    -n --no-delay
      Transmit immediately, do not wait for a TX window. Used
      for testing only.

  Frequencies can be specified either as an absolute TX carrier frequency, or
  using one of the following strings. If a string is used, the transmission
  will happen in the middle of the region of the selected band.
    160m 80m 60m 40m 30m 20m 17m 15m 12m 10m 6m 4m 2m

  Transmission gaps can be created by specifying a TX frequency of 0

******
Radio licensing / RF:
******
  In order to transmit legally, a HAM Radio License is REQUIRED for running
  this experiment. The output is a square wave so a low pass filter is REQUIRED.
  Connect a low-pass filter (via decoupling C) to GPIO4 (GPCLK0) and Ground pin
  of your Raspberry Pi, connect an antenna to the LPF. The GPIO4 and GND pins
  are found on header P1 pin 7 and 9 respectively, the pin closest to P1 label
  is pin 1 and its 3rd and 4th neighbour is pin 7 and 9 respectively. See this
  link for pin layout: http://elinux.org/RPi_Low-level_peripherals
  
  Examples of low-pass filters can be found here:
    http://www.gqrp.com/harmonic_filters.pdf
  TAPR makes a very nice shield for the Raspberry Pi that is pre-assembled,
  performs the appropriate filtering for the 20m band, and also increases
  the power output to 20dBm! Just connect your antenna and you're good-to-go!
    https://www.tapr.org/kits_20M-wspr-pi.html

  The expected power output is 10mW (+10dBm) in a 50 Ohm load. This looks
  neglible, but when connected to a simple dipole antenna this may result in
  reception reports ranging up to several thousands of kilometers.

  As the Raspberry Pi does not attenuate ripple and noise components from the
  5V USB power supply, it is RECOMMENDED to use a regulated supply that has
  sufficient ripple supression. Supply ripple might be seen as mixing products
  products centered around the transmit carrier typically at 100/120Hz.

  DO NOT expose GPIO4 to voltages or currents that are above the specified
  Absolute Maximum limits. GPIO4 outputs a digital clock in 3V3 logic, with a
  maximum current of 16mA. As there is no current protection available and a DC
  component of 1.6V, DO NOT short-circuit or place a resistive (dummy) load
  straight on the GPIO4 pin, as it may draw too much current. Instead, use a
  decoupling capacitor to remove DC component when connecting the output dummy
  loads, transformers, antennas, etc. DO NOT expose GPIO4 to electro- static
  voltages or voltages exceeding the 0 to 3.3V logic range; connecting an
  antenna directly to GPIO4 may damage your RPi due to transient voltages such
  as lightning or static buildup as well as RF from other transmitters
  operating into nearby antennas. Therefore it is RECOMMENDED to add some form
  of isolation, e.g. by using a RF transformer, a simple buffer/driver/PA
  stage, two schottky small signal diodes back to back.

******
TX Timing:
******
  This software is using system time to determine the start of
  transmissions, so keep the system time synchronised within 1sec precision,
  i.e. use NTP network time synchronisation or set time manually with date
  command. A broadcast takes 15 seconds and starts at :00,:15,:30,:45.
  
  Reception reports can be viewed on https://pskreporter.info/

******
Calibration:
******
  As of 2017-02, NTP calibration is enabled by default and produces a
  frequency error of about 0.1 PPM after the Pi has temperature stabilized
  and the NTP loop has converged.

  Frequency calibration is REQUIRED to ensure that the transmission
  occurs within the narrow band. The reference crystal on your RPi might
  have an frequency error (which in addition is temp. dependent -1.3Hz/degC
  @10MHz). To calibrate, the frequency might be manually corrected on the
  command line or a PPM correction could be specified on the command line.

  NTP calibration:
  NTP automatically tracks and calculates a PPM frequency correction. If you
  are running NTP on your Pi, you can use the --self-calibration option to
  have this program querry NTP for the latest frequency correction before
  each transmission. Some residual frequency error may still be present
  due to delays in the NTP measurement loop and this method works best if your
  Pi has been on for a long time, the crystal's temperature has stabilized,
  and the NTP control loop has converged.

  AM calibration:
  A practical way to calibrate is to tune the transmitter on the same frequency
  of a medium wave AM broadcast station; keep tuning until zero beat (the
  constant audio tone disappears when the transmitter is exactly on the same
  frequency as the broadcast station), and determine the frequency difference
  with the broadcast station. This is the frequency error that can be applied
  for correction while tuning on a frequency.

  Suppose your local AM radio station is at 780kHz. Use the --test-tone option
  to produce different tones around 780kHz (eg 780100 Hz) until you can
  successfully zero beat the AM station. If the zero beat tone specified on the
  command line is F, calculate the PPM correction required as:
  ppm=(F/780000-1)*1e6 In the future, specify this value as the argument to the
  --ppm option on the comman line. You can verify that the ppm value has been
  set correction by specifying --test-tone 780000 --ppm <ppm> on the command
  line and confirming that the Pi is still zero beating the AM station.

******
PWM Peripheral:
******
  The code uses the RPi PWM peripheral to time the frequency transitions
  of the output clock. This peripheral is also used by the RPi sound system
  and hence any sound events that occur during a transmission will
  interfere with transmissions. Sound can be permanently disabled
  by editing /etc/modules and commenting out the snd-bcm2835 device.

******
Example usage:
******
  Brief help screen
    ./ft8 --help

  Transmit a constant test tone at 780 kHz.
    sudo ./ft8 --test-tone 780e3

  Call CQ for callsign N9NNN at grid square EM10 on the 20m band with a
  random frequency offset:
    sudo ./ft8 -o 20m CQ N9NNN EM10

  Reply to N9NNN with callsign Z3ZZ and signal report: (escape leading "-")
    sudo ./ft8 20m -- N9NNN Z3ZZ -12
      OR
    sudo ./ft8 20m "N9NNN Z3ZZ -12"

******
Reference documentation:
******
  http://www.raspberrypi.org/wp-content/uploads/2012/02/BCM2835-ARM-Peripherals.pdf
  http://www.scribd.com/doc/127599939/BCM2835-Audio-clocks
  http://www.scribd.com/doc/101830961/GPIO-Pads-Control2
  https://github.com/mgottschlag/vctools/blob/master/vcdb/cm.yaml
  https://www.kernel.org/doc/Documentation/vm/pagemap.txt

******
Credits:
******
  This code is based on WsprryPi [4] and adapted to FT8 by
  KO4KOP using the ft8_lib library [5].

  Credits goes to Oliver Mattos and Oskar Weigl who implemented PiFM [1]
  based on the idea of exploiting RPi DPLL as FM transmitter.

  Dan MD1CLV combined this effort with WSPR encoding algorithm from F8CHK,
  resulting in WsprryPi a WSPR beacon for LF and MF bands.

  Guido PE1NNZ <pe1nnz@amsat.org> extended this effort with DMA based PWM
  modulation of fractional divider that was part of PiFM, allowing to operate
  the WSPR beacon also on HF and VHF bands.  In addition time-synchronisation
  and double amount of power output was implemented.

  James Peroulas <james@peroulas.com> added several command line options, a
  makefile, improved frequency generation precision so as to be able to
  precisely generate a tone at a fraction of a Hz, and added a self calibration
  feature where the code attempts to derrive frequency calibration information
  from an installed NTP deamon.  Furthermore, the TX length of the WSPR symbols
  is more precise and does not vary based on system load or PWM clock
  frequency.

  Michael Tatarinov for adding a patch to get PPM info directly from the
  kernel.

  Retzler András (HA7ILM) for the massive changes that were required to
  incorporate the mailbox code so that the RPi2 and RPi3 could be supported.

  [1] PiFM code from
      http://www.icrobotics.co.uk/wiki/index.php/Turning_the_Raspberry_Pi_Into_an_FM_Transmitter
  [2] Original WSPR Pi transmitter code by Dan:
      https://github.com/DanAnkers/WsprryPi
  [3] Fork created by Guido:
      https://github.com/threeme3/WsprryPi
  [4] WsprryPi, a WSPR transmitter:
      https://github.com/JamesP6000/WsprryPi
  [5] Uses FT8 library code from Kārlis Goba:
      https://github.com/kgoba/ft8_lib