1 Direct Digital Synthesis (DDS) and NCO

Direct Digital Synthesis is a technique of generating an analog waveform, generally of sinusoidal wave shape from a time varying signal in its digital form and a DAC.

The NCO module operates on the principle of DDS by repeatedly adding a fixed value to an accumulator. The accumulator is 20 bits in length and additions occur at the input clock rate, which can be a maximum of about 16 MHz. The accumulator will overflow with a carry bit set periodically, and this will produce a transition in the output of the NCO module.

The NCO module can operate in two modes: fixed duty cycle PWM and frequency controlled Pulse mode. With such an arrangement, the response will be very linear across a wide range of frequencies, ranging from 0 kHz up to 500 kHz using a clock of 16 MHz. The frequency resolution that can be obtained is precise and is in steps of 15 Hz across this entire frequency range. The linear frequency control and the increased frequency resolution are the key distinguishing factors when compared to the traditional PWM-based frequency control. Figure   1 illustrates the internal block diagram of the NCO module.

The NCO module generates precisely controllable output frequencies using the DDS technique. The DDS technique essentially provides a clock with carefully controlled jitter on it. Therefore, it is necessary that the signal be aggregated on the frequency domain.

Figure   2 illustrates the typical output spectra when generating 50% duty cycle square wave using the NCO module. The sideband noise generated by the jitter is insignificant in comparison to the fundamental frequency. When plotted on a logarithmic scale, the NCO output compares to that of a perfect square wave.