Case Study: DDC/NCO IP Core for Zynq-7020

High-Performance Digital Down Conversion on AMD Xilinx PL

Challenge: Processing high-speed RF samples requires a highly efficient Digital Down Converter (DDC) and Numerically Controlled Oscillator (NCO) running at line rate. Software-based SDR architectures bottleneck on memory bandwidth and CPU cycles when dealing with continuous multi-megasample data streams. The objective was to offload the entire DDC/NCO mixing and decimation pipeline into the Programmable Logic (PL) of a Zynq-7020 SoC, freeing the Processing System (PS) for higher-level analysis.

Architecture & Implementation: The IP core was developed in Verilog, utilizing a quarter-wave Look-Up Table (LUT) NCO architecture to minimize block RAM utilization while maintaining high Spurious-Free Dynamic Range (SFDR). The mixer outputs feed into a cascaded CIC (Cascaded Integrator-Comb) and FIR (Finite Impulse Response) decimation filter chain. The module interfaces with the rest of the system using AXI4-Stream for high-throughput I/Q data and AXI4-Lite for PS-side configuration (frequency tuning words and phase offsets).

Performance: By migrating the DDC/NCO pipeline directly into the Zynq-7020 PL fabric, the design achieved continuous, real-time RF baseband translation with deterministic latency. The AXI DMA offloads the decimated, baseband I/Q data directly into DDR memory, completely removing the ARM Cortex-A9 from the real-time sample processing loop and maximizing system throughput.

System Architecture Diagram

NCO Architecture

graph LR
    A[ADC Data Stream] --> B[AXI4-Stream Interface]
    B --> C[Mixer]
    D[Quarter-Wave LUT NCO] --> C
    E[AXI4-Lite Config] --> D
    C --> F[CIC Decimation Filter]
    F --> G[FIR Filter]
    G --> H[AXI DMA to PS DDR]

Client Outcome

Delivered a drop-in, AXI-compliant IP core that resolved real-time processing bottlenecks. The hardware-accelerated DDC enabled the client to process wider bandwidths at lower power, achieving the required throughput while freeing up 90% of the Zynq PS CPU cycles for application-level logic.