Monday, April 17, 2006

Direct Digital Synthesis

As all the engineers know, the waveform is generally analog in nature. It is then converted into digital form for processing ease. To test our digital circuits, we usually get an analog wave from an audio frequency oscillator (AFO) and feed it to ADC to get the digital output.

But today, the technology has changed paving way to a digital synthesis of analog waveforms. They calls this as direct digital synthesis chip or simply DDS. DDS consists of a single chip that generates sine wave, triangular wave, square wave and whatever a bulky AFO can do. The wonder is, it is a digital IC. Inside the circuit, the samples of these waves are first generated and passed on to the DAC to get the output as analog waves.

DDS has a lot of advantages. First is that the entire generator has shrinked down to a chip from the size of a shoe box. The frequency of the wave and the shape of the wave can be changed at very high speed - faster than what we can do with a completely analog AFO. This finds a lot of applications in cryptic communications. Also, DDS comes with a built in passband modulator. This means phase shift keying and frequency shift keying can be done with in this chip. But everything with in a few megahertz. DDS is clocked - meaning more than one DDS chip can be synchronized through a common clock. An "out of the head" application of the same is a separate modulation of I and Q. Above all these, DDS is programmable. You can program the different sequences in which you want the frequency of the sine wave to change with time.

With all these advantages, DDS also has a few cons. The demerits include the presence of phase noise, jitter and spur in the output. But these demerits are clearly out-weighed by the advantages it has.