Raspberry Pi bareback LF/MF/HF/VHF CW (Morse code) transmitter

Makes a very simple Morse Code transmitter from your RasberryPi by connecting
GPIO port 4 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.

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Installation / update:
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  Simple instructions (see BUILD file for more information):
    sudo apt-get install git
    git clone https://github.com/JamesP6000/PiCW.git
    cd PiCW
    make
  Note that compiling takes about 60 seconds on the RPi v1!

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

  Uninstall:
    sudo make uninstall

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Example usage:
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  Brief help screen
    ./PiCW --help

  Send the Morse code message "TEST DE N9NNN" on carrier frequency 10.140 MHz
  using the default rate of 20 WPM:
    sudo ./PiCW --freq 10.140e6 TEST DE N9NNN

  Transmit an endless series of dits at 60 WPM. Can be used to measure the
  worst case frequency domain performance of the transmitter.
    sudo ./PiCW --freq 10.140e6 --ditdit --wpm 60

  Transmit a continuous tone at 10.140 MHz
    sudo ./PiCW --freq 10.140e6 --test-tone

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"PiCW --help" output:
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  Usage:
    PiCW [options] "text to send in Morse code"

  Options:
    -h --help
      Print out this help screen.
    -f --freq f
      Specify the frequency to be used for the transmission.
    -w --wpm w
      Specify the transmission speed in Words Per Minute (default 20 WPM).
    -p --ppm ppm
      Known PPM correction to 19.2MHz RPi nominal crystal frequency.
    -s --self-calibration
      Call NTP periodically to obtain the PPM error of the crystal (default).
    -n --no-self-cal
      Do not use NTP to correct frequency error of RPi crystal.
    -d --ditdit
      Transmit an endless series of dits. Can be used to measure TX spectrum.
    -t --test-tone
      Continuously transmit a test tone at the requested frequency.

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Radio licensing / RF:
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  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 a ground
  pin of your Raspberry Pi, then 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.qrp-labs.com/LPF
    http://www.qrp-labs.com/ULPF
    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 100mW (+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
  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 to
  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.

  In transmitting Morse code, the CW carrier must be turned on and off.
  To avoid abruptly turning the carrier on and off, the drive strength
  of the GPIO pin used for transmission is gradually increased from 0 to
  16ma with the idea being that this will reduce the frequency spurs created.
  Specifically, the current is increased up to maximum current by following
  the shape of a raised cosine curve.

  Furthermore, a random amount of time domain jitter is added to the turn on
  and turn off ramps to again reduce the spurs created by the turn on/ off
  transients.

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Calibration:
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  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 HIGHLY recommended to ensure that your
  transmissions lie within the CW band you are targetting.

  NTP calibration:
  NTP automatically tracks and calculates a PPM frequency correction. If your
  Pi is connected to the internet and you are running NTP, you can use the
  --self-calibration option to have PiCW periodically querry NTP for the latest
  frequency correction. Some residual frequency error may still be present
  due to delays in the NTP measurement loop. 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 you 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 WSPR 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 command line.

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PWM Peripheral:
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  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 transmission will
  interfere with CW transmissions. Sound can be permanently disabled
  by editing /etc/modules and commenting out the snd-bcm2835 device.

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Reference documentation:
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  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

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History/Credits:
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  Credits go to Oliver Mattos and Oskar Weigl who implemented PiFM based on
  the idea of exploiting RPi DPLL as FM transmitter.
  http://www.icrobotics.co.uk/wiki/index.php/Turning_the_Raspberry_Pi_Into_an_FM_Transmitter

  Dan MD1CLV combined this effort with WSPR encoding algorithm from F8CHK,
  resulting in WsprryPi a WSPR beacon for LF and MF bands.
  https://github.com/DanAnkers/WsprryPi

  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.
  https://github.com/threeme3/WsprryPi

  James Peroulas <james@peroulas.com> AB0JP added several command line options,
  a makefile, improved frequency generation precision, and a self calibration
  feature where the code attempts to derrive frequency calibration information
  from an installed NTP deamon.
  https://github.com/JamesP6000/WsprryPi

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

  James Peroulas <james@peroulas.com> AB0JP created PiCW.
  https://github.com/JamesP6000/PiCW

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