How DDS Works

The basic principle of DDS is to use the sampling theorem to generate waveforms through table lookup. There are many types of DDS structures, and its basic circuit principle can be represented by Figure 3.

The phase accumulator is composed of an N-bit adder and an N-bit accumulator register in cascade. For each clock pulse FS, the adder adds the frequency control word K and the accumulated phase data output by the accumulator register, and sends the result to the data input of the accumulator register. The accumulator register feeds back the new phase data generated by the adder after the last clock pulse to the input of the adder, so that the adder continues to add the frequency control word under the action of the next clock pulse. In this way, the phase accumulator continuously adds the frequency control word under the action of the clock.

Linear phase accumulation. It can be seen from this that the phase accumulator accumulates the frequency control word once for each clock pulse input. The data output by the phase accumulator is the phase of the synthesized signal, and the overflow frequency of the phase accumulator is the signal frequency output by the DDS. The data output by the phase accumulator is used as the phase sampling address of the waveform memory (ROM), so that the waveform sampling value (binary code) stored in the waveform memory can be found through the lookup table to complete the phase to amplitude conversion. The output of the waveform memory is sent to the D/A converter, and the D/A converter converts the waveform amplitude in digital form into an analog signal of the required synthesized frequency. The low-pass filter is used to filter out unnecessary sampling components in order to output a sine wave signal with a pure spectrum. DDS far exceeds the transmission in a series of performance indicators such as relative bandwidth, frequency conversion time, high resolution, phase continuity, orthogonal output and integration.

It exceeds the level that can be achieved by traditional frequency synthesis technology, providing the system with performance superior to that of analog signal sources.

(1) The output frequency is relatively wide relative to the bandwidth

The output frequency bandwidth is 50%fs (theoretical value). However, considering the characteristics and design difficulty of the low-pass filter and the suppression of output signal spurious, the actual output frequency bandwidth can still reach 40%fs.

(2) Short frequency conversion time

DDS is an open-loop system without any feedback link. This structure makes the frequency conversion time of DDS extremely short. In fact, after the frequency control word of DDS changes, it takes one clock cycle to accumulate according to the new phase increment to achieve frequency conversion. Therefore, the frequency conversion time is equal to the transmission time of the frequency control word, that is, the time of one clock cycle. The higher the clock frequency, the shorter the conversion time. The frequency conversion time of DDS can reach the nanosecond level, which is several orders of magnitude shorter than using other frequency synthesis methods.

(3) Extremely high frequency resolution

If the frequency of the clock fs remains unchanged, the frequency resolution of the DDS is determined by the number of bits N of the phase accumulator. As long as the number of bits N of the phase accumulator is increased, any smaller frequency resolution can be obtained. Currently, the resolution of most DDS is on the order of 1 Hz, and many are less than 1 mHz or even smaller.

(4) Continuous phase change

Changing the DDS output frequency actually changes the phase increment of each clock cycle. The curve of the phase function is continuous, but the frequency changes suddenly at the moment of changing the frequency, thus maintaining the continuity of the signal phase.

(5)Flexibility of output waveform

As long as the corresponding control is added inside the DDS, such as frequency modulation control FM, phase modulation control PM and amplitude modulation control AM, the frequency modulation, phase modulation and amplitude modulation functions can be conveniently and flexibly realized to generate FSK, PSK, ASK and MSK signals. In addition, as long as different waveform data are stored in the waveform memory of the DDS, various waveform outputs can be realized, such as triangle wave, sawtooth wave and rectangular wave or even arbitrary waveform. When the waveform memory of the DDS stores the sine and cosine function tables respectively, two orthogonal outputs can be obtained.

(6) Other advantages

Since almost all components in DDS are digital circuits, they are easy to integrate, have low power consumption, small size, light weight, high reliability, easy program control, and are quite flexible to use, so they are extremely cost-effective.