http://nr8o.dhlpilotcentral.com/?p=83

diff --git a/AD9850/AD9850.ino b/AD9850/AD9850.ino
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+/*
+ * A simple single freq AD9850 Arduino test script
+ * Original AD9851 DDS sketch by Andrew Smallbone at www.rocketnumbernine.com
+ * Modified for testing the inexpensive AD9850 ebay DDS modules
+ * Pictures and pinouts at nr8o.dhlpilotcentral.com
+ * 9850 datasheet at http://www.analog.com/static/imported-files/data_sheets/AD9850.pdf
+ * Use freely
+ */
+ 
+ #define W_CLK 8       // Pin 8 - connect to AD9850 module word load clock pin (CLK)
+ #define FQ_UD 9       // Pin 9 - connect to freq update pin (FQ)
+ #define DATA 10       // Pin 10 - connect to serial data load pin (DATA)
+ #define RESET 11      // Pin 11 - connect to reset pin (RST).
+ 
+ #define pulseHigh(pin) {digitalWrite(pin, HIGH); digitalWrite(pin, LOW); }
+ 
+ // transfers a byte, a bit at a time, LSB first to the 9850 via serial DATA line
+void tfr_byte(byte data)
+{
+  for (int i=0; i<8; i++, data>>=1) {
+    digitalWrite(DATA, data & 0x01);
+    pulseHigh(W_CLK);   //after each bit sent, CLK is pulsed high
+  }
+}
+ 
+ // frequency calc from datasheet page 8 = <sys clock> * <frequency tuning word>/2^32
+void sendFrequency(double frequency) {
+  int32_t freq = frequency * 4294967295/125000000;  // note 125 MHz clock on 9850
+  for (int b=0; b<4; b++, freq>>=8) {
+    tfr_byte(freq & 0xFF);
+  }
+  tfr_byte(0x000);   // Final control byte, all 0 for 9850 chip
+  pulseHigh(FQ_UD);  // Done!  Should see output
+}
+ 
+void setup() {
+ // configure arduino data pins for output
+  pinMode(FQ_UD, OUTPUT);
+  pinMode(W_CLK, OUTPUT);
+  pinMode(DATA, OUTPUT);
+  pinMode(RESET, OUTPUT);
+ 
+  pulseHigh(RESET);
+  pulseHigh(W_CLK);
+  pulseHigh(FQ_UD);  // this pulse enables serial mode - Datasheet page 12 figure 10
+}
+ 
+void loop() {
+  sendFrequency(10.e6);  // 10.e6 freq
+  while(1);
+}