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DDS (Direct Digital Synthesis) is a modern signal generation technique that produces waveforms directly from the phase domain. It integrates advanced digital processing methods into the field of signal synthesis, offering high precision and flexibility in frequency control. The DDS signal generator uses Direct Digital Synthesis technology to achieve frequency stability and accuracy comparable to the reference frequency, allowing for fine-tuned frequency adjustments across a wide range. This type of signal source can operate in modulation mode, enabling users to adjust output levels or generate various waveform types.
The function of a DDS signal generator relies on a lookup table storing one cycle of a waveform. A phase accumulator tracks the current phase of the output. For low-frequency signals, the phase increment (Δ) between samples is very small, such as 1 degree for a slow sine wave. Each sample corresponds to a specific point on the waveform, and after 360 samples, a full cycle is completed. For higher frequencies, the Δ value increases, reducing the number of samples per cycle. The frequency is determined by the phase increment, and this can be rapidly adjusted using a frequency register. By defining a frequency table with different segments, complex signals like frequency sweeps or hopping can be generated. DDS also allows for continuous phase changes, making it highly versatile for applications in research, communications, consumer electronics, aerospace, defense, and emerging technologies like software-defined radio and wireless sensing.
The working principle of a DDS signal generator follows the Nyquist sampling theorem. It samples, quantizes, and encodes a sinusoidal signal based on the phase Φ of the continuous signal, then stores the resulting waveform in ROM or RAM. The phase accumulator controls the frequency by adjusting the phase increment. Different increments result in different sampling points within a cycle. Since angular frequency ω = Δφ × Δt, changing the frequency control word alters the phase and amplitude of the signal without affecting the sampling rate. After digital-to-analog conversion and low-pass filtering, a clean analog signal is produced.
The basic block diagram of a DDS system includes four main components: the phase accumulator, which determines the frequency range and resolution; the waveform memory (sine lookup table), which stores the sampled amplitudes of the waveform; the D/A converter, which generates the analog signal; and the low-pass filter, which reduces noise and eliminates spurious components. A stable crystal oscillator serves as the reference clock, ensuring the synthesized signal has the same frequency stability as the oscillator.
The AD7008 is a monolithic DDS chip that contains a 32-bit phase accumulator, sine and cosine lookup tables, a 10-bit DAC, and additional features like frequency, phase, and amplitude modulation. It supports both parallel and serial interfaces for microcontroller communication. The phase accumulator’s 32-bit architecture allows for precise control over the output frequency, calculated as:
$$ f_{out} = \frac{\Delta \text{phase} \times f_{clock}}{2^{32}} $$
In practice, the maximum output frequency is limited to around 40% of the clock frequency to avoid excessive phase noise and spurious signals. The Δphase value can be selected from two registers, FREQ0 and FREQ1, depending on the desired output. Although the phase accumulator is 32 bits, the output is only 12 bits, so the full resolution isn’t always necessary. The 12-bit data is used to address the sine lookup table, producing a 10-bit amplitude signal. If amplitude modulation is not required, the IQ multiplier can be bypassed, and the sine wave can be directly sent to the DAC. The DAC’s full-scale output current can be adjusted via an external resistor Rset, using the formula:
$$ I_{out}(\text{mA}) = \frac{6233 \times V_{ref}(\text{V})}{R_{set}(\Omega)} $$