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• 1

  Telegraph. Theory.

  If you just turn on the transmitter (generator) at a certain frequency, it will not transmit any useful information. To transmit information, the radio signal must to be modulated. There are many different kinds of modulation.

  In HAM-radio practice, the simplest mode of communication is Telegraph (CW). Telegraph is a unique type of communication. Telegraph can be considered the first digital form of communication. However, a trained person is able to decode this digital signal by ear. Now to decode the Telegraph signal can be used decoder software. But the human ear is still considered to be the most accurate instrument for receiving a Telegraph signal. And also the telegraph is one of the most "long-range" types of communication. Due to the transmission at one frequency and low transmission speed, all signal energy is concentrated on one frequency, it has a positive effect on the transmission distance. Narrowband filters can be used for reception. A Telegraph signal consists of dots and dashes. Each character in the Telegraph alphabet is a combination of dots and dashes. The most commonly used letters consist of fewer elements.

  Dots and dashes are the facts of turning on the transmitter. In the intervals between the dots and the dash, the transmitter turns off, the signal is not transmitted to the air. There are rules by which the dash must have a duration equal to three dots. The space between dots or dashes in one character must be equal to the duration of one dot. The space between the letters in one word must be three dots. The space between words must be at least seven dots.

  If we connect a short-length wire (as an antenna) to the FPGA pin and send a clock signal to this pin, we will be able to receive this signal as an unmodulated carrier frequency. Turning on or off the signal to the antenna, we will modulate the carrier. And forming the correct time intervals for dots and dashes, we will form a Telegraph radio signal.

• 2

  Telegraph signal generation

  To form time intervals in fractions of a second, we need to divide the clock signal. In my case, the frequency of the clock generator is equal to 25175000 Hz. I decided to take a 23 bit binary counter. The frequency obtained as a result will be equal to:

  the counter is 2^22 = 4194304 (steps)
  clock frequency 25175000 Hz
  result frequency 25175000 / 4194304 = 6 Hz

   The resulting frequency of 6 Hz is sufficient to form a single dot interval. Now let's try to form something very simple. For example, SOS signal: three dots, three dashes and three dots. Draw a time diagram:

  From the diagram we can see that we need 30 steps. 30(dec) = 11110(bin). That's 5 bit's. That is, we add another 5 bits to the 22 bits of the counter. And we get 27 bits.
  
  Now the logic is very simple (because complex logic we can not put in such a small FPGA): when the value of the counter is equal to 0, 2, 4, 8, 9, 10... (and forth on diagram) we are set 1, otherwise we set 0. This signal (CW) can already be output from the FPGA and controlled, for example, by flashing the led. And to modulate the radio signal, we will output the clock signal to the output only if the value of the CW signal is equal 1.

  module epm7064_test(clk, out_lf, out_rf);
  input wire clk; // clock input
  output wire out_lf; // low frequency cw output
  output wire out_rf; // modulated radio frequency cw output
  
  reg [27:0] cnt; initial cnt <= 28'd0;
  
  always @(posedge clk) cnt <= cnt + 1'b1;
  
  // 5 higth bits of the counter for generate symbols
  wire [4:0] hi_bits = cnt[26:26-4];
  
  wire cw = (hi_bits == 5'd0) ||
            (hi_bits == 5'd2) ||
            (hi_bits == 5'd4) ||
            (hi_bits == 5'd8) ||
            (hi_bits == 5'd9) ||
            (hi_bits == 5'd10) ||
            (hi_bits == 5'd12) ||
            (hi_bits == 5'd13) ||
            (hi_bits == 5'd14) ||
            (hi_bits == 5'd16) ||
            (hi_bits == 5'd17) ||
            (hi_bits == 5'd18) ||
            (hi_bits == 5'd22) ||
            (hi_bits == 5'd24) ||
            (hi_bits == 5'd26);
               
  assign out_lf = cw;
  assign out_rf = cw & clk;
  
  endmodule
  

   That's all, it's time to turn on the receiver!
  

• 3

  Receive signal from our transmitter

  Unfortunately, we cannot use a standard radio receiver to receive our transmitter. First, because of the frequency bands-the frequency of the transmitter is outside the known broadcast bands. Second, because of the type of modulation used. The broadcast radio receiver receives in the mode of amplitude modulation (AM). If you receive a Telegraph signal to the AM receiver, when the CW signal is not, the receiver will receive the air noise, and when there is a carrier, the receiver will be quiet (because the carrier is not modulated by amplitude). As a result, the AM receiver will make noise, interrupted by silence at the dots and dashes.

  What is the receiver suitable for the reception of the Telegraph? To receive a Telegraph, you need a single-band receiver. When I chose the frequency of the transmitter, I cheated a bit. I have a USB SDR receiver, the lower limit of reception is 24 MHz, so the frequency of the generator I chose taking into account that the signal can be taken to this receiver. This receiver can be bought on ebay/aliexpress is very cheap. Search for words R820T2. As a program for receiving I am using the SDRSharp.

  
  

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