Software implementation

To run this software on an Arduino Diecimila or Duemilenove connect a potentiometer to +5Volt and Ground and the wiper to analog 0. The frequency appears on pin 11 where you can connect active speakers or an output filter described later.


/*
*
* DDS Sine Generator mit ATMEGS 168
* Timer2 generates the 31250 KHz Clock Interrupt
*
* KHM 2009 / Martin Nawrath
* Kunsthochschule fuer Medien Koeln
* Academy of Media Arts Cologne

*/

#include "avr/pgmspace.h"

// table of 256 sine values / one sine period / stored in flash memory
PROGMEM prog_uchar sine256[] = {
127,130,133,136,139,143,146,149,152,155,158,161,164,167,170,173,176,178,181,184,187,190,192,195,198,200,203,205,208,210,212,215,217,219,221,223,225,227,229,231,233,234,236,238,239,240,
242,243,244,245,247,248,249,249,250,251,252,252,253,253,253,254,254,254,254,254,254,254,253,253,253,252,252,251,250,249,249,248,247,245,244,243,242,240,239,238,236,234,233,231,229,227,225,223,
221,219,217,215,212,210,208,205,203,200,198,195,192,190,187,184,181,178,176,173,170,167,164,161,158,155,152,149,146,143,139,136,133,130,127,124,121,118,115,111,108,105,102,99,96,93,90,87,84,81,78,
76,73,70,67,64,62,59,56,54,51,49,46,44,42,39,37,35,33,31,29,27,25,23,21,20,18,16,15,14,12,11,10,9,7,6,5,5,4,3,2,2,1,1,1,0,0,0,0,0,0,0,1,1,1,2,2,3,4,5,5,6,7,9,10,11,12,14,15,16,18,20,21,23,25,27,29,31,
33,35,37,39,42,44,46,49,51,54,56,59,62,64,67,70,73,76,78,81,84,87,90,93,96,99,102,105,108,111,115,118,121,124

};
#define cbi(sfr, bit) (_SFR_BYTE(sfr) &= ~_BV(bit))
#define sbi(sfr, bit) (_SFR_BYTE(sfr) |= _BV(bit))

int ledPin = 13; // LED pin 7
int testPin = 7;
int t2Pin = 6;
byte bb;

double dfreq;
// const double refclk=31372.549; // =16MHz / 510
const double refclk=31376.6; // measured

// variables used inside interrupt service declared as voilatile
volatile byte icnt; // var inside interrupt
volatile byte icnt1; // var inside interrupt
volatile byte c4ms; // counter incremented all 4ms
volatile unsigned long phaccu; // pahse accumulator
volatile unsigned long tword_m; // dds tuning word m

void setup()
{
pinMode(ledPin, OUTPUT); // sets the digital pin as output
Serial.begin(115200); // connect to the serial port
Serial.println("DDS Test");

pinMode(6, OUTPUT); // sets the digital pin as output
pinMode(7, OUTPUT); // sets the digital pin as output
pinMode(11, OUTPUT); // pin11= PWM output / frequency output

Setup_timer2();

// disable interrupts to avoid timing distortion
cbi (TIMSK0,TOIE0); // disable Timer0 !!! delay() is now not available
sbi (TIMSK2,TOIE2); // enable Timer2 Interrupt

dfreq=1000.0; // initial output frequency = 1000.o Hz
tword_m=pow(2,32)*dfreq/refclk; // calulate DDS new tuning word

}
void loop()
{
while(1) {
if (c4ms > 250) { // timer / wait fou a full second
c4ms=0;
dfreq=analogRead(0); // read Poti on analog pin 0 to adjust output frequency from 0..1023 Hz

cbi (TIMSK2,TOIE2); // disble Timer2 Interrupt
tword_m=pow(2,32)*dfreq/refclk; // calulate DDS new tuning word
sbi (TIMSK2,TOIE2); // enable Timer2 Interrupt

Serial.print(dfreq);
Serial.print(" ");
Serial.println(tword_m);
}

sbi(PORTD,6); // Test / set PORTD,7 high to observe timing with a scope
cbi(PORTD,6); // Test /reset PORTD,7 high to observe timing with a scope
}
}
//******************************************************************
// timer2 setup
// set prscaler to 1, PWM mode to phase correct PWM, 16000000/510 = 31372.55 Hz clock
void Setup_timer2() {

// Timer2 Clock Prescaler to : 1
sbi (TCCR2B, CS20);
cbi (TCCR2B, CS21);
cbi (TCCR2B, CS22);

// Timer2 PWM Mode set to Phase Correct PWM
cbi (TCCR2A, COM2A0); // clear Compare Match
sbi (TCCR2A, COM2A1);

sbi (TCCR2A, WGM20); // Mode 1 / Phase Correct PWM
cbi (TCCR2A, WGM21);
cbi (TCCR2B, WGM22);
}

//******************************************************************
// Timer2 Interrupt Service at 31372,550 KHz = 32uSec
// this is the timebase REFCLOCK for the DDS generator
// FOUT = (M (REFCLK)) / (2 exp 32)
// runtime : 8 microseconds ( inclusive push and pop)
ISR(TIMER2_OVF_vect) {

sbi(PORTD,7); // Test / set PORTD,7 high to observe timing with a oscope

phaccu=phaccu+tword_m; // soft DDS, phase accu with 32 bits
icnt=phaccu >> 24; // use upper 8 bits for phase accu as frequency information
// read value fron ROM sine table and send to PWM DAC
OCR2A=pgm_read_byte_near(sine256 + icnt);

if(icnt1++ == 125) { // increment variable c4ms all 4 milliseconds
c4ms++;
icnt1=0;
}

cbi(PORTD,7); // reset PORTD,7
}

Results

In the upper part of the picture you see PWM signal on pin 11 and in the lower part what the filter makes out of it. The sinewave looks not so clean but thats mainly the limited resolution of the digital oscilloscope.

The spectrogram shows a surprisingly good result. The big peak is the output frequency of about 1000 Hz. All unwanted distortions are below 50 dB which is roughly what we expect from a signal what is generated by an 8-bit DAC. ( 1/256 = 48dB).

DDS Spreadsheet

dds_calc A little worksheet around the DDS formula to calculate the tuning word.

PWM Output lowpass Filter

For a start you can just connect the output pin 11 to active speakers. But usually you need lowpass filter is to get rid of the 32KHz sampling frequency in the output signal. A Chebyshef lowpass with a cutoff at 12 KHz build with standard component values is shown here.

PWM DDS dedicated Hardware

This software implementation of DDS has of course several drawbacks in case of signal purity and frequency range due to the limited speed of the software algorithm and analog capabilities of the atmega chip. DDS modules which are using dedicated DDS chips are the state of the art in signal generation and offer a frequency coverage from zero up into the 100MHz range.
WSPR Application

WSPR “Weak Signal Propagation Reporter” pronounced Whisper is a system which reports the propagation of very weak radio signals over the world. The DDS Method was used here to generate a tone sequence where four frequencies (1497,8 1499,3 1500,7 1502,2 Hz) are used code a message in a very robust manner. This message is transmitted with a low power radio beacon and can be observed worldwide via wspr.net .