AD9851 Direct Digital Synthesis Arduino Library

AD9851 is a chip from Analog Devices for direct digital sythesis of radio frequency signals. This is a template class for controlling the AD9851.

Setup Functions

Instantiate the AD9851 template with the appropriate parameters. You must provide a Reset pin and a digital pin number to be used for chip select (FQ/UD):

#define AD9851_RESET_PIN      3
#define AD9851_FQ_UD_PIN      2

class MyAD9851 : public AD9851<AD9851_RESET_PIN, AD9851_FQ_UD_PIN> {};
MyAD9851 dds;

The third template parameter is the core clock frequency, after the 6x multiplier. Common modules use a 30MHz reference clock and so a 180MHz core clock. a calibration adjustment in parts-per-billion.

The fourth template parameter is the SPI data rate, default 2Mb/s. Hardware SPI means physical SCK (13) and MOSI (11) pins will be used automatically.

The AD9851 reset will be applied at the time the constructor is run. Alternatively, you can reset the chip at any time later:

dds.reset();

Calibrating the frequencies

If you have a good frequency counter, you apply a calibration for your AD9851 using setClock:

setClock(uint32_t calibration = 0);

To calibrate, use a zero calibration value, set some frequency and measure it as accurately as you can with a frequency counter. Convert that value to parts-per-billion error (positive if the frequency is high, negative if low) and provide that integer to setClock() as the calibration value instead. For example if you program 10MHz, but measure 1000043.2Hz, your calibration factor should be 4320.

You must setFrequency() after calling setClock with a new calibration, and recalculate any stored frequency deltas.

Setting the frequency

The setFrequency() function tells the module what frequency to generate.

dds.setFrequency(7140000UL);

Rapid frequency modulation

If you have to change frequency often, use frequencyDelta() to compute a delta value, and apply that with setDelta():

uint32_t div = dds.frequencyDelta(7140000UL);
dds.setDelta(div);

To make an FM signal, convert your centre frequency to a delta, and convert your 100% deviation to another delta. Then add or subtract a fraction of the deviation delta:

uint32_t centre_delta = dds.frequencyDelta(27245000UL);	// Citizen's band channel 25
uint32_t deviation_delta = dds.frequencyDelta(25000UL);	// +-25KHz deviation

void set_modulated_frequency(short d = 0)	// Range: -128..+128
{
  // This deviation calculation will not overflow up to 700KHz deviation
  dds.setDelta(centre_delta+(d*deviation_delta/128));
}