138 8-bit DDS sine wave generator
138 : 8-bit DDS sine wave generator
- Author: Abhinav Prasad, Steven O'Shea
- Description: A simple direct digital synthesizer with 8-bit frequency control and external R-2R output
- GitHub repository
- Open in 3D viewer
- Clock: 66000000 Hz
How it works
This project implements a basic 8-bit Direct Digital Synthesizer (DDS) that outputs a digitized sine wave with a user-defined frequency.
- The user provides an 8-bit phase increment (
phase_inc) via theui_in[7:0]pins - A rising edge on
uio_in[0](load_freq) loads this value into DDS - If no
load_freqtrigger is applied, the DDS runs at a default low frequency (corresponding tophase_inc = 8'd1)
Internally, a phase accumulator adds phase_inc to a phase register on each clock cycle. The 8-bit output of the accumulator directly indexes a sine look-up table (LUT) with 256 entries. Each LUT value corresponds to a digtized amplitude sample of a sine wave. These values are sent to uo_out[7:0], which can be converted to an analog waveform using an external R-2R DAC.
The output frequency is given by the standard DDS formula:
f_out = (phase_inc / 2^N) * f_clk,
where,
phase_incis the user-defined tuning word (8 bits)N = 8is the size of the phase accumulator (for this simplified implementation)f_clk = 66 MHzis the system clock
This design is based on principles described in Analog Devices’ application note:
"Fundamentals of Direct Digital Synthesis (DDS)" — Analog Devices https://www.analog.com/media/en/training-seminars/tutorials/MT-085.pdf
How to test
- Connect an 8-bit DIP switch or microcontroller output to
ui_in[7:0]to set the frequency - Pulse
uio_in[0]high momentarily to load the new value - Connect
uo_out[7;0]to an R-2R DAC to view the generated sine wave - You can low pass filter the DAC output to generate a smooth wave
Example:
- Phase increment =
8'b00000001produces a wave of ~258 kHz - Phase increment =
8'b10000000produces a wave of ~33 MHz (Nyquist limit)
External hardware
- DIP switches or a microcontroller can be used to control the frequency (
ui_in) and trigger (uio_in[0]) - An 8-bit R-2R resistor ladder DAC is required to convert
uo_out[7:0]into an analog sine waveform
IO
| # | Input | Output | Bidirectional |
|---|---|---|---|
| 0 | phase bit 0 | sine amplitude bit 0 | load_freq trigger (active high) |
| 1 | phase bit 1 | sine amplitude bit 1 | |
| 2 | phase bit 2 | sine amplitude bit 2 | |
| 3 | phase bit 3 | sine amplitude bit 3 | |
| 4 | phase bit 4 | sine amplitude bit 4 | |
| 5 | phase bit 5 | sine amplitude bit 5 | |
| 6 | phase bit 6 | sine amplitude bit 6 | |
| 7 | phase bit 7 | sine amplitude bit 7 |