Spartan-6 Microboard: FM All-Band CQ Transmitter

Here we go! This time we’re starting something a little bit more advanced. Lets check out the digital clock manager (DCM), which allows us to make custom clocks that run faster or slower than the actual physical clock. So what are we going to do with this? We’re going to make an FM transmitter that can transmit on any amateur radio band* and, of course, the standard FM radio band.

* please note this is totally illegal if you’re not a licensed HAM. So if you get caught, that’s on you.

I got most of this code from hamsterworks. It was a short simple piece of code that would type out S O S on 91MHz. I made it slightly less ridiculous I having a type out CQ (this is used to seek out contacts.) At least that way, my neighbors aren’t taking an axe to their dashboards trying to find a little man screaming for help. My neighbors are weird like that.

I put a ~1M long piece of 22Ga wire in the output pin, and get at least 10M range. No idea what kind of power I’m putting out (microWatts at most) and I fixed the frequency drift by finding the exact frequency given by the DCM.

Since this code had to be modified (only slightly, I won’t lie) I did some of my best commenting; I know that would have helped me. (Mind the scrollbar at the bottom.)


library IEEE;

entity fm_xmit is
    Port ( clk : in  STD_LOGIC;
           antenna : out  STD_LOGIC;
	   rst : in STD_LOGIC;
	   sw : in std_logic_vector(3 downto 0)	
end fm_xmit;

architecture Behavioral of fm_xmit is
   component fast_clock
   port ( CLK_IN1  : in  std_logic; --These are the signals from our Digital Clock Manager
          CLK_OUT1 : out std_logic; --Make sure the names are the exact same (not case sensitive)
	  RESET    : in std_logic   --Or this won't work at all
   end component;

   signal clk320            : std_logic;
   signal shift_ctr         : unsigned (4 downto 0) := (others => '0');
   signal phase_accumulator : unsigned (31 downto 0) := (others => '0');
   signal beep_counter      : unsigned (19 downto 0):= (others => '0'); -- gives a 305Hz beep signal
   signal message           : std_logic_vector(33 downto 0) := "1110101110100001110111010111000000"; --gives CQ in morse

	signal band_160m			: std_logic_vector (3 downto 0) := "1111"; --These are the 
	signal band_80m				: std_logic_vector (3 downto 0) := "1110"; --positions of 
	signal band_60m				: std_logic_vector (3 downto 0) := "1101"; --the 4 onboard
	signal band_40m				: std_logic_vector (3 downto 0) := "1100"; --dip switches,
	signal band_30m				: std_logic_vector (3 downto 0) := "1011"; --giving us a
	signal band_20m				: std_logic_vector (3 downto 0) := "1010"; --binary band 
	signal band_17m				: std_logic_vector (3 downto 0) := "1001"; --select.
	signal band_15m				: std_logic_vector (3 downto 0) := "1000";
	signal band_12m				: std_logic_vector (3 downto 0) := "0111";
	signal band_10m				: std_logic_vector (3 downto 0) := "0110";
	signal band_6m				: std_logic_vector (3 downto 0) := "0101";
	signal band_2m				: std_logic_vector (3 downto 0) := "0100";
	signal band_1_25m			: std_logic_vector (3 downto 0) := "0011";
	signal band_fm				: std_logic_vector (3 downto 0) := "0000";
	shared VARIABLE upper_side_signal: 	INTEGER; --these variables allow us to calculate
	shared VARIABLE lower_side_signal: 	INTEGER; --our clock divider ratio from the base 
	shared VARIABLE current_freq: 		INTEGER; --320MHZ (319996800Hz) to get our square
	shared VARIABLE center_signal: 		INTEGER; --wave period right


clock320 : fast_clock PORT MAP(
clk_in1 => CLK,
clk_out1 => CLK320,
reset => rst

   antenna <= std_logic(phase_accumulator(31));

   process(clk320, sw)
	IF    (sw = band_160m) 		THEN 	current_freq := 1810000; --these are the standard morse
	ELSIF (sw = band_80m) 		THEN 	current_freq := 3560000; --calling frequencies in the USA
	ELSIF (sw = band_60m) 		THEN 	current_freq := 5403500; --modify them to whatever you like
	ELSIF (sw = band_40m) 		THEN 	current_freq := 7040000; --in Hz.
	ELSIF (sw = band_30m) 		THEN	current_freq := 10106000;
	ELSIF (sw = band_20m) 		THEN 	current_freq := 14060000;
	ELSIF (sw = band_17m) 		THEN 	current_freq := 18080000;
	ELSIF (sw = band_15m) 		THEN 	current_freq := 21060000;
	ELSIF (sw = band_12m) 		THEN 	current_freq := 24910000;
	ELSIF (sw = band_10m) 		THEN 	current_freq := 28060000;
	ELSIF (sw = band_6m) 		THEN 	current_freq := 50090000;
	ELSIF (sw = band_2m) 		THEN 	current_freq := 144100000;
	ELSIF (sw = band_1_25m) 	THEN 	current_freq := 222100000;
	ELSIF (sw = band_fm) 		THEN 	current_freq := 100100000; --100.1 MHz, open freq here
    current_freq := 91000000;

	upper_side_signal := (current_freq/319996800*(2**32)) + 75000; --This exact clock frequency fixes
	lower_side_signal := (current_freq/319996800*(2**32)) - 75000; --the 'drift' shown in the video
	center_signal := (current_freq/31996800*(2**32));

      if rising_edge(clk320) then
         if beep_counter = x"FFFFF" then
            if shift_ctr = "00000" then
               message <= message(0) & message(33 downto 1);
            end if;
            shift_ctr <= shift_ctr + 1;
         end if;      

         if message(0) = '1' then
            if beep_counter(19) = '1' then
               phase_accumulator <= phase_accumulator + upper_side_signal; --+75kHz signal               
               phase_accumulator <= phase_accumulator + lower_side_signal; -- -75kHz signal
            end if;
            phase_accumulator <= phase_accumulator + center_signal; -- center frequency signal
         end if;

         beep_counter <= beep_counter+1;
      end if;
   end process;
end Behavioral;


NET "clk" LOC = C10;
NET "antenna" LOC = F15;
NET "rst" LOC = V4;
NET "sw[0]" LOC = B3 | PULLDOWN;
NET "sw[1]" LOC = A3 | PULLDOWN;
NET "sw[2]" LOC = B4 | PULLDOWN;
NET "sw[3]" LOC = A4 | PULLDOWN;

Teaser: Spartan-6 LX9 Microboard

Sparta-6 LX9 Microboard from AVNET and Xilinx

Sparta-6 LX9 Microboard from AVNET and Xilinx

This is teasing you as much as it is me. This is the Microboard. it’s an FPGA development board with onboard ram, 10/100 ethernet, and USB (among the various blinky switchy things all development boards must have) for the low, low cost of around $90.

Or if you know someone like Vlad a bit cheaper. Thanks for the donation!

For my first project, I’m looking at a simple FM (re)transmitter.  I want to make a local area delay filter. Think about it.

Part 1: Three-Band Tone Control

Digging through datasheets will always, ALWAYS give you great ideas. Looking in the datasheet for the TL082 dual JFET opamp (oddly available even at my podunk radioshack) found me this little gem.

Tone Control Schematic

Three Band Tone Control Schematic

Nice and simple, eh? Awesome, I know right? But for what use? Obviously for audio, but modern audio doesn’t require this, unless your building amplifiers…or radio transmitters…more after the break…

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