The frequency is generated by the AD9850 and the modulation is done by the arduino
The full code is here:
http://pastebin.com/YMErwwK3
The FM band channels are centered at certain frequency, in our case 90Mhz, any slight deviation of that freq (by a max of +/- 75Khz) will move the membrane of the speaker backward or forward, here is the bit of code that implements that. And this is the (unoptimized) modulation routine.
void playTone(unsigned int note, unsigned int duration )
{
const double freq = 90 * 1e6; // transmitting at 90Mhz
// play silence
if ( note == 0)
{
ad.setfreq(freq);
delay(duration);
return;
}
unsigned long currentMillis = millis() + duration;
while( millis()< currentMillis)
{
ad.setfreq(freq + 75e3/2); // speaker's membrane backward
delayMicroseconds(note);
ad.setfreq(freq - 75e3/2); // speaker's membrane forward
delayMicroseconds(note);
}
ad.setfreq(freq );
}
Credits go to:
Brett Hagman for the RTTTL tune player
Poul-Henning Kamp for the AD9850 lib
