Arduino FM Modulation with AD9850

//More info http://zissisprojects.wordpress.com/arduino-sdr-ad9850

#include <SPI.h>
uint32_t frequency = 694000;                  //The desired frequency must be divided by 50 ex. 34,7MHz/50 = 694000
uint32_t tword = frequency * 3436 / 100;      //tuning word calculation
byte W[5] = {0,0,0,0,0};
const unsigned char PS_128 = (1 << ADPS2) | (1 << ADPS1) | (1 << ADPS0);  //these are the prescalers for the ADC sampling rate.
const unsigned char PS_32 = (1 << ADPS2) | (1 << ADPS0);
const unsigned char PS_16 = (1 << ADPS2);
int mic = 512;

void setup()
          {

            DDRB = B01100000;                          //portb has outputs, PB5(may be used for reset) PB6(FU_UD pin)
            PORTB = 0x00;
           // Serial.begin(115200);                    // everything can be controlled by serial comm
             pinMode (A0,INPUT);                       // here comes the audio frequency in
            SPI.setDataMode(SPI_MODE0);                // mode 0 seems to be the right one
            SPI.setClockDivider(SPI_CLOCK_DIV2);       // this is pretty fast
            SPI.setBitOrder(LSBFIRST);                 // AD9850 wants LSB first
            SPI.begin();
                                                       // set up the ADC
            ADCSRA &= ~PS_128;                         // remove bits set by Arduino library
            ADCSRA |= PS_32;                           // setting the sampling rate at 16MHz/32 this makes the analogRead() complete in around 40μs

          }

void loop()
  {    mic = analogRead(A0);                           //reading the AF signal
       frequency = 694000 + 3*mic-1536;                //this is the Frequency Modulation. Desired frequency is divided by 50 and then around 75khz deviation is calculated depending on the input amplitude
       tword = frequency * 1718;                       //calculating the tuning word for AD9850
              W[0] = (byte) tword;
              W[1] = (byte) (tword >> 8);
              W[2] = (byte) (tword >> 16);             //converting it to bytes
              W[3] = (byte) (tword >> 24);
              W[4] = 0; //phase zero
                                                       //start sending with spi interface
                 PORTB = B01000000;
                 PORTB = 0x00;                         //pulse FU_UD
                for (int j = 0; j<5;j++)
           {
             SPI.transfer(W[j]);                       //send the word
           }
                 PORTB = B01000000;
                 PORTB = 0x00;                         //pulse FU_UD

  }