Vitis HLS: Direct Digital Synthesizer implementation

The Direct Digital Synthesizer (DDS) is a critical component in radar signal generation, providing precise and programmable frequency and phase signals essential for radar chirps and waveforms. Using Vitis High-Level Synthesis (HLS), I would like to explore how to efficiently design and implement a DDS for FPGA platforms, enabling high-performance, low-latency signal synthesis.

Architecture Overview :

The DDS HLS IP core consists of the following functional blocks:

• Read Axis: Handles data input via the AXI streaming interface.
• Phase Dithering: Applies noise to reduce phase quantization errors.
• Quantizer: Manages amplitude quantization, balancing precision and resource efficiency.
• Lookup Table: Retrieves precomputed waveform values (e.g., sine or cosine).
• Taylor Series Correction: Improves waveform accuracy with optional higher-order corrections.
• Write Axis: Outputs the generated waveform data through the AXI streaming interface.

Synthesis and Implementation Report :

The implementation was evaluated for 8192 input samples, with three configurations:

• Truncated: Focused on minimizing hardware usage by truncating lower bits in computations.
• Dithered: Introduced dithering to reduce quantization errors while maintaining moderate resource efficiency.
• Taylor Corrected: Enhanced accuracy using Taylor series correction but at the cost of higher resource utilization.

Key metrics from the synthesis and implementation reports include:

• DSP Utilization: Taylor correction uses the most DSPs due to increased precision, while truncation minimizes DSP usage.
• Latency: The latency increases with added correction methods, from 8192 cycles (Truncated) to 8210 cycles (Taylor Corrected).
• BRAM and LUT Usage: Higher precision configurations (Dithered, Taylor Corrected) demand more memory and logic resources.

DDS Performance Evaluation :

The performance evaluation highlights:

• DSP Utilization: Truncated implementation uses just 8 DSPs, while Taylor correction requires up to 31 DSPs for higher accuracy.
• Latency: Truncated DDS achieves the lowest latency, while Taylor correction has the highest due to additional processing.
• Resource Trade-offs: Dithering strikes a balance between accuracy and resource usage, offering a middle ground between truncation and Taylor correction.

Performance evaluation was conducted for the following Hardware Architecture:

Hardware Architecture :

Programmable Logic (PL):

Contains multiple DDS IP cores with different correction methods:

• HLS DDS Truncated: Optimized for minimal resource usage.
• HLS DDS Dithered: Reduces quantization errors using dithering.
• HLS DDS Corrected: Uses Taylor series correction for high accuracy.
• Features AXI Stream Switches for seamless data routing between IP cores.
• Includes AXI DMA v7.1 for efficient data transfer between the PL and PS.

Processing System (PS):

• Powered by the Arm Cortex-A9 for managing configuration, runtime control, and data processing.
• Interacts with DDR memory for storing input and output data streams.

The Hardware Architecture features:

• Implementation of fully pipelined DDS IP cores in Vitis HLS.
• Support for phase truncation, dithering, and Taylor series correction.
• Integration of Xilinx AXI DMA and AXIS Switch IP cores for flexible data handling.
• Spurious Free Dynamic Range (SFDR) estimation for evaluating DDS performance.
• Linux Deployment for Zynq-7000 using Buildroot.

The plot evaluates the Spurious Free Dynamic Range (SFDR) of a Direct Digital Synthesizer (DDS) implemented using Vitis HLS with three different correction methods: truncated, dithered, and Taylor-corrected outputs.

Key Observations :

DDS Truncated Output:

• Represents the DDS output with no phase or amplitude correction applied.
• The spurious tones are relatively high, resulting in an SFDR of 72.3 dB.
• This configuration is resource-efficient but produces significant harmonic distortion, limiting its use in applications requiring high spectral purity.

DDS Dithered Output:

• Introduces phase dithering to reduce quantization errors.
• This significantly suppresses spurious tones, improving the SFDR to 91.8 dB.
• Offers a balance between spectral performance and resource usage, making it suitable for mid-range applications.

DDS Corrected Output:

• Applies Taylor series correction to achieve high accuracy.
• Produces the cleanest spectrum with spurious tones minimized, achieving an SFDR of 101.6 dB.
• Ideal for applications requiring the highest spectral purity, such as precision radar or communication systems, but comes at the cost of increased resource usage.

For more details about implementation of Direct Digital Synthesizer in Vitis HLS please visit my website.