DDS Function Generator

Vassilis Papanikolaou   vpapanik@gmail.com     
685

A new implementation of the AVR DDS signal generator v2.0 found at: http://www.scienceprog.com/avr-dds-signal-generator-v20

Introduction

This is a new implementation of the AVR DDS signal generator v2.0, already published in scienceprog.com.  It is obvious that full credit for the original schematic and the firmware goes to its original creator. Presented here is a different PCB that is compact, single sided with only through-hole components for easy construction.

The function generator features two BNC outputs : one for the high speed [1 to 8 MHz] square signal (BNC1) and another for the DDS signal (BNC2). Offset and amplitude can be regulated by two potentiometers : offset in range of +5V to -5V (POT1) and amplitude in range of 0 to 10V (POT2). Up and down arrow buttons are used for changing the function type (sine, triangle etc.) while left and right arrow buttons are used for changing the frequency value. There is also a separate menu for changing frequency step. When the middle button is pressed, the signal generation starts. Middle button is pressed again for stopping the signal. More details can be found in the original site.

Schematic

Schematic

EAGLE Schematic (only a status led and an on/off switch was added)

Parts

Part Value
R1 470 Ω ½W 5%
R2 10 ΚΩ ¼W 5%
R3 100 ΚΩ ¼W 1%
R4 20 ΚΩ ¼W 1%
R5 20 ΚΩ ¼W 1%
R6 10 ΚΩ ¼W 1%
R7 20 ΚΩ ¼W 1%
R8 10 ΚΩ ¼W 1%
R9 20 ΚΩ ¼W 1%
R10 10 ΚΩ ¼W 1%
R11 20 ΚΩ ¼W 1%
R12 10 ΚΩ ¼W 1%
R13 20 ΚΩ ¼W 1%
R14 10 ΚΩ ¼W 1%
R15 20 ΚΩ ¼W 1%
R16 10 ΚΩ ¼W 1%
R17 20 ΚΩ ¼W 1%
R18 10 ΚΩ ¼W 1%
R19 20 ΚΩ ¼W 1%
R20 100 Ω ¼W 5%
R21 100 ΚΩ ¼W 1%
R22 12 ΚΩ ¼W 1%
R23 150 Ω ¼W 5%
POT1 1 ΚΩ linear potentiometer
POT2 47 KΩ linear potentiometer
POT3 10 ΚΩ trimmer
C1 100 nF MKT/polyester
C2 100 nF MKT/polyester
C3 18 pF ceramic
C4 18 pF ceramic
Q1 16 MHz crystal
IC1 ATMEL ATMEGA16P
IC2 LM358N
BNC1 – BNC2 BNC female connector
S1 – S6 Push button
LCD Header Female header 16 pin for LCD
LCD Module HD44780-based 2×16 character LCD
ISP Male header 2×3 for ISP
POWER Female header 4-pin for power as follows :
PIN1 : +12V
PIN2 : -12V
PIN3 : GND
PIN4 : +5V
LED1 3 mm green led
ON/OFF Miniature on/off switch

Power Supply

For powering up the function generator, a PC ATX power supply unit was used, where all voltages are already available (+12V, -12V, +5V). The wiring is shown in the following image, taken from scienceprog.com

molex_ATX

Photos of factory-made PCB and parts

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Video

Photos of assembled PCB

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Programming of the ATMEGA16 with usbtiny programmer

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The device firmware can be downloaded from :

http://www.scienceprog.com/avr-dds-signal-generator-v20

Changes in firmware

Because of the present LCD character orientation, which is different from the original implementation (180 degrees), the following changes in main.c were made :   

Buttons LEFT and RIGHT were reversed :
#define LEFT 3//PORTD
#define RIGHT 1//PORTD
Buttons TOP and BOTTOM were reversed :
#define DOWN 4//PORTD
#define UP 0//PORTD
For the latest version of AVR-GCC compiler, the following changes should be made (according to Geoff comment on scienceprog.com) :
struct signal{
volatile uint8_t mode; //signal
volatile uint8_t fr1; //Frequency [0..7]
volatile uint8_t fr2; //Frequency [8..15]
volatile uint8_t fr3; //Frequency [16..31]
volatile uint32_t freq; //frequency value
volatile uint8_t flag; //if 0 generator is OFF, 1 is ON
volatile uint32_t acc; //accumulator
volatile uint8_t ON;
volatile uint8_t HSfreq; //high speed frequency [1...4Mhz]
volatile uint32_t deltafreq; //frequency step value
}SG;
The ATMEGA16 fuses should be :
HIGH = 0×59
LOW = 0xCF
This is interpreted to the following options checked (all others unchecked) :
OCDEN
SPIEN
BOOTSZ1
BOOTSZ2
SUT1
SUT0

Photos of the device operation

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PCB

2 Response on “DDS Function Generator

  1. I just bought this on Ebay http://www.ebay.com/itm/1PC-DDS-Function-Signal-Generator-Module-Sine-Square-Sawtooth-Triangle-Wave-Kit-/251768575779?hash=item3a9e939723 which appears to be a fully built kit for sale, using the circuit described on this webpage. Unfortunately, the frequency output is half what it should be (1000Hz signal of any waveform setting gives me a 500Hz signal instead). Is this most likely something that was done wrong by the person who built the kit, or maybe in the firmware for the device has a glitch to start with? Would correcting it be as simple as adjusting the 10kohm trimmer resistor? Or would I need to reprogram the microcontroller to fix this issue? Any help with this would be great. Thanks in advance.

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