DDS Signal Generator Based on FPGA

 

A DDS (Direct Digital Synthesis) Signal Generator based on FPGA is a high-speed and flexible method for generating accurate waveforms such as sine, square, triangle, or arbitrary signals. Using an FPGA for DDS provides precise frequency control, phase resolution, and the ability to generate complex modulations or custom waveforms in real time.

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 Key Concepts of DDS in FPGA

 1. Basic DDS Architecture

The core components of a DDS generator include:

mathematica

   
   ┌────────────┐      ┌────────────┐      ┌────────────┐
   │ Phase                  │      │ Waveform               │      │ DAC or                 │
   │ Accumulato             │──▶│ Lookup                 │──▶│ PWM/Output             │
   │ (Adder)                │      │ Table (ROM)            │      │ Stage                  │
   └────────────┘      └────────────┘      └────────────┘

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 2. Components Explained

• Phase Accumulator: Adds a fixed phase increment to its value on each clock cycle. The increment determines the output frequency.

• Phase-to-Amplitude Converter: Maps the accumulator value to a waveform amplitude using a lookup table (typically a sine wave ROM).

• Digital-to-Analog Converter (DAC): Converts the digital output into an analog signal. Can be external (real DAC) or internal (e.g., PWM or resistor DAC).

• Clock: Higher clock → finer resolution and smoother waveform.

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 DDS Frequency Formula

$$f_{out} = \frac{\Delta_{Phase} \times f_{clk}}{2^N}$$

• $\Delta_{Phase}$: Phase increment

• $f_{clk}$: Clock frequency

• $N$: Bit width of the phase accumulator

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 How to Implement DDS in FPGA (e.g., Xilinx/Intel)

Step 1: Design Phase Accumulator

vhdl

signal phase_acc : std_logic_vector(N-1 downto 0);
phase_acc <= phase_acc + phase_inc;

Step 2: ROM for Sine Wave

• Create a ROM using Block RAM or case statement.

• Use the upper bits of phase_acc to index the ROM.

Step 3: Digital Output

• Output the ROM value to DAC or use PWM modulation.

• Optionally apply filtering (low-pass) to smooth the output.

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 Optional Features

Feature	Description
Modulation	AM/FM/PM via dynamic phase increment
Multiple Channels	Multi-tone or quadrature signal gen
Arbitrary Waveforms	Load waveform samples into RAM
User Interface	Control via UART, SPI, switches, etc.

 FPGA Advantages for DDS

• High frequency resolution (up to MHz–GHz)

• Real-time waveform switching

• Easily scalable to multiple channels

• Ideal for SDR, signal processing, instrumentation

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 Practical Tools & Platforms

• FPGA Board: Xilinx Artix-7, Spartan-6, Intel MAX10, etc.

• DAC Options:

  • Onboard (e.g., AD9708)

  • External I²C/SPI DACs (e.g., MCP4921)

  • PWM + RC Filter (simple, cost-effective)

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 Example Application

FPGA DDS Signal Generator for Lab Use:

• Output: 1 Hz – 1 MHz sine and square waves

• Control: Frequency and waveform selection via UART

• Display: OLED/7-Segment showing current output

• Output via: SPI DAC and low-pass filter