-- Model of Binary Frequency Shift Keying (BFSK) modulator
-- with digital input and analog output


library IEEE;
use IEEE.electrical_systems.all;
use IEEE.STD_LOGIC_1164.all;
use IEEE.MATH_REAL.all;

entity bfsk is
  generic (
    fc      : real    := 455.0e3;   -- Mean carrier frequency
    delta_f : real    := 5.0e3;     -- Difference between low and high carrier frequency
    amp     : voltage := 1.0;       -- Amplitude of modulated signal
	offset  : voltage := 0.0        -- output offset voltage
    );

  port (
    d_in           : in std_logic;       -- digital input
    terminal a_out :    electrical       -- output terminal 
    );
end entity bfsk;

architecture behavioral of bfsk is

  quantity vout across iout through a_out;      -- output branch
  quantity phi : real;                          -- free quantity for angle in radians
  constant wc : real := math_2_pi*fc;           -- convert fc to rad/s
  constant delta_w : real := math_2_pi*delta_f; -- convert delta_f to rad/s

begin

  if (d_in = '0') use
    phi'dot == wc;               -- set to carrier frequency
  elsif (d_in = '1') use
    phi'dot == wc + delta_w;	 -- set to carrier frequency + delta
  else
    phi'dot == 0.0;
  end use;

  vout == offset + amp*sin(phi); -- create sinusoidal output using phi

end architecture behavioral;